The exit pupil is the bright circle that can be seen in the center of each eyepiece when you hold the binoculars about 12 inches away from your eyes with the objective lenses pointed toward a bright light. The larger the diameter is, the brighter the viewfield is, which is an important consideration when using binoculars in dark situations and for astronomical observation.
Exit pupil = The effective diameter of the objective lens ÷ Magnification
- With 8x42 binoculars, the formula is 42 ÷ 8 = 5.3. Therefore, the diameter of the exit pupil is 5.3mm. The adjacent image indicates the brightness of the image in view.
The relationship between the dilation and contraction of your eyes (pupil size) and the size of the Exit Pupil determines light delivery potentials.
- Smaller exit pupils affect brightness enormously, because the pupil of the human eye controls the amount of light entering our eye, by shrinking when light is brighter, or growing when light dims.
The human eye pupil diameter ranges from about 2mm in bright light, to a maximum of about 7mm in total darkness.
- For our optics-aided eyeball to take advantage of available light, the exit pupil in a binocular should be at least the size of our own eye’s pupil in any given situation. If smaller, your eye will be “light starved” (not enough light reaches your eye). This normally happens at dawn and dusk.
Of course, during daylight hours, or in a well-illuminated venue (stadium, arena, or concert hall), your pupil can contract to be as small as 2mm or 3mm. In these usages, a 3mm exit pupil is sufficient and allows the use of a smaller, more portable binocular.