Telescopes and binoculars concentrate the light gathered by the objective into a beam of light - "the exit pupil". The exit pupil diameter is the objective diameter divided by the magnifying power.
For maximum effective light-gathering and brightest image, the exit pupil should equal the diameter of the fully dilated iris of the human eye — about 7mm, reducing with age.
See also:
- What do the numbers on my scope mean?- What is magnification?- What is the field of view- What is an objective lens?- What is eye relief?- What is the exit pupil?- Twilight Factor- Relative Brightness