Glossary of Terms for Binoculars, Spotting Scopes and Telescopes
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.
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.
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.
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.
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 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.
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.
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 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.
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.
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.
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 EE210.
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.
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.
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 10x50 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 PLÖSSL. Amongst them, PLÖSSL 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 PLÖSSL 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) – 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, the Nipon 26-78x78 scope can be fitted with a digital eyepiece or other standard 1.25” eyepieces (eg. the PLÖSSL eyepiece set) for a wider range of applications.
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 - Cups on the eyepieces of binoculars that allow for positioning of the eyes and provide optimal eye relief. 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.
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 – 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, a 10x42 binocular is a full-sized binocular, while a 10x25 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”.
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 center-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.
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.
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.
Near Focus – See “Close Focus”.
Nitrogen-Purged - 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.
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 - The small lens in the eyepiece. In some cases (as in some roof-prism binoculars), this lens can be in the same size as the objective lens.
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.
Primary Mirror – The principal light gathering mirror in a reflecting telescope.
Prism Glass - 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.
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.
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.
Spyglass – This is another term for handheld telescope. One example can be found by clicking here.
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”.
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.
Variable Power (Zoom Lens) – Variable-power scopes or binoculars have a control that allows the user to adjust the magnification over a predetermined range.
Waterproof / Fogproof – The binoculars or 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.
Wedge – 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.
Zoom Lens – See “Variable Power”.
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