EXIT PUPIL of the binocular

The exit pupil is actually not a physical feature like it sounds; it is the diameter of the stream of light flowing through the binoculars at the eyepieces into the viewers’ eyes and is calculated by dividing the diameter of the objective lenses by the magnification.

Example: Objective lens diameter: 40 mm; magnification: 8x. Exit pupil: 5. For viewing in dim light the exit pupil should not be less than 5mm, but if you’re still young, it should be at least 7mm – unless you realize that you will sacrifice clarity in fading light or focusing on animals in the shade. Pocket binoculars never have exit pupils bigger than 3 mm. This means that they will not be suitable in low light conditions, even though they do have other advantages. For a thorough discussion of “exit pupil”, please consult “Understanding binoculars 2” and “Best low light binoculars”.
This shows how important it is not to go for too much power.

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