...your guide to using optics.

Binocular Terms: (A - C)


The diameter of the binocular lens, measured in millimeters.

Aspherical Lenses

If all other binocular features are equal, aspherical lenses provide the binocular with brighter, sharper images.

BaK-4 Glass

Binoculars using this fine, high density barium crown glass, eliminate internal light scattering so the images seen through these binoculars are sharper.

Center Focus Binoculars

Binoculars using a center focus mechanism allow both eyepieces to be adjusted simultaneously for rapid focus.

Chromatic Aberration

This is a defect of optical lenses used in binoculars. Different wavelengths (producing different colors) are diffracted, or bent, at different angles and produce colored halos around images.

Close Focus

The close focus (near focus) of a binocular will vary between binoculars. If in viewing things like butterflies you need to be close to the subject, choose a binocular with a low close focus number.


A thin layer of an anti-reflection coating is applied to the glass surface in a binocular. This coating reduces the amount of light lost as the light passes through the glass surface. The result is that more light will actually passes through your binocular and reaches your eye. Better coatings with more complete and complex coating schemes, improve the brightness and overall optical performance of the binocular.


Collimation refers to the proper alignment of lenses in the optical system of the binocular.

Compact Binoculars

Binoculars so small they fit in a pocket or purse are called compact binoculars. The small, compact nature of these binoculars makes them ideal for day use. Available as roof prism binoculars or reverse-Porro prism binoculars.

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