Exit pupil

The exit pupil can be seen by holding the binoculars at arm’s length and looking through the eyepieces. The pencil of light you see is the exit pupil. The diameter of the exit pupil determines how much light is transmitted to your eye. The actual diameter of the exit pupil is computed by dividing the diameter of the front objective lens (in millimeters) by the magnification of the binocular. So, in the case of a 7×50 binoculars, the diameter of the exit pupil is 7.1. The diameter of the exit pupil is important in fading light. If it’s less than 7 mm, your binoculars will not allow enough light through to your eyes (in fading light, that is), unless you’re an elderly person, in which case 5 mm. will be enough.

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