Originally Posted By: Russ
At night, a large objective lens is what makes binoculars amazing or inadequate. The exit pupil is determined by dividing the obj lens diameter by the power. 40/7=5.7 vs 50/7=7.1
A 7mm exit pupil vs slightly less than 6mm exit pupil. Given the same glass, 7x50's will beat 7x40's every time.

Remember that is the maximum possible exit pupil, it's likely the optical design restricts the exit pupil to less than this - to reduce aberation. Although on good quality optics it will be as close as possible.

It also depends on the user. A pupil of 7mm is really about the maximum for a child's perfect and dark adapted eye. By middle age it is closer to 6.0mm even in the best conditions.
An exit pupil larger than this just wastes light.

A good estimate is pupil = 7.0mm * e(0.5 * (100/age)^2 ) for the geeks among us!