The exit pupil of a telescope eyepiece combination is the diameter of the image coming out of the eyepiece. It is found by dividing the focal length of the eyepiece by the focal ratio of the telescope.
Exit Pupil = ( ƒ_ep ) ÷ ( f-ratio_scope )
Small exit pupils (1-2 mm) can bring out more details when observing the moon, planets, and some dense globular clusters. Larger exit pupils tend to be better for dimmer objects. This is because the brightness of these dim DSO’s (deep sky objects) varies as the exit pupil squared.
Finding the exit pupil for two separate eyepieces: a 17 mm and a 8 mm eyepiece. Using my Celestron 130 SLT with a focal ratio of f/5…
Exit Pupil_1 = ( 17 mm ) ÷ ( 5 ) = 3.4 mm
Exit Pupil_2 = ( 8 mm ) ÷ ( 5 ) = 1.6 mm
Calculating the brightness difference between objects in the two eyepieces…
( 3.4 )² = 11.56 and ( 1.6 )² = 2.56
This means that the object in the 3.4 mm exit pupil will be 4.52 times brighter than the in the 1.6 mm exit pupil.