Glossary of Binocular Terms

Confused about all the jargon? Want to make buying binoculars easier? Find out everything you need to know from our useful list of binocular terms:

Aperture (See also objective lens)

When referring to binoculars the aperture is the size of the objective lens which is the large lens at the opposite end of the binoculars to the eyepiece. The larger the aperture the greater the ability to gather light therefore a larger aperture is preferable for low light conditions.

You will easily find the size of aperture (objective lens) from the binocular description. When it is stated that a pair of binoculars are 10x42 (for example) the first number refers to magnification and the second (i.e. 42) to objective lens. 

Angular Field of View (Apparent)

The Angular field of view (apparent) is the real field of view multiplied by the binoculars magnification power so continuing to use the Canon 12x36 IS II Binoculars as our example, they have a real field of view is 5^o  and a magnification power of 12, 12 x 5 = 60 ^o .   

Angular Field of View (Real)

The Angular field of view is measured in degrees. You can roughly calculate the linear field of view (field of view at 1000mtrs) by multiplying the angular field of view (real) by 17.5. Take for example Canon 12x36 IS II Binoculars. The real field of view is 5^o so if you multiply 5 x 17.5 you come up with the linear field of view which is 87.5mtrs.

Aspherical Lens

An aspherical lens is a lens that doesn’t have a single radius (i.e. is not a portion of a sphere). The edges of an aspherical lens are flattened and designed to reduce aberrations providing clearer, sharper images.

Bak-4 Glass Prisms

Prisms are basically made using BaK-4 or BK7 glass. BK7 glass is usually used in lower priced binoculars and produces an inferior quality image to binoculars with BaK-4 prisms. Bak-4 prisms provide greater clarity of image and edge to edge sharpness.

BK7 Prisms

BK7 prisms are generally found in lower priced binoculars (see Bak-4 Prisms).

Brightness

The level of brightness of an image when looking through binoculars is dependent of several factors such as the binoculars ability to gather light (based on the size of the objective lens), the level of magnification (lower magnification binoculars tend to deliver brighter images) and the size of the exit pupil (large exit pupils will help improve the brightness of an image).

Other factors that will have an influence on how bright and image will appear is lens quality, the type of lens coating (good quality binoculars should have fully multi-coated lenses) and the type of prisms used.

The level of brightness when observing objects with binoculars will also have an impact on the clarity of colour, the brighter the image the greater the colour differentiation is likely to be.

 

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