THE EXIT PUPIL
The exit pupil of a pair of binuculars is the diameter of the beam of light leaving each eye-piece and entering each eye. It is easy to calculate—simply divide the aperture by the power. For a 7 x 50 model, the 50 mm aperture divided by the 7 power equals a 7.1 mm exit pupil.
At night, under ideal conditions, the pupil of a dark-adapted human eye opens to 7 mm. Binoculars with a 7 mm exit pupil— a 7 x 50, an 8 x 56, or a 9 x 63 model—produce as wide a beam of light as the average human eye can accept, yielding maximum image brightness. Such models are commonly recommended for astronomy—indeed, the 7 x 50 model is sometimes called a night glass.
So why consider a 10 x 50 model? The nighttime viewing conditions most of us contend with are far from pitch black, so our eyes never fully dilate to 7 mm. Age also takes its toll. The pupils of most people over 30 open to only 6 mm in the dark. By the age of 50, our pupils' maximum aperture may be no more than 4 to 5 mm, so the light from binoculars with a 7 mm exit pupil is simply wasted, reducing the effective aperture of the binoculars. For many skywatchers, especially city dwellers, binoculars with a 5 or 6 mm exit pupil (a 10 x 50, or a lighter-weight 7 x 42) are a better choice.