Understanding optics...

Exit Pupil

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