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.

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.

Focus: 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.

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

Interpupillary Distance (IPD): The distance between the exit pupils of a binocular instrument. This usually is adjustable so as to accommodate the observer’s interocular distance, a normal range being from 55 mm to 75 mm.

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 (virtual): An optical image as seen directly by the viewer’s eye is said to be “virtual” because it doesn’t exist where it appears to be. A virtual image cannot be captured on a screen or film. The images seen in a plane mirror, which appear to be located behind the surface, are likewise classified as virtual.

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.
Light-Gathering Power: A term used to describe the capacity of objective lenses to receive light. Light-gathering power can be calculated as the square of the objective’s diameter in millimeters. For example, doubling the diameter of the objective, quadruples the light gathering power.

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.

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. It is designated by the number preceding the “X” in optical specifications, i.e., a 10X 42 mm binocular makes objects appear 10-times larger.

Monocular: 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.

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

Objective: The optical element (front lens) that receives light from viewed objects and forms the first or primary image in an optical instrument. The number following the “X” in optical specification numbers is the objective diameter in millimeters, i.e., a 10X 42 mm binocular has 42 mm objectives. Larger objectives gather more light, relative to their surface areas, and have better resolution, relative to their diameters.

Parallax: Parallax is an optical phenomenon that exists in telescopic sights when the image plane of the target—the location of which varies longitudinally within the scope depending on the target distance—does not fall exactly on the reticle plane. This is seen as a relative motion between the target and the reticle when the eye is moved from side to side in the exit pupil. Low-power scopes are adjusted at the factory to be parallax-free at one specific distance, such as 100 or 150 yards for riflescopes, 75 yards for shotgun scopes and 50 yards for rimfire scopes. Scopes with magnifications exceeding 10X usually have special focusing adjustments that eliminate parallax and simultaneously assure maximum image sharpness by refocusing the target image to precisely coincide with the reticle plane. These adjustments are located either on the scope’s objective bell, called “adjustable objective” (AO), or on the adjustment turret, called “side focus.”

Prisms: 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.

Refraction: 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.

Relative brightness: Also called geometrical luminosity factor. The ability of an exit pupil to transmit available light, which is calculated as the square of the diameter in millimeters. Accordingly, an instrument with a 2.5 mm exit pupil has a relative brightness of 6.25, while a 7 mm exit pupil has a relative brightness of 49, which corresponds to the maximum dilation of the pupil of the human eye.

Resolving Power (Resolution): The ability of an optical instrument to reproduce the points, lines and surfaces in an object as separate entities in the image. Resolution is measured in seconds of angle using a target containing a series of sets of lines at progressively smaller spacing.

Reticle: 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.

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.

Telescope: 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. Terms used to describe various types of telescopes include:

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

•    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.

•    Refracting Telescope: Also called a refractor, a telescope that uses a conventional objective lens to focus an image of a distant object at a focal point. Small astronomical telescopes, practically all spotting scopes, telescopic sights and binoculars are refractors.