Pupil size - Binoculars
There are many different factors that cause pupils to dilate or contract. For binocular enthusiasts, the most important factor here is the quantity of external light. In dark conditions, the pupils will widen as much as possible so that the maximum amount of light can enter the eye. When there is a lot of light, the pupils will constrict and become smaller in order to protect the eye from 'over-exposure' in much the same way a diaphragm in a camera does.
When looking through binoculars, it is important that the human pupil is preferably smaller or in any case not bigger than the exit pupil of the binoculars. To calculate this measurement, see Exit pupil.
The human pupil size ranges between about 2 and 7 millimetres. As we age, the flexibility of the pupil decreases and the maximum size is approximately 5 millimetres.