Relative Brightness
A simple low-light index derived from the exit pupil diameter, where a larger pupil delivers a brighter image to a dark-adapted eye.
Relative brightness is a quick comparative number found by squaring the exit pupil diameter in millimeters, so a 5 mm exit pupil yields an index of 25. The idea rests on the fact that a wider cone of light leaving the eyepiece can fill more of a dilated pupil, which in dim conditions means a brighter, easier image. Because it depends only on the exit pupil, the figure is easy to calculate from a scope’s magnification and objective size.
The index is a rough guide rather than a complete answer, and it pairs with the twilight factor, which weighs resolution alongside brightness for low-light use. Neither number accounts for light transmission, the share of light that actually survives the glass and coatings, so two optics with the same relative brightness can still differ at dusk. A larger objective lens raises the exit pupil at a given power, which is the most direct way to push the index up.