Exit Pupil: This is the small disk of light seen when looking at the eyepiece lens of an optical instrument that is held at arm’s length and pointed toward a bright background. Though the exit pupil may appear to be within the eyepiece, it is actually located behind the eyepieces at the point where the eye must be positioned to see the full field of view. Exit pupil diameters, in millimeters, are calculated by dividing the objective diameter by the magnification, e.g. a 10X 50 mm binocular has 5 mm exit pupils.

Eyepiece (ocular): The lens or lens group between the final image in a visual optical instrument and the viewer’s eye. It serves to magnify that image, thereby determining the magnifying power.

Eye Pupil: The apparently black circular opening in the iris of the eye that permits light to pass through the eye lens to the retina. The human eye pupil dilates automatically from a diameter of about 2 mm in bright sunlight to 7 mm in the dark.

Eye Relief: The distance from the ocular (rear) lens of an optical system to a point at which the user’s eye can see the full field of view. “Long-eye-relief” binoculars and spotting scopes allow users to see the entire field of view while wearing eyeglasses. Telescopic sights used on hard-kicking firearms must have especially long eye relief to reduce the likelihood of the ocular striking the shooter’s eye or face.

Field of View (FOV): The maximum width of the areas that can be seen through the eyepiece of an optical instrument, as measured either angularly in degrees or linearly in feet or meters at specified distances. For example, a 10X binocular with a six-degree angular field has a linear FOV of 315 feet at 1,000 yards. For conversion purposes, one degree equals 52.36 feet at 1,000 yards.