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Glossary: Binocular Terms
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Glossary: Binocular Terms

Binoculars for photographers

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A guide for the optically perplexed.


Angle of view: The angle between the left- and right-hand edges of the field of view at 1000 yards measured from the binocular’s point of view.

Apparent angle of view: This is simply the angle of view with the binocular’s magnification figured in. For a 10X magnification and a 5-degree angle of view, the apparent angle of view would be 50 degrees.

Brightness index: The square of the exit pupil diameter. A binocular with a 4mm exit pupil has a brightness index of 16. A common way to measure the brightness of an instrument.

BaK-4 glass: A high-quality, high density barium crown glass used in prisms to minimize internal light scattering, thereby yielding sharper images. Fine quality binoculars generally use BaK-4 glass instead of lower quality BaK-7.

Center focus binoculars: Binoculars using a central focus-control mechanism to allow both eyepieces to be adjusted simultaneously for quick focusing. Virtually all general-purpose binoculars now have this feature.

Close focus: The ability of a binocular to focus closer than the “average” close distance of about 10 feet. A low close-focus number is useful when viewing small objects such as butterflies.

Coatings: All modern binoculars have anti-reflection coatings on lenses and prisms, but multicoating and other complex coating methods reduce internal reflections and increase light transmission for a brighter view. Many companies claim their proprietary coasting systems enhance performance.

Diopter adjuster: A separate adjuster on one eyepiece (usually the right) that allows the user to compensate for the difference between eyes. It’s a feature of all better-quality binoculars.

Exit pupil: The point at which all the light rays passing through the binoculars exit through the eyepiece. To calculate the exit pupil, divide the diameter of the objective lens by the magnification. For example with an 8x40 binocular, the exit pupil is 40 over 8, which equals 5mm. A large exit pupil is important for low-light viewing.

Eye relief: The distance behind the ocular lens at which the image is projected to its focal point; it varies from about 5mm to 23mm. The greater the eye relief, the easier it is for eyeglass wearers to see the entire field.

Field of view: The diameter of the circular viewing field seen through a binocular. It’s usually listed on the binocular, either in degrees or feet, measured at 1000 yards. One degree equals 52.5 feet over 1000 yards.

Full-size binoculars: These have large objective (front) lenses that provide better light-gathering ability than a compact binocular with the same magnification, e.g. an 8x42 full-size binocular is much brighter than a compact 8x25.

Image stabilization: An advanced feature designed to compensate for shake when handholding a binocular, a significant factor with binoculars of 10X or greater magnification, or when they’re used on board a boat or helicopter.

Interpupillary distance: The distance between the viewer’s pupils, which can be accommodated by adjusting the barrels of the binocular inward or outward. Often listed as a range, e.g., ID 56-72, referring to the minimum and maximum distances available, in millimeters.

Magnification: Also called power, it’s the first number listed in basic binocular specs, e.g. 8x42, An 8X or 8- power binocular will magnify the image 8 times, so an object that’s 800 feet away will appear as if it were only 100 feet away.

Nitrogen purging: The process of replacing the air within a binocular with nitrogen, to prevent mold, mildew, or acid etching of lenses. It also helps prevent fogging due to the moisture in atmospheric air.

Objective lens: The lens at the front end of the binocular away from the eye that gathers the light presented to the eye. The diameter of the objective lens is the second number listed  in binocular specs—a 10x42 binocular has 42mm-diamter objective lenses.

Ocular lens: The lens in the eyepiece, It’s usually smaller than the objective lens except in the case of some roof prism binoculars.

Porro prism binoculars: These have the classic binocular appearance with body extending wide of the eyepieces due to the offset prisms used for image erecting. Said to provide better 3-D viewing.

Phase correction: The application of special coatings to bring the two out-of-phase light beams produced by roof prism binoculars back into phase, thus increasing brightness, contrast, resolution and color fidelity.

Relative brightness: A number indicating the size of the light shaft that reaches the eyes. Brightness factors up to 10 are okay for daylight use; figures from 10-16 are adequate for dusk or cloudy days, 25-50 for nighttime use.

Roof prism binoculars: These are generally more compact, with a slimmer body design, since their image-erecting prisms are lined up. Often more costly, roof prism binoculars are said to provide superior structural rigidity.

Twilight factor: Another factor used in comparing low light performance, it’s calculated by multiplying the magnification by the objective lens diameter and finding the square root of the result. For example, an 8x58 binocular has as twilight factor of 21.2, very good for low-light viewing, but an 8x30 has a twilight factor of 15.5, less suitable for low light use.

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