Abbe Number

AKA the Abbe constant, this is the quantification of the amount of dispersion, or variations in index, for a certain spectral range. A high Abbe number means there will be less separation between wavelengths of light because the various wavelengths have nearly the same index of refraction in that medium. This means there will be less color dispersion and color aberration.


The Abbe-Koenig prism is a common type of roof prism used in many hunting optics to invert an image or rotate it 180º. This type of prism is named after Ernst Abbe and Albert Koenig. It is constructed from two glass prisms that are optically cemented together in a symmetric, shallow V-shaped formation. Light enters one face of the prism assembly at normal incidence and is then internally reflected from a 30º slope face. After that, the light is once again reflected from a “roof” section made up of two faces meeting at a 90º angle. Next, the light is reflected from the opposite 30º face and exits the construction at normal incidence. Thus, the image is flipped 180º without changing its handedness, and so the prism is able to be used as an erecting system. The Abbe-Koenig design offers the highest light transfer of the roof prism designs and is less bulky than the double Porro design.


Correcting lens aberration is vital to ensuring the best possible view through a hunting optic while maintaining a bright, sharp image. The best optics will correct Astigmatism, Chromatic Aberration, Coma, Curvature of Field, Distortion, and Spherical Aberration.


Also known as an achromatic lens. A lens consisting of two or more elements, usually of crown and flint glass, that has been corrected for chromatic (color) aberrations with respect to two selected colors, usually blue and red.

Achromatic Doublet Lens

Such a lens is designed to limit the effects of chromatic and spherical aberration by using a strong positive lens made from a low dispersion glass, like crown glass, coupled with a weaker high dispersion glass such as flint glass. This combination can correct the chromatic aberration for two colors, usually red and blue. The doublets are often optically cemented together (called achromat doublets) and are often used in compound lenses. An achromat doublet cannot completely correct chromatic aberration, but does eliminate it for two colors. To eliminate chromatic aberration for three colors, an apochromat triplet can be used.

Additive Color

A primary light color - red, blue, or green; these three colors produce white light when added together.

Altazimuth Mount

This usually refers to telescope mount which allows movement in two directions: azimuth (horizontally) and elevation (vertically).

Angle Of Incidence

The angle between a wave striking a barrier and the line perpendicular to the surface.

Angle of incidence (AOI)

The angle formed between a ray of light striking a surface and the line perpendicular to the surface.

Angle Of Reflection

The angle between a reflected wave and the normal to the barrier from which it is reflected.


An angstrom is 1/100,000,000 of a centimeter.

Angular Measurements

The performance characteristics of optical instruments are often expressed as angular measurements. Field of view is measured in degrees; windage and elevation adjustments are calibrated in minutes of angle or fractions thereof; resolution is measured in seconds of angle, and the center-to-center spacing of dots on mil-dot reticles equals one milliradian (mil.).

Anomalous Partial Dispersion Glass

This is a type of extra-low dispersion (ED) glass also referred to as HD glass. It has a non-linear refraction of color wavelengths, which allows the lens designer to combine different types of glass together, getting the desired level of color correction. Using ED glass, a lense designer can reduce or even completely eliminate residual chromatic aberration, something that cannot be achieved with conventional optical glass.

Antireflection (AR) coatings

Used to reduce reflections from glass-to-air surfaces, thereby, reducing flare and improving light transmission. Better instruments have multilayer coatings on all surfaces, and are referred to as being "fully multicoated."

Antireflection coating (AR)

Dielectric thin film coating applied to an optical material to increase the transmittance by reducing the surface reflection.


The diameter of the binoculars' or scope's objective lenses, measured in mm.


Also known as an apochromatic lens. A lens system in which the three primary colors (blue, green and red) have been brought to a common focus through the use of special optical glasses, thus eliminating practically all chromatic aberration. Abbreviated terms used to designate instruments with apochromatic optics include: ED (Extra-low Dispersion), HD (High Definition), FL (Fluorite) and APO (Apochromatic).

Apochromatic Lens (APO)

Apochromats, or apochramtic lenses (APOs), are lenses comprised usually of three elements that bring the light from three different frequencies to a common focal point. This provides better correction of chromatic aberration and spherical aberration than the more common achromat lenses.

Apochromatic Triplet

In an apochromatic triplet lens (see Apochromatic Lens above), three elements are used to reduce chromatic aberrations, which provides better correction levels than achromat doublets.

Apparent magnitude

This refers to how bright the star appears to the naked eye. The difference between the apparent brightness of two stars follows a logarithmic ratio of 2.512. Therefore, a star that is three magnitudes less than another is (2.512) 3, or about 16 times brighter. Using this system, stars can also have negative magnitude values, and these are the brightest we see in the sky. See also Magnitude.

Aspheric Lenses

A lens with a surface that is not perfectly spherical, convex, or concave is called an aspheric lens. In other words, different areas of the lens have different degrees of curvature, producing a more complex surface that can reduce or eliminate spherical aberration while also reducing other optical aberrations better than a simple lens. A single aspheric lens can also sometimes replace a much more complex multi-lens system, because of the use of different degrees of curvature. This means a smaller, lighter instrument that is sometimes less expensive. This performance and quality is not guaranteed just because it has the aspheric label, though, because many low end optics are made using aspheric lenses that are poured into a mold rather than being conventionally ground, which reduces the quality. The higher end of the quality spectrum sees aspheric lenses ground into much more sophisticated shapes than a spherical lens, a very expensive and difficult process but one that pays off with incredible results.

Aspheric vs. Spherical

Just like a sphere has a constant curvature, spherical lenses have the same curvature throughout their entire surface. Thus, the power of the lens at different points is variable, which results in peripheral light rays becoming defocused relative to the more centralized light rays. These lenses are easier to make, but their optics are worse. An aspheric lens, on the other hand, maintains a constant power even at the periphery of the lens, because the curvature is varied. This means the same focus for all light rays, both peripheral and central. These lenses are more difficult and expensive to make, but they yield better optics.

Aspherical Lens

A lens with flattened edges, useful for a clearer, sharper image.


When light rays are coming from off-axis, this forms an ellipse on the surface of the lens instead of a circle. This means that a dot can appear to be more like a line than a dot, resulting in image distortion. Astigmatism is similar to coma, except that coma aberrations are affected by the size of the aperture.

Bak-4 Glass

Premium, high-density barium crown glass that minimises internal light scattering so the images seen through these lenses are sharper. See also "Prism Glass".

BaK–4 prisms

BaK–4 prisms, manufactured from Baritleichkron, a Barium Crown glass, has a higher refractive index (1.569) than BK–7 prisms, so less peripheral light is “lost” through the non-total internal reflection. Optics made with BaK–4 prisms thus have higher light transmission and are better suited to low-light conditions than those using BK–7 prisms.

Bandpass filter

Optical filter or interference filter that allows transmission over a particular wavelength range while restricting the transmission of wavelengths above and below this range.


Spectral width of wavelengths transmitted by a bandpass filter. Usually specified as Full Wave Half Maximum (FWHW) or the width at 50% of peak transmittance.

Barlow Lens

An extra lens used in conjunction with a telescope's eyepiece to increase the magnification, usually by 2 or 3 times. This is named after the English physicist Peter Barlow. View Barlow lenses in this shop.


Relating to, used by, or involving both eyes at the same time, as in binocular vision. n. A pair of telescopes mounted side-by-side, usually by means of a centrally located hinge, so that the eyepiece spacing can be adjusted to match the viewer’s interocular distance. Binoculars are rated by their magnifying power followed by the objective diameter in millimeters (e.g., 8X 42 mm)

Binocular tripod adaptor

An L-shaped adapter that connects a binocular to the pan head of a standard tripod.

Binocular, Porro Prism

This classic binocular design has a Z-type configuration that positions the objective lenses further apart than the eyepieces, significantly increasing the viewer's stereoscopic depth perception.

Binocular, Roof Prism

This type is recognizable by having its objective and eyepiece lenses in a straight line. The stereopsis is then equal to that of the viewer's unaided eyes as enhanced by the magnification. Although, some roof prism binoculars, particularly those having Abbe-style roof prisms, do have the objectives slightly offset.


Borosilicate crown optical glass with low bubble content and high homogeneity. BK7 is an excellent visible to NIR material for many optical components.

Bk-7 glass

Also known as "borosilicate" glass. Most optical prisms are made of BK-7 glass.

BK–7 prisms

Found in lower-priced optics, BK–7 prisms have a refractive index of 1.5168, allowing more peripheral light to be lost than the more expensive BaK–4 prisms. Edge-to-edge sharpness and light transmission suffer at least slightly with BK–7 prisms.

Broadband Anti-Reflective (AR) Coatings

This is a relatively new optical coating technology to provide anti-reflective properties over much wider spectral range for improved image quality.

Broadband AR coating (BBAR)

A coating designed to provide increased transmittance and reduced reflectance over a very wide (broad) band of wavelengths. Typically from 425-675nm, however, can be designed for any wavelength range.


A reflecting scope comprising a primary mirror with a central hole through which the light from the primary mirror is reflected to an eyepiece at the focus, the cassegrain focus, beyond the primary mirror. The design is often used in compact and portable telescopes.

Catadioptric Design

In a catadioptric system, the image is brought to your eye using both refraction and reflection. This is achieved in most cases by way of curved mirrors, called catoptrics, and lenses referred to as dioptrics. Catadioptric systems are used primarily in focusing systems like search lights, headlamps, early lighthouse focusing systems, optical telescopes, microscopes, and telephoto lenses. Celestron also uses the design in a small number of its spotting scopes.

Catadioptric Telescope

A telescope that uses a combination of mirrors and lenses to focus an image of a distant object at a focal point.

Center-Focus binoculars

A mechanism that allows both eyepieces to be adjusted at the same time, useful for rapid focus. Center focusing is the most common and convenient and generally the most preferred way of focusing. See also Diopter Adjuster and Individual Focus.

Central Focusing Wheel

A wheel mounted in the middle of the binoculars for focus adjustment.

Central wavelength

Wavelength at the midpoint of a coating design.


The mechanical axis and the optical axis exactly coincide in a perfectly centered optic.


A ground beveled edge on an optic and useful in preventing chipping.

Chromatic aberration

This is a defect of optical lenses used in binoculars. Different wavelengths (producing different colours) are diffracted, or bent, at different angles and produce coloured halos around images.

Clear aperture

The usable area, within published specifications, of an optic. Typically 90% if there is no clear aperture specified.

Close Focus (Near Focus)

The closest you can be from an object and still get a clear, focused view through the binoculars or the scope. For example, the close focus of Nipon 10x50 binoculars is around 7m; the close focus of NIPON 26-78x78 is about 10m, suitable for birding at close range.

Coated / Multi-Coated Glass

Thin layer(s) of coating applied to the glass surface to help reduce light reflections. This coating reduces the amount of light lost as the light passes through the glass surface.

Coated Optics

Optical elements having refractive or reflective surfaces that have been coated with one or more layers of either dielectric or metallic materials that serve to reduce or increase reflections, eliminate phase interference, decrease external fogging or protect the surfaces from abrasion. Magnesium fluoride, silicon oxide, zinc sulfide, aluminum and silver are a few of the most commonly used coating materials.

Coatings, Antireflection

These coatings, whether single layer or multilayered, reduce reflections from optical surfaces, thus increasing the percentage of the available light transmitted through optical instruments. The results are improved image brightness and contrast.

Coatings, Phase-Correction (P)

When applied to the roof surfaces of roof prisms, phase-correction coatings reduce the image-degrading affects of destructive interference that occur when out-of-phase light waves from the opposite roof surfaces reunite to form images.

Coatings, Reflection

These coatings improve the reflectance of the various types of mirrored surfaces used in optical instruments. Any light lost in the process of reflection reduces an instrument's image brightness.


The process of aligning the optical system of a scope or binocular so that the light is brought to a focus at the correct position.


An optical device consisting of a well-corrected objective lens with a reticle at its focal plane. An example being the lens systems used in optical bore sighters.


Coma is an aberration that forces rays from an off-axis point of light to create a trailing “comet-like” blur moving away from the optic axis. What this often looks like is an image with a sharp center, but increasing blur toward the edges of the image. Bending the lens can correct this with a single-lens setup, but a combination of lenses symmetric around a central stop provides more complete correction.

Compact Binoculars or Scopes

Small binoculars or scopes that can fit in a pocket or handbag and are convenient to carry around. Compact binoculars are roof prism binoculars or reverse-Porro prism binoculars. Compact scopes are those with Maksutov-Cassegrain System, such as the NIPON 26-78x78 scope model.

Compass Binoculars

Binoculars with a compass built in - perfect for finding your way back to the campsite after a long day of bird-watching or hunting.


Describing a hollow curved surface; curved inward.

Concave Lens

A lens that is thinner in the middle than at the edges; used to correct nearsightedness.


Good image contrast is desirable for viewing low contrast objects such as the targets in low lighting condition or the lunar surface and planets.


Denoting a spherically shaped surface; curved outward.

Convex Lens

A lens that is thicker in the middle than at the edges; used to correct farsightedness.


Also known as reticle, it is a system of cross wires, dots, or rings in the focus of a finder scope or eyepiece for target centering purposes.

Crown Glass

There are two types of glass used in manufacturing achromatic lenses, crown glass and flint glass. Crown glass is harder than flint glass, and has a lower index of refraction as well as a lower dispersion. Crown glass is produced from alkali-lime (RCH) silicates, which have approximately 10 percent potassium oxide. Crown glass is one of the earliest low dispersion glasses.

Curvature of Field

Curvature of field, also known as Petzval field curvature, is a very common optical problem. Basically, it causes a flat object to appear sharp in just certain parts of the image, instead of being uniformly sharp across. This happens because the optical elements are curved, and project the image in a curved manner instead of flat.


A system for measuring the altitude of a celestial object expressed as degrees north, or south, of the celestial equator. Angles are positive if a point is North of the celestial equator, and negative if South. It is used, in conjunction with Right Ascension, to locate celestial objects.

Degree of Angle

A unit of angular measure equal to 1/360 of a circle. One degree subtends 62.83 inches at 100 yards or 52.36 feet at 1,000 yards.

Depth of Field

This is the distance from near to far that displays in crisp focus at a given setting of the focus adjustment or at a certain distance. Typically, depth of field will decrease as the magnification increases in an optical system, which is a distinct disadvantage of high magnification hunting optics. This is why depth of field is normally more important when comparing spotting scopes, for example, than binoculars. Depth of field also changes with the distance observed, decreasing as the distance observed increases.

Dew shield

A covering of ABS plastic wrapped snuggly around the tube assembly and extending beyond the aperture of a telescope to prevent dew from forming on the objective lens of a refractor or correcting plate of a Schmidt-Cassegrain or Maksutov telescope.

Diagonal, correct-image

A 45 degree or 90 degree diagonal used primarily for terrestrial viewing because it renders images as the unaided eye sees them - upright and left-to-right. Some resolution is lost when using a correct-image diagonal, so it is generally not recommended for astronomical viewing.

Diagonal, mirror

An accessory that fits into a telescope's focuser and diverts incoming light at a right angle. This is for viewing at a more comfortable angle when using a refractor or catadioptric telescope. Mirrors are used to redirect the light within the diagonal.

Diagonal, prism

An accessory that fits into a telescope's focuser and diverts incoming light at a right angle. This is for viewing at a more comfortable angle when using a refractor or catadioptric telescope. Prisms are used to redirect the light within the diagonal.


A material that does not readily conduct electricity. Used in vacuum-deposited layer form to construct interference filters and other thin-film optical coatings.

Dielectric coating

High-reflectance or low-reflectance coating composed of alternating layers of non-conducting inorganic materials with higher and lower indices of refraction than the substrate. Produced via electron beam gun with ION assist with both optical and crystal monitoring.

Diffraction Grating

A piece of transparent or reflecting material, which contains many thousands of parallel lines per centimeter; used to produce a light spectrum by diffraction.

Digital Camera Binoculars

Binoculars with a digital camera built in - useful for taking clear, magnified pictures.

Digital Eyepiece

A digital eyepiece can be attached to a scope using a specially made eyepiece adapter to take pictures and even video footage through the scope. One example is digital eyepiece EE300.


A measurement unit that expresses the refractive power of a lens. A lens with a focal length of 1 meter has a refractive power of one diopter. Lenses with shorter focal lengths have greater power in diopters. Accordingly, a lens with a focal length of one-quarter meter has a power of four diopters. The refractive power of converging (convex) lenses is positive; the refractive power of a diverging (concave) lens is negative.

Diopter (US) / Dioptre (UK)

A unit of measurement, this reflects the optical power of a lens or curved mirror. One diopter is equal to the reciprocal of the focal length, as measured in meters. A 4-diopter lense brings parallel rays of light to focus at 1/4 meter, for example.

Diopter Adjuster

A separate eyepiece-focusing tool, usually on the right lens, that allows the user to adjust the lenses separately to allow for eyesight differences.

Diopter Scale

A focusing scale—such as on the eyepieces of binoculars—which serves to indicate the degree of convergence or divergence of the light rays emerging from the instrument.

Dioptric Correction

The capability for adjusting the hunting optics to the varying visual acuity of a person’s eyes. This allows one lens to be adjusted to provide compatible focus when the viewer's eyes have different visual capabilities. On single-lens optics, dioptric correction allows eyeglass wearers to use the optic without their eyeglasses.


The breaking of white light into its component colors when it passes through one medium, like air, into another medium, such as glass. Dispersion is what causes chromatic aberration in lenses.


There are a number of different types of distortion possible in optics, including aberrations like spherical and chromatic aberrations. In addition, though, there are other types of distortion, such as Pincushioning or Barreling. In these types of distortion, straight lines can look like they are curved when looking through the device. * Barrel Distortion – Straight lines appear to curve outward * Pincushion Distortion – Straight lines appear to curve inward * Roller Ball Distortion – Sometimes, if there is neither barreling or pincushioning and you have a totally flat image, a ball seems to roll across an image as you pan across it.

Dove prism

It resembles half of a common right-angle prism in which a ray entering parallel to the hypotenuse face is reflected internally at that face and emerges parallel to its incident direction. One of the incident rays emerges along a continuation of its incident direction, and if the prism is rotated about that ray through some angle, the image rotates through twice that angle. A Dove prism must be used in parallel light.

ED glass

Short for "Extra-low dispersion", an optical glass that has superior refractive properties compared to standard optical glass. Lenses made with ED glass typically exhibit less chromatic aberration than lenses made with standard glass.

Electromagnetic Spectrum

Transverse radiant energy waves, ranging from low frequency to very high frequency, which can travel at the speed of light.

Electromagnetic Wave

A wave that does not have to travel through matter in order to transfer energy.

Electron-beam (EB) gun evaporation

It is a form of physical vapor deposition in which a target anode is bombarded with an electron beam given off by a charged tungsten filament under high vacuum. The electron beam causes atoms from the target to transform into the gaseous phase. These atoms then precipitate into solid form, coating everything in the vacuum chamber (within line of sight) with a thin layer of the anode material.


A substance that cannot be broken down into simpler substances by ordinary means.

Equilateral prism

A dispersing type prism typically used for wavelength separation applications. The three sides are of equal length. A light ray is twice refracted passing through the prism with total deviation a function of prism material index of refraction, and hence wavelength.

Erecting System

Lenses or prisms that serve to erect the inverted (upside down and laterally reversed) image formed by objective lenses. All telescopic sights and many telescopes use lenses to erect the image, while binoculars and prismatic spotting scopes use either Porro or roof prisms. Astronomical telescopes don't need and, therefore, seldom have erecting systems.

Erector Lens

Certain combinations of objective and ocular lenses yield an inverted image. An erector lens incorporated into the system serves to reorient the image right side up. In binoculars and scopes, prisms are often used to 'erect' the image.

Evaporated coating

Thin precisely controlled layers of solid material(s) deposited on a substrate after being vaporized under high-vacuum conditions.

Exit pupil

The small circle of light that can be seen behind the eyepiece when an optical instrument is held at arm's length and pointed toward a bright background. It is located at the point where the eye must be placed to see the full field of view (FOV). Exit-pupil diameters are calculated by dividing the objective lens diameter in millimeters by the magnification number. Therefore, a 10x50 binocular has 5mm exit pupils (50 ÷ 10 = 5).

Extra Low Dispersion Glass (ED Glass)

ED glass is used on the lenses of some high-end optics to alleviate the effects of chromatic aberration. The designers of multi-element objective lenses have more options with ED glass, allowing them to focus and direct the wavelengths of light more effectively. This way, they can minimize aberrations, particularly chromatic aberration. The better aberrations are controlled, in general, the cleaner and brighter the image will appear. Optics manufactured using ED glass typically transmit clearer and sharper images to your eyes.

Eye Pupil

The apparently black circular opening in the iris of the eye that permits light to pass through the eye lens to the retina. The human eye pupil dilates automatically from a diameter of about 2 mm in bright sunlight to 7 mm in the dark.

Eye Relief

Shown in parentheses in the test tables, eye relief is the distance in millimeters between the eyepiece lens and the exit pupil, where the eye must be placed to see the full FOV. An eye relief of about 20mm is needed to enable eyeglass wearers to see a full FOV. Eye relief for telescopic sights is measured in inches, with 3 inches the minimum for hard-kicking firearms.


Related to eye relief, eyecups keep the distance between the oculars and your eyes, while also keeping stray light from entering your eyes when you are using the optic. Many eyecups are made from rubber, rolling up or down depending on whether or not you wear eyeglasses. Unfortunately, constant rolling tends to break the rubber eyecups. Some eyecups are made to slide rather than roll. These, unfortunately, can be hard to keep in place. Yet a third type of eyecup twists up and down (often called helicoid eyecups), and can be left in any position from all the way up to all the way down. These sometimes have click stops at certain intervals and are labeled with the eye relief distance for each stop, allowing you to achieve the perfect eye relief for your eyes.

Eyecups (or eyeguard)

Cups on the eyepieces of binoculars that allow for positioning of the eyes and provide optimal eye relief. It improves viewing comfort and helps block extraneous spripheral light. Some eyecups come in a rubber version that the user can fold down to accommodate eyeglasses. Other binoculars use cups such as 'twist-up' or 'pop-and-lock' that are more adjustable for any user.


Sometimes known as an ocular. This is a system of lenses closest to the eye. Its purpose is to magnify the image at the focus of the scope. The magnification of an eyepiece can be obtained by dividing its focal length into that of the scope. Note: There are various types of eyepiece designs, such as Kellner, Orthoscopic, Erfle, and PLOSSL. Amongst them, PLOSSL eyepieces are considered to be a good compromise and offer the best all-around price and performance. According to the scope manufacturer, a set of these PLOSSL eyepieces with 16mm, 26mm and 32mm focal length would serve a wide range of observation purposes.

Eyepiece (ocular)

The lens or lens group between the final image in a visual optical instrument and the viewer’s eye. It serves to magnify that image, thereby determining the magnifying power.

Eyepiece Adapter (Adaptor)

Eyepiece adapters are used to adapt from one eyepiece size/format to another. This device will make it possible for the same scope to use different types of eyepieces. For example, with a specially designed eyepiece adapter, both the Nipon 20-60x70 spotting scope and Nipon 26-78x78 scope can be fitted with a digital eyepiece or other standard 1.25" eyepieces (eg. the PLOSSL eyepiece set) for a wider range of applications.

Eyepiece Sizes

There are three sizes of scope eyepieces, i.e., 0.965", 1.25" and 2". The sizes are determined by the diameter of the eyepiece barrel that fits into the telescope. 1.25" is regarded as a standard eyepiece size and almost all telescopes are designed to be used with 1.25" diameter eyepieces.

Eyepiece, Zoom

Provides a continuous magnification range and hence the option of using a single eyepiece versus switching from one to another. The less expensive zooms sometimes suffer from internal reflections, unless they've been properly coated and their internal barrels blackened or glare-threaded.


Also called the f-stop, focal ratio, f-ratio, or relative aperture, the F-number is the ratio of the lens's focal length to the pupil entrance diameter. To calculate the F-Number, you must divide the focal length of an optic by the diameter of the entrance pupil.

Field Flattener Lenses

These improve the sharpness of an image around the edges, lowering distortion by reducing the curvature of the field aberrations that can occur when focus is placed on the center of the field of view. Focusing on the center of the field of view causes the edges to go out of focus, or for the center to go out of focus when focusing on the images. Field flattener lenses produce sharper, clearer images even at the lens periphery, and are used in most high-end optics.

Field Glass

A type of binocular that uses a second lens (instead of a set of prisms) to magnify an object. Field glasses are more durable than prism binoculars, although the magnification strength tops out at about 5x.

Field of View (FOV)

The maximum width of the areas that can be seen through the eyepiece of an optical instrument, as measured either angularly in degrees or linearly in feet or meters at specified distances. For example, a 10X binocular with a six-degree angular field has a linear FOV of 315 feet at 1,000 yards. For conversion purposes, one degree equals 52.36 feet at 1,000 yards.

Field Stop

An aperture located at the image plane of an optical instrument that determines the extent and shape of the field of view, which is usually round in viewing optics and either square or rectangular in cameras.

Field, Apparent

The angular subtense of the FOV as it appears to the eye as the result of magnification. Apparent field is calculated by multiplying the true angular field by the magnification. Accordingly, a 10X binocular with a six-degree true field has a 60-degree apparent field.


A screen that allows only certain colors to pass through it; a transparent material that separates colors of light.

Filter, colour

Glass filters, each of a specific color, which screw onto eyepiece barrels for enhancing lunar and planetary detail. Various color filters reduce other interfering or scattered wavelengths that blur certain wavelength-specific features. Red filters, for example, bring out Martian surface detail while green increases contrast of Jupiter's Red Spot. Also called planetary filters.

Filter, light-pollution

A filter that threads onto an eyepiece or rear cell of a Schmidt-Cassegrain that blocks wavelengths of light pollution sources such as mercury vapor and high-pressure sodium, but pass wavebands specific to deep-sky objects, such as hydrogen alpha, hydrogen beta, and oxygen III.

Filter, moon

A glass filter in an aluminum cell that threads onto an eyepiece barrel and reduces the Moon's glare so that it can be comfortably observed. Without the eye being overwhelmed by moonlight, more lunar detail becomes apparent.

Filter, planetary

Glass filters, each of a specific color, which screw onto eyepiece barrels for enhancing lunar and planetary detail. Various color filters reduce other interfering or scattered wavelengths that blur certain wavelength-specific features. Red filters, for example, bring out Martian surface detail while green increases contrast of Jupiter's Red Spot. Also called color filters.

Filter, solar

A glass filter that fits snugly over the aperture of a telescope and allows the photospheric surface of the sun - sunspots and solar faculae - to be observed comfortably and safely. A good solar filter blocks some 99.99% of the sun. Observing the sun without a solar filter may cause serious damage to the eye.

Filter, Variable-polarizing

Variable-polarizing filters act as dimmer switches to bright celestial objects, including the Moon or a planet. The filter, which threads onto 1.25" eyepiece barrels, consists of two pieces of polarized glass mounted in an aluminum cell that, depending on how much it is rotated, varies light transmission from 1% to 40%.

Finder (or Finder Scope/Finderscope)

A small telescope, with a wide field of view, mounted on the main telescope tube to enable an observer to easily locate celestial objects, and place them within the field of view of the main telescope. Note: In the 'red dot finder scope', you see a LED red dot in the center of the finder's visual field, which helps to locate the target.


Flare is the scattering of non-image forming light within an optical system, which appears as stray reflections, ghost images or a veiling glare that obscures details. Aberrations are optical defects that degrade image quality. These include: Astigmatism, Chromatic Aberration, Coma, Curvature of Field, Distortion and Spherical Aberration.

Flint Glass

Flint glass is the other of the two types of optical glass used to manufacture achromatic lenses. Flint glass has a low Abbe number, a higher dispersion, and a higher index of refraction than crown glass. A concave lens of flint glass is often used alongside a crown glass convex lens to manufacture an achromatic doublet lens, because the two types of glass compensate each other; optical properties. This reduces chromatic aberration.

Fluoride Glass

This is a class of non-oxide optical glasses, composed of the fluorides of various metals. Optical elements made from calcium fluoride are often used in telephoto lenses to correct color aberration. However, fluoride glasses are being replaced with the use of low dispersion glasses, because these glasses possess higher refraction index, better dimensional stability, and are less fragile.

Focal Length

The distance between the principal focus of a lens or mirror and its optical center.

Focal Plane

The plane where the image formed by the lens or lens system is in sharp focus. In a camera, the focal plane is the sensitized surface of the film.

Focal Point

This is a point where the light rays from an image come sharply into view after passing through the binocular or scope.

Focal Ratio (f-ratio)

Defined as f value. This is the focal length of a lens (or mirror) divided by its diameter. A focal ratio of 8 is written as f/8.


To adjust the eyepiece or objective of an optical instrument so that the image appears sharp and well defined to the observer. Changes in viewing distances usually require changes in focus.

Focusing Range

All binoculars or scopes have the ability to be focused for infinity. So a primary point of distinction between product models is the minimum focus range (see "Close Focus").


The mechanism which holds the eyepiece and allows adjustment for focussing the image.

Folded Light Path

A combination optical configuration using lenses and mirrors to create a total scope length much shorter than the total focal length of the system. This provides a compact design yielding long focal length performance.

For Riflescopes Only Mounting length

The maximum length of a scope's main tube to which mounting rings can be attached. For easy mounting on full-length bolt-action rifles, this area should be at least 5 inches long.


The number of waves that pass a point in a given unit of time.

Full Size Binoculars

In comparison with compact binoculars, full-size binoculars offer better light gathering ability because of a relatively larger objective lens. For example, a 10x42 binocular is a full-sized binocular, while a 10x25 binocular is considered as a compact binocular.

Fully coated optics (FC)

All glass surfaces that have any vulnerability to air are coated.

Fully Multi-Coated (FMC)

All glass surfaces susceptible to air are multi-coated.

Fully Multi-Coated Optics

Binoculars or scopes that have fully multi-coated optics have multiple coatings on all air-to-glass surfaces. See also "Coated/Multi-Coated Glass".

Fused quartz

It is manufactured by fusing (melting) naturally occurring quartz crystals of high purity at approximately 2000 °C, using either an electrically heated furnace (electrically fused) or a gas/oxygen-fuelled furnace (flame fused). Fused quartz is normally transparent. The optical and thermal properties of fused quartz are superior to those of other types of glass due to its purity.

Fused silica

Glass consisting of high-purity silica in amorphous (non-crystalline) form. It is different from traditional glasses in that no other ingredients are added to lower the melt temperature. Fused silica, therefore, has much higher working and melting temperatures than soda-lime or borosilicate glasses.


Full width at half maximum. The width of the transmitting band of an optical filter, measured at 50% of the peak transmission.

Galilean Binoculars

Galilean binoculars are so named because the concept was first used in the telescopes made by Galileo Galilei in the 17th Century. These types of binoculars use convex objective lenses with concave eyepiece lenses, so prisms are not needed to erect the images. These are also known as opera glasses and are used for viewing objects not very far away.

Gamma Ray

High-energy wave of high frequency and with a wavelength shorter than an x-ray; released in a nuclear reaction.

Giant Binoculars

Any binocular with an objective of 60mm or more are called giant binoculars. Giant binoculars are often used for astronomical viewing.


Light scattered by particulate matter in the atmosphere, such as dust or moisture droplets. Haze lens a foggy or cloudy appearance to distant objects or scenes, subduing colours and contrast. Note: Haze effects are more apparent when using high magnification optical instruments than when viewing with lower-power optics, and are more pronounced at long range than short range under a given set of atmospheric conditions.

Helicoid Eyecups

These are also referred to as twist-up eyecups because they twist up and down using a helicoid mechanism. This allows them to be left at any position from fully up to fully down, and some even have click stops at regular intervals with the eye relief distance for each stop marked on the cup. This allows you to get the perfect eye relief for your vision.


The reproduction of an object formed with lenses or mirrors.

Image (real)

An optical reproduction of an object formed by a lens or mirror. A real image can be captured on a screen of film.

Image or Focal Plane

A plane at right angles to the principal axis of a lens or mirror on which the best image of an object is focused.


Binoculars with a self-steadying feature, designed to counteract any hand-shaking of the user.

Index of Refraction

The amount that light is refracted when it enters a substance; given as the ratio of speed of light in a vacuum to its speed in a given substance.

Individual Focus

A type of binocular focusing system in which each eyepiece is adjusted independently each time the instrument is refocused.

Infrared (IR)

The long wavelength invisible portion of the electromagnetic spectrum between ~2.5 µm and 14.0 µm.

Infrared (IR) Illuminator

This provides a light source for the optical system to amplify, yielding enhanced images in very low light conditions (such as with night vision systems) where no ambient light is available for amplification.

Infrared Radiation

Invisible radiation with a longer wavelength than red light and next to red light in the electromagnetic spectrum; used in heat lamps, to detect heat loss from buildings, and to detect certain tumors.

Inter-Pupillary Distance (IPD)

IPD is the distance (in mm) between the centers of the pupils in each eye. This measurement is sometimes provided in binoculars descriptions to define a range of user populations the binoculars can fit. For British adults (5th-95th percentile, 18-64 years old), the IPD measurement is in a 54mm-68mm range, with an average value of 61mm.


The addition by crossing wave patterns of a loss of energy in certain areas and reinforcement of energy in other areas.

Interpupillary distance

Sometimes shortened to IPD, this is the distance between the exit pupils of a pair of binoculars. It needs to correspond with the distance between the center of one pupil and the other in your eyes (also called interpupillary distance). IPD is crucial in the design of binoculars since both of the user's pupils need to be positioned within the exit pupils of the bins' viewing system. Binoculars manufacturers can get data for setting the interpupillary distance of their binoculars from a variety of anthropometric databases such as Military Handbook 743A and the 1988 Anthropometric Survey of US Army Personnel. These particular databases provide the IPD for each gender and sample size with mean and standard deviation, minimums and maximums, as well as percentiles.


A toy in which reflections from mirrors make patterns. It was invented in 1819 by David Brewster.

Laser (Light Amplification by Stimulated Emission of Radiation)

A device that produces a highly concentrated, powerful beam of light which is all one frequency or color and travels only in one direction.

Law Of Reflection

The angle of incidence equals the angle of reflection.


A curved, transparent object; usually made of glass or clear plastic and used to direct light.

Lens coatings (anti-reflection)

Most high-quality hunting optics have at least a single layer of anti-reflective coating on the air-to-glass surfaces to help with light transmission, producing brighter images with improved contrast. Please see our full article explaining anti-reflection lens coatings.


Light is a form of energy, traveling through the universe in waves. The wavelengths of visible light range from less than 4,000 angstroms to more than 7,000 angstroms.

Light Transmission

The percentage of light passing all the way through an optical system relative to the amount that entered. This can range anywhere from as low as 50 percent for instruments with uncoated optics to about 95 percent for those with high-quality, multilayer anti-reflection coatings.

Light-Gathering Power

The light-gathering power of a binocular or scope is determined by the surface area of its objective lens.

Limiting Magnitude

The faintest object that can just be detected by a telescope.

Low Dispersion Glass (LD Glass)

This describes a class of glass with low dispersion properties. Crown glass, for example, is a relatively inexpensive LD glass, with Abbe numbers around 60. Thus, crown glass is often used for making optical lenses.

Magnesium fluoride (MgF2)

Magnesium Fluoride is a birefringent crystal optical material that exhibits high transmission from VUV to IR wavelengths (~110nm to 6.0 µm). MgF2 is a relatively hard material resistant to mechanical and thermal shock. MgF2 is typically oriented along the optical axis to avoid birefringent effects and provide maximum transmission.


Also called power, magnification is the extent to which objects appear larger when viewed through optical instruments. It is expressed by the number (or numbers) proceeding the "X" in specification numbers. An 8x32 binocular makes objects appear eight times larger.

Magnification (Power)

The power of the binoculars or scopes. It tells you how many times bigger an image can be seen through the scope (or how many times the target can be 'brought' closer) than you would see it with the unaided eye. Note: the stronger the magnification, the smaller the field of view.

Magnifying Power

This is the ability of an optical instrument to make objects appear larger. An instrument that makes objects appear eight times higher and eight times wider is said to have eight-power or 8X magnification.

Magnitude, Absolute

A measure of a star's true or intrinsic brightness. Essentially, astronomers decide this by gauging how bright the star would appear to the eye if brought to a standard distance of 10 parsecs, or 32.6 light-years. Alnitak, the easternmost star in Orion's belt, has an apparent magnitude of 2.05 but an absolute magnitude of -5.9, because that's how bright it would appear if it lay 10 parsecs away. The Sun, with an apparent magnitude of -26.7 has an absolute magnitude of 4.8.

Maksutov (MAK)

A catadioptric reflecting telescope similar to a Schmidt, except that it employs a deeply curved full-aperture lens called a meniscus to correct for spherical aberration. Maksutovs utilize spherical mirrors and can be designed with a Cassegrain configuration, in which they are called Maksutov-Cassegrains, or as Newtonians, in which they are called Makstutov-Newtonians (or MAK-Newts, for short).

Maksutov-Cassegrain Telescope

Most of the spotting scopes on the market utilize lenses and prisms, but the Maksutov-Cassegrain design often referred to as a Mak, is quite a bit different. Usually seen in telescopes, the Mak utilizes lenses and mirrors to bring the image to your eye, one of several catadioptric designs. One of the lenses, called a corrector lens, helps to eliminate aberrations in the image that’s delivered to the eyepiece. Maks are very uncommon in spotting scopes, although Celestron does manufacture a few. These scopes are well-suited to visual use and planetary imaging, making them great for amateur and backyard astronomy.

Micron (µm)

A unit of measure in the metric system equal to 10-6 meter. Also abbreviated µ or µ. Usually used in the measurement of near-infrared (NIR) to far-infrared wavelengths of light (~1.0 µm to 14.0 µm)


A unit of angular measure that is subtended by one unit of length at a distance of 1,000 units of length, such as one meter at a distance of 1,000 meters. The spaces between dot centers on mil dot reticles subtend 3.6 inches at 100 yards, 7.2 inches at 200 yards, 10.8 inches at 300 yards and so forth.

Minute of Angle (m.o.a.)

A unit of angular measure equal to 1/60 of a degree, or 60 seconds. One minute subtends almost exactly 1 inch (1.047") at 100 yards, 2 inches at 200 yards, 3 inches at 300 yards, and so forth—making it a criterion for firearm accuracy.


Optical phenomenon that occurs when air near the ground is significantly denser than the air above, creating visible reflected images of distant objects or targets.


adj. Of, relating to, or intended for use by only one eye. n. Usually, a small handheld telescope that resembles one-half of a binocular.

Multi-layer coated (MC)

One or more glass surfaces are coated multiple times.

Nanometer (nm)

Unit of length in the metric system equal to 10-9 meter. Usually used in the measurement of vacuum ultraviolet (VUV) to near-infrared (NIR) wavelengths of light. One nm = 10 angstroms.

Narrowband AR coating

Also known as a V-coat this AR coating is designed to provide increased transmittance and reduced reflectance over a very restricted (narrow) band of wavelengths.

Near infrared (NIR)

The long wavelength Red to Invisible portion of the electromagnetic spectrum between ~700 nm and 2500 nm (2.5 µm).

Neutral density filter

A light filter that decreases (attenuates) the intensity of light without altering the relative spectral distribution.

Nitrogen Purging

This is the process of filling an optical instrument, such as a spotting scope, binocular, or riflescope, with zero grade or almost pure nitrogen, purging the optical chamber of all moisture and oxygen. This is done to prevent virtually any fogging or oxidation of the optics.

Nitrogen-Purged (or nitrogen filled)

The atmospheric air inside the binocular or scope tubes is replaced with nitrogen, which prevents mildew, mold or acid inside the tubes. Nitrogen-Purged binoculars are commonly known as water & fog-proof, as with the Nipon 10x42 binoculars. (Note: In rare situations such as extreme humidity and elevation changes, some internal fogging may occur, though the fogging usually clears on its own after a few minutes.)


A line perpendicular to a surface.

O-Ring Sealed

A special sealant on binoculars that makes them waterproof.


Anything that is observed through or imaged by an optical system.


The optical element (front lens) that receives light from viewed objects and forms the first or primary image in an optical instrument. Larger objectives gather more light, relative to their surface areas, and have better resolution, relative to their diameters.

Objective Lens

In any hunting optic, this is the large lens at the end of the device, opposite the eyepiece and closest to the viewed subject. This is the lens that gathers light into the optic, combining convex and concave lenses to minimize chromatic aberration, giving you a clearer image.

Ocular Lens

The ocular lens is the small lens in the eyepiece of any optic device. Usually, this is the lens closest to the eye and is often smaller than the objective lens. This is not always the case, though, as seen in some porro prism binoculars and spotting scopes where the ocular lens is the same size as the objective lens. The ocular lens usually consists of several “lens elements” contained within a housing with a barrel at one end. The ocular lens magnifies the image created by the objective lens. Field-flattener lenses are often used on eyepiece lenses to correct lens aberrations in which the peripheral area of an image is out of focus.


Not transparent; no light passes through the material.

Optical Axis

The line straight out from the center of a parabolic mirror; straight line through the center of a lens.

Optical coatings

Thin layers of various materials applied to lenses and/or prisms to improve optical performance.

Optical Fiber

A thin strand of glass that transmits light down its length.

Optical Telescope

A tube with magnifying lenses or mirrors that collect, transmit, and focus light.

Optical tube assembly (OTA)

The main tube of a telescope including the primary mirror or objective lens, focuser, and finder scope. The optical tube assembly does not include a mount or tripod.

Optical window

Flat polished glass (typically BK-7 or fused silica) materials. Generally used for view ports or sealing and or protecting other components within an optical assembly, instrument or laser.


A curved line representing the path of a projectile; the shape of the surface of a parabolic mirror.

Parabolic Mirror

A curved mirror.


A condition that exists when the image of the target (the location of which varies axially with the viewing distance) and the image of the reticle are on different focal planes. When parallax is present, you'll notice an obvious shift in the target laterally relative to the reticle as the eye is moved from side to side.

Parallax/focusing adjustment

This adjustment, consisting of a ring on the scope's objective bell (AO) or as a knob on the left side of the W/E turret, eliminates parallax and simultaneously focuses the image for distances ranging from about 10 yards to infinity.


The angular wedge difference between two opposing optical surfaces. Usually measured as arc minutes or arc seconds.


The range of wavelengths transmitted by an optical filter.

PC-3 Phase coating

This chemical coating is applied to the prisms to enhance resolution and contrast. It is found on some roof prism binoculars, but it would not provide an advantage on porro prism models.

Peak transmittance

The maximum transmittance achieved within the passband.

Peak wavelength

The wavelength of maximum transmittance within the passband. Differs from the central wavelength only in filters exhibiting asymmetrical band shapes.

Penta prism

A five-sided prism containing two reflecting surfaces at 45° to each other, and two refracting faces perpendicular to the entering and emerging beams. The deviation angle of 90° is independent of any rotation of the prism about an axis parallel to the line of intersection of the two reflecting faces. It is commonly used as an end-mirror in a rangefinder and as an "optical square" in surveying and alignment machinery.

Phase Correction

A coating applied to the prisms of roof prism binoculars to prevent the light beam from splitting into two out-of-phase beams of light. This enhances color fidelity and reduces image contrast and gives a clearer view.

Phase Correction Coatings

In order to keep light in correct color phases, phase correction coatings are used on roof prisms to enhance resolution, contrast, and color fidelity. These coatings keep light in the correct color phases, so they produce images with better contrast and resolution, along with better color reproduction. You'll only find this on high end optics.

Phase Shift

Any change that occurs in the phase of one quantity is called phase shift, as is the difference between two or more quantities of a wave or phase. Manufacturers of high end roof prism optics will use phase correction coatings to minimize this, preventing the deterioration in an image sharpness caused by light waves reflecting at the opposite faces of a roof prism.

Phase-correction (P) coatings

Used on the roof surfaces of roof prisms to reduce the image-degrading phase interferences that occur when light waves from the opposite roof surfaces combine to form images. P-coating is not needed on Porro prisms.


A material that absorbs certain colors of light and reflects other colors.

Plane Mirror

A mirror with a flat surface.


A lens or mirror surface that is perfectly flat.

Point of aim (POA) Shift

Also called "track-out," this is an insidious condition, peculiar to variable-power scopes, in which the POA shifts during power changing.

Polarized Light

Light in which all waves are vibrating in a single plane.

Porro Prism

This design was invented by Ignazio Porro in mid–19th-Century Italy. One advantage of this design is that all of its reflective surfaces are completely reflective, so there is no light loss and such binoculars are easy to produce. The optical path, however, is bent like the letter Z, making the system take up considerable space and cause the bins to be larger than those with a roof prism. You can tell a Porro prism design from a roof prism design because the objective lenses and eyepieces will not be in line with each other.


This refers to a telescope's magnification (i.e., 80x can be referred to as 80 "power").

Primary Mirror

The principal light-gathering mirror in a reflecting telescope.


A transparent material with two or more straight faces at an angle to each other.

Prism Glass

A solid glass figure cut with flat surfaces. Most optical prisms are made from borosilicate (BK-7) glass or barium crown (BaK-4) glass. BaK-4 is a higher quality glass yielding brighter images and high edge-to-edge sharpness.

Prism Systems

The prism system turns what would otherwise be an upside-down image right-side-up.

Prismatic Binoculars

Binoculars that use internal prisms instead of a second lens to magnify an object. These binoculars aren't ideal for heavy-duty use, as the prisms can be broken or knocked out of alignment due to rough handling. However, the magnification strength of prismatic binoculars is much better than that of traditional field glasses.


As used in binoculars and spotting scopes, prisms serve to erect the inverted image formed by the objective lenses while simultaneously folding the light path, permitting the instruments to be made much shorter than if erecting lenses were used. The commonly used types of prisms are porro prisms and roof prism, neither of which has a commanding advantage. Porro prisms, with their Z-type configuration, are less expensive to manufacture. Therefore, quality being equal, Porro prism instruments tend to be better buys. Roof prism instruments, with their in-line configuration, tend to be more compact and slightly more shock-resistant.

Rack-and-Pinion Focuser

A device into which an eyepiece is inserted and adjusted to bring a telescopic image to focus. A focuser can be as simple as a manual drawtube, but the more efficient type is the "rack-and-pinion" design, whereby a threaded axle affixed with knurled knobs at each end meshes with a threaded drawtube, enabling it to be moved up or down through the focal plane.

Radius (of curvature)

The measurement of a curved surface as applied mainly to lenses.

Rangefinder Binoculars

Binoculars with a rangefinder built right in. It is a tool used to calculate the exact distance between you and the object in focus.

Real Image

An image that can be projected onto a screen; formed by a parabolic mirror or convex lens.

Reflecting Telescope

A telescope in which magnification is produced by a parabolic mirror.

Reflecting Telescope

Also called a reflector, a telescope that uses a reflecting objective (mirror) to focus an image of a distant object at a focal point. Large astronomical telescopes are usually of this type.


The light or image you see when light bounces off a surface; bouncing a wave or ray off a surface.


A telescope in which the main light gathering element is a mirror.


The bending of light rays as they pass from a medium of one density into a medium of a different density, e.g., from air to glass or vice versa. The extent to which a ray is bent depends on 1), the angle of incidence and 2) the difference in density between the two mediums.

Refraction and Partial Reflection

Light bends, or refracts, when it travels through two transparent substances of different densities. For instance, when light leaves air and enters glass, or when it leaves glass and enters air, the light will refract. Under ideal conditions, all light will bend equally, but real life conditions dictate that light bends at around a 95 percent quantity, with 5 percent of the light reflecting back into the first surface. To combat the consequences of partial reflection, scientists devised chemical coatings to apply to the lens coatings, increasing the ability of the lens to transmit light.

Relative Brightness

This is a term to quantify the "brightness" of scope sights and binoculars to facilitate comparison. The relative brightness number is the square of the diameter of the scope's exit pupil, expressed in mm.


The ability of an optical system to produce separate and distinct images of two closely spaced objects, such as the points on a deer's antlers. We measured resolution in seconds of angle (SOA) using a USAF Resolving Power Test Target, consisting of progressively smaller sets of black lines on a gray background.

Resolution (Resolving Power)

Resolution or definition is the ability of a binocular or spotting scope to distinguish fine detail and retain clarity.


Aiming indicators, measuring scales, dots, posts or other distinct points placed on the focal planes of telescopes. In telescopic sights, reticles are used as aiming indicators that sometimes double as stadia-type rangefinders, an example being mil-dot reticles. Binoculars and spotting scopes may also be equipped with rangefinding reticles.

Reticle Focus

Not to be confused with the eyepiece focusing on binoculars and spotting scopes, this adjustment on eyepieces of telescopic sights exists for the sole purpose of making the reticle appear sharp to the shooter’s eye. It should not be used for focusing the image of the object being viewed.

Rich-Field telescope

A short focal-length telescope designed for sweeping very large regions of sky such as star fields (hence the name "rich"). Also known as wide-field telescopes.

Right angle prism

A type of 45-90-45 degree prism typically used to bend a beam of light through a right angle with the surfaces forming the 90-degree angle acting as the transmitting faces. The most common type of prism for many applications.

Roof Prism

Sometimes referred to as a Dach prism because of the German word for roof, this describes the roof-like shape of a prism. Optics with a roof prism design maintain the objective lens and eyepiece in line with one another, aligning the internal prisms instead of offsetting them, as in Porro prisms. The drawback to the roof prism system is that the first prism has one surface without total internal reflection, resulting in the loss of some light. Coatings are sometimes used to increase the reflectivity of the surface, known as dielctric coatings. Also, highly advanced technology is used to produce a roof surface of the second system with as precise an edge as possible, so that it delivers sharp images without a flare or a double image. This is why it is important not to try economizing on roof prism bins, instead buying the best you can afford.

Roof prism vs. Porro prism

Binoculars, in particular, but also spotting scopes, tend to come in two main design styles depending on the type of prism system used. This is either a Porro prism design or a roof prism design. It is quite easy to tell the two apart from one another: if the objective lenses and the eye pieces are in line with one another, you're looking at a roof prism design. On the other hand, if the two are offset from each other, the optic is using the Porro prism design. Roof prism optics tend to be more compact, but to achieve the same optical quality as Porro prism models they can be more expensive to manufacture. This is less the case now than it used to be, but you still find some roof prism optics priced higher than Porro prism designs.

Ruby Coatings

The objective lenses of a binocular with ruby coatings will be a bright reddish-orange. Since red light is reflected the colors seen through binoculars with ruby coatings are skewed to the cool end of the spectrum. Note: Another result of using ruby coatings is a shortened color spectrum which may increase the contrast and resolution of a binocular.


The spreading out of light by intersecting objects, whose size is near the wavelength.


A wide field reflecting telescope which uses a special mirror and correcting plate instead of a parabolic mirror. Mainly used for photographic sky surveys.

Second of Angle (s.o.a.)

A unit of angular measure equal to 1/60 of a minute or 1/3600 of a degree, which subtends 0.01745 inches at 100 yards. Resolving power is usually stated in seconds of angle.


The surface of a lens or mirror that is part of a sphere.

Spherical Aberration

A bundle of light rays coming from one point on the optical axis is often focused at a different place than the focused point, depending on how far it is from the optical axis when the light incidents. This deviation is caused by variations in angles of each incident light ray, and can cause something that should be a focused point appear as a blurred circle, instead. Making lens diameters smaller reduces such spherical aberration.

Spotting Scope

Originally, a telescope used at shooting matches to spot hits made on distant targets. Currently, the common name for small telescopes used by shooters, hunters, bird watchers and other outdoor enthusiasts.


This is another term for a handheld telescope.

Stereoscopic Vision (stereopsis)

The three-dimensional perception of depth that results from the spacing between the eyes that allows objects to be seen from two slightly different points of view, which is a primary advantage of binocular vision.


The underlying material to which an optical coating is applied. Typically an optical material such as BK7 Glass or UV Fused Silica.

Subtractive Color

One of the three pure pigment colors—magenta, yellow, cyan; these pigment colors produce black when mixed.

Surface figure

The accuracy with which an optical surface conforms to its intended shape. Usually measured with an interferometer or reference test plate by forming an interference pattern through variance of the optical path length across the surface. Specifications are usually presented in fractions of a wavelength, typically at the helium/neon laser line of 632.8 nm.

Surface quality (or finish)

Allowable cosmetic flaws in an optical surface by comparison to reference standards of quality. Usually made up of two types of standards defining long defects (such as scratches) and round defects (such as digs & pits).

Synthetic fused silica

It is made from a silicon-rich chemical precursor usually using a continuous flame hydrolysis process which involves chemical gasification of silicon, oxidation of this gas to silicon dioxide, and thermal fusion of the resulting dust (although there are alternative processes). This results in a transparent glass with an ultra-high purity and improved optical transmission in the deep ultraviolet.


A camera adapter that attaches to the body of a 35mm camera (without the lens) and then connects to the focuser for prime-focus astrophotography.


Converts the lens mount on a camera body to a standard "T-thread" that can accept a T-adapter or universal camera adapter for either prime focus or eyepiece projection photography.


An optical device made up of lenses or mirrors, usually with a magnification of greater than unity (1X), that renders distant objects more distinct, by enlarging their images on the viewer’s retina.

Telescopic sight

A small refracting telescope equipped with a reticle and used as a sighting device for a firearm. Common names include handgun scopes, shotgun scopes and riflescopes, depending on the type of firearm used.


Refers to bird watching, landscape or seascape daytime observing with a telescope, binoculars, or spotting scopes. A terrestrial scope is used during the day or in low light to observe terrestrial fields of view. Applicable to birding, sightseeing, and nature study.


A riflescope's ability to respond precisely to changes in the windage and elevation (W/E) adjustments.


Semitransparent; a material that admits some light.


The admission of light into a medium with the passage of this light through the medium without reflection back to the source or absorption in the medium.


As light travels through binoculars or scopes, a certain percentage of that light is lost through absorption and reflection at each air-to-glass surface or inside the prism system itself. The term used to describe this percentage of light that is not lost through the optical system is transmittance. See also "Light Transmission".


See-through; light can go through.


A three-legged stand with a swivel or pan head upon which a camera, spotting scope, or binocular can be attached. (Also see 'binocular tripod adaptor')

True Image

A true image is the way other people see us. It is the opposite of the image that is seen in a mirror.

Twilight Factor

A numerical guide for comparing the low-light performance of optical instruments, which takes magnification into account. To calculate twilight factor, multiply the diameter of the objective lens by the magnification and extract the square root of the product. Note that twilight factor does not take into account optical qualities, such as light transmission, resolution and image contrast.

UD Lens (Ultra Low Dispersion lens)

A lens made of special optical glass possessing optical characteristics similar to fluorite. UD lens elements are especially effective in correcting chromatic aberrations in super-telephoto lenses.

Ultra wide Band Coating

This is an anti-reflection coating process customized for every lens element in the optical path. This allows the best possible light from the front glass all the way to the eyepiece, giving optimum brightness and true color across the entire light spectrum.

Ultraviolet (UV)

The short wavelength Violet to an Invisible portion of the electromagnetic spectrum between ~400 nm and ~200 nm.

Ultraviolet Radiation

Radiation that has a shorter wavelength than visible light; next to violet light in the electromagnetic spectrum.

Universal Digital Camera Adaptor

It holds your digital camera next to your telescope's eyepiece so that you can take pictures of an object through the scope. The adaptor can be adjusted in 3 directions for different camera sizes and for camera lens positioning. See some typical universal camera adaptors.

UV grade fused silica

Synthetic form of silicon dioxide made from SiCl4 , which transmits light over a very broad range of wavelengths from ~ 170 nm to 2.5 µm. UV grade fused silica has excellent thermal stability and is used as a UV to NIR material for many optical components.

Variable Optics (zooms)

An optical system of variable focal length used to change the magnifying power within a predetermined ratio, such as three-to-one or four-to-one, an example being a 3-9X riflescope.

Variable Power (Zoom Lens)

Variable-power scopes or binoculars have a control that allows the user to adjust the magnification over a predetermined range.


When the eye relief is too short, you can get a dark area around the edges or periphery of your field of view. This reduction of an image's brightness or saturation at the periphery compared to the image center is known as vignetting. It's a great effect to apply to a photograph, but you don't want it in your binoculars or riflescopes.

Virtual Image

An image formed by a mirror or lens that cannot be projected onto a surface.

Visible (VIS)

The region of the electromagnetic spectrum that can be seen by the human eye. Generally considered to be the ~400 nm to ~700 nm wavelength portion of the spectrum.

Visible Light Spectrum

Band of visible colors produced by a prism when white light is passed through it.

Waterproof / Fogproof

The binoculars or scopes that are sealed with O-rings and nitrogen purged for waterproof and fog proof protection. These products are able to withstand complete immersion and remain dry inside.


The total linear length of one wave crest and trough.


A device used to attach a fork mounted telescope to a tripod.

Wide-Angle Binoculars

Binoculars with a wider field of view (generally described as greater than 6.5 degrees). For example, NIPON 7x50 binoculars have a wide field of view at 7.5 degrees (130m/1000m), convenient for target search.


A piece of glass with plane parallel surfaces used to admit light into an optical system and to exclude dirt and moisture.


Invisible electromagnetic radiation of great penetrating power.

Zoom Lens

See "Variable Power".

Not seeing your word?

Go ahead and ask us directly