...your guide to using optics.

Binocular Terms: (D - F)

Depth of Field

This refers to what you actually see in focus at any one time. It is the area in focus between the foreground and background of the viewing area.

Diopter Adjuster

All better quality binoculars will allow for separate adjustment of the focus for one eyepiece (usually the right). This is done to compensate for differences between eyes.

Exit Pupil

The exit pupil of a binocular is the point at which all of the light rays that entered the objective lens and passed through the binocular exit through eyepiece to form a magnified, circular image. If you hold the binocular away from your eyes and look through the ocular lens, you will be able to see the clear circular exit pupil.

To Calculate the Exit Pupil of a Binocular

Divide the measurement of the binocular's objective lens by the magnification of the binocular. For example, with an 8x40 binocular you would divide the 40mm objective lens by the magnifcation of 8 power for an exit pupil of 5mm.

Eye Relief

Eye relief in a binocular refers to the distance images are projected from the ocular lens to their focal point. The eye relief of a binocular can vary from 5mm to as much as 23mm.
Note: Without proper eye relief, the eyeglass wearer will not see the full field of view.

Field of View

The field of view is the widest dimension of the circular viewing area seen through a binocular. This measurement may be listed on the binocular in either degrees or feet, measured at 1000 yards.

Note: One degree equals 52.5 feet/1000 yards.

Focal Point

This is a point where the light rays from an image come sharply into view after passing throught the binocular.

Full Size Binoculars

Full size binoculars offer better light gathering ability than compact binoculars because of a relatively large objective lens. For example, an 8x42 binocular is a full-sized binocular (an 8x25 binocular is called a compact binocular).

Fully Multi-Coated Optics

Binoculars that have fully multi-coated optics have multiple coatings on all air-to-glass surfaces.

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