Eye Relief and adjustable Eyepieces
This describes the distance from the eyepiece to the surface of the eye where the full image can be viewed.
If you wear glasses, it is best to choose a pair of binoculars with long eye relief. Adjustable eyepieces can be twisted up and
down to further enhance comfortable viewing . See the illustration
Exit pupil size
Exit pupil size refers to the diameter of the image that emerges from the eyepiece. The larger the image, the greater
your ability to see in low-light situations. Exit pupil size can be determined by dividing the diameter of the objective lens by
the magnification. This gives you the size of the exit pupil in millimetres. An exit pupil size of 2 or 3 millimetres is sufficient
for viewing subjects in bright light, but if low-light performance is an issue, look for a binocular with an exit pupil size of 5-7 millimetres.
In case of 8x42 binoculars: 42/8 = 5.3 . exit pupil = 5.3 mm