Glossary of Binoculars & Telescope Terms


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

Aperture – The diameter of the binoculars’ or scope’s objective lenses, measured in mm.

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.

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

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”.

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.

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

Bk-7 glass – Also known as “borosilicate” glass. Most optical prisms are made of BK-7 glass.

Cassegrain – 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, such as NIPON Trophy MC800x80.

Center-Focus binoculars – A mechanism that allows both eyepieces to be adjusted at the same time, useful for rapid focus. Centre 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.

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.

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 10×50 binoculars is around 7m; the close focus of NIPON 26-78×78 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.

Note – Types of coating:
Coated optics (C) – one or more glass surface is coated.
Fully coated optics (FC) – all glass surfaces that have any vulnerability to air are coated.
Multi-layer coated (MC) – one or more glass surfaces are coated multiple times.
Fully Multi-Coated (FMC) – all glass surfaces susceptible to air are multi-coated.
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.
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.

Collimation – 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.

Compact Barlow Lens – A small Barlow lens that can be attached to the end of telescope eyepieces, often with standard 1.25″ eyepiece fitting diameter. This ensures perfect alignment between the centre of the Barlow lens and the eyepieces. For digi-scoping, the compact lens also serves the purpose of adjusting the image focal point and enables the camera to get a focused image through a wide range of telescopes.

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 MC800x80 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.

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

Crosshairs – 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 centring purposes.

Declination – 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.

Depth of Field – This refers to the distance from “near” to “far” that is in focus at a certain setting of the focus or at a certain distance. In a given system, as the magnification increases, depth of field decreases. Depth of field also changes with the distance observed, usually reducing in depth as the distance decreases.

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.

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.

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.

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

DSLR Adapters – these refer to camera adapters to attach DSLR cameras to telescopes. Various types of adapter designs have become available to fit different types of DSLR cameras and telescopes.

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.

Erector Lens (Erecting Eyepiece) – 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.

Exit pupil – The amount of light rays that enter the objective lens and exit the ocular lens (eyepiece) to form a magnified, circular image. The measurement is achieved by dividing the lens aperture by the magnification. For example: In the NIPON 10×50 binoculars, the exit pupil is 50mm/10=5mm. A higher exit pupil means the binoculars will work efficiently in dim light. For well-lit surroundings, an exit pupil of 2.5 to 4 is sufficient. If you hold a pair of binoculars away from your eyes and look through the eyepiece, you’ll be able to see the clear circular exit pupil.

Eyepiece – 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 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 Adapter (Adaptor) – Telescope eyepieces can have different fitting diameters, such as 0.965″, 1.25″ and 2″. 0.965″ eyepieces are found with some older and smaller scopes; 1.25″ eyepieces are considered to be ‘standard’ size which are used in most telescopes nowadays, and 2″ eyepieces are used in large telescopes. Eyepiece adapters are used to adapt or convert from one eyepiece size to another. This device will make it possible for the same telescope to use different types of 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.

Eye Relief – The distance images are projected from the ocular lens to their focal point, measured in mm. This is the distance a binocular or scope can be held away from the eye and still present the full filed of view. The eye relief of a binocular can vary from 5mm to as much as 23mm. A typical range of eye relief is 8-13 mm which is considered to be appropriate to enable eyeglass wearers to see the whole field of view.

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.

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 size of the image you can see while looking through binoculars or a scope. It is defined by the width in feet or metres of the area visible at 1000 yards or metres. It can also be defined as an angle in degrees (1 degree of field=52.5 ft/1000 yards).

Note: A wide FOV is better for following fast-moving target or scanning for wildlife. In general, the higher the magnification, the narrower the field of view.

Filter, colour – Glass filters, each of a specific colour, which screw onto eyepiece barrels for enhancing lunar and planetary detail. Various colour 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 on to an eyepiece or rear cell of a Schmidt-Cassegrain that blocks wavelengths of light pollution sources such as mercury vapour 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 aluminium 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 colour, which screw onto eyepiece barrels for enhancing lunar and planetary detail. Various colour 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 colour 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 on to 1.25″ eyepiece barrels, consists of two pieces of polarized glass mounted in an aluminium 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 centre of the finder’s visual field, which helps to locate the target.

Focal Length – The distance between the objective lens (or primary mirror) and its focus (or focal plane).

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 sensitised 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.

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”).

Focusser – 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.

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, the 10×50 and 12×50 binoculars are a full-sized binoculars, while a 10×25 binocular is considered as a compact binocular.

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”.

Giant Binoculars – Binoculars with large objective lenses and high power are often known as giant binoculars. These binoculars are suitable for medium-to-long-range terrestrial observations, as well as for star gazing. A typical example is the NIPON 20×80 giant observation binoculars. For their size and weight, these binoculars should be mounted on a sturdy tripod while in use.

Haze – 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.

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

Individual Focus – Unlike centre-focus binoculars which adjust both eyepieces at the same time, individual-focus binoculars focus each eyepiece separately. This allows for extra-precise focus adjustment for each eye.

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.

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.

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

Light Transmission – The ratio of the total amount of light passing through the objective lens to the eye. Better coatings on the optics increase the amount of light that reaches the eye.

Light-Gathering Power – The ability of the binoculars to collect light. This measurement is directly proportional to the size of the objective lens of the binoculars.

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

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.

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).

Mirage – 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.

Monocular – A single “pocket-sized” telescope used as a handy spotting scope.

Near Focus – See “Close Focus”.

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 proof & fog-proof.
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.

Objective Lens – The large lens at the end of the binocular or scope away from the eyepiece. This lens gathers light into the eye. The larger the objective lens, the more light that enters the scope and the brighter the image.

Ocular Lens – An alternate term for eyepiece.

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.

O-Ring Sealed – A special sealant on binoculars that makes them waterproof.

Parallax – Apparent shift in position of a viewed object attributable to the difference between two separate and distinct points of view. In a scope sight, parallax can cause an aiming error, or parallax error, when the target image is not formed in the same plane as the reticle.

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 colour fidelity and reduces image contrast and gives a clearer view.

Porro Prism – The objective or front lens is offset from the eyepiece (as opposed to the aligned roof prisms). Porro prisms have objective lenses spaced wider than roof prisms, and can provide greater depth perception and generally offer a wider field of view.

Power – 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.

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.

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.

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.

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

Refractor – A telescope in which the main light gathering element is a lens, known as the objective lens.

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.

Resolution (Resolving Power) – Resolution or definition is the ability of a binocular or spotting scope to distinguish fine detail and retain clarity.

Reticle – In a rifle or handgun scope, the reticle is an aiming reference consisting of crosswires, dot, pointed post or other distinct shape that appears superimposed on the field of view. The reticle is positioned within the optical system to coincide with the plane of focus of the objective lens.

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.

Roof Prism – The prisms overlap closely, allowing the objective lenses to line up directly with the eyepiece (as opposed to the off-set porro prisms). This result in a slim, streamlined shape of binocular or scope. The top models of the roof-prism and porro-prism binoculars are now generally considered to have equal optical quality.

Ruby Coatings – The objective lenses of a binocular with ruby coatings will be a bright reddish-orange. Since red light is reflected the colours 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 colour spectrum which may increase the contrast and resolution of a binocular.

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

Spotting scope – A small, portable telescope used primarily for terrestrial observing, such as nature study and bird watching. Most spotting scopes use prisms to provide an image that matches the naked eye.

Spyglass – This is another term for handheld telescope, often found with some vintage brass pull-out telescopes.

T-Adapter – 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.

T-Ring – 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.

Terrestrial – 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.

Transmittance – 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”.

Tripod – 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’)

Twilight Factor – Most often associated with binoculars, this is a numerical expression of the telescope effect in dim light. It may also be calculated for scope sights.

Note: The twilight factor is derived by multiplying the magnification by the useful objective diameter (mm), and then extracting the square root. This factor assumes realistically that in dim light, all other factors being equal, viewing instruments with higher magnification and larger objective lenses will outperform those with lower power and lesser light gathering capability.

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.

Universal Digital Camera Adapter – 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 adapters.

Variable Power (Zoom Lens) – Variable-power scopes or binoculars have a control that allows the user to adjust the magnification over a predetermined range.

Vintage binoculars: A range of vintage binoculars are available here, and information on “How to identify the production dates of vintage binoculars“.

Waterproof / Fogproof – The binoculars or spotting scopes that are sealed with O-rings and nitrogen-purged for waterproof and fogproof protection. These products are able to withstand complete immersion and remain dry inside. One example is the NIPON 25-125×92 spotting scope.

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

Wide-Angle or Wide-Field Binoculars – Binoculars with a wider field of view (generally described as greater than 6 degrees). For example, NIPON 12×50 military binoculars have a wide field of view at 6 degrees, convenient for target search. Some vintage binoculars, especially for military use, have wide field designs with large eyepieces. These binoculars can be found in German, British and Japan product categories.

Zoom Lens – See “Variable Power”.


Please send your comments and suggestions to:

Thank you!

Relevant links:

Glossary of Optical Terms

Glossary of Binocular Terms

Binoculars Glossary