What is the exit pupil of my spotting scope?

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	What is the exit pupil of my spotting scope?

Answer The exit pupil is width of the beam of light leaving the eyepiece. It’s usually measured in millimeters. A large exit pupil is advantageous under low light conditions and at night because the larger the exit pupil, the brighter the image. For astronomical applications, the exit pupil of the scope should correspond with the amount of dilation of your eye's pupil after it is fully dark-adapted. This number will be between 5mm and 9mm. 9mm of dilation is the maximum amount for the human eye. Maximum dilation tends to decrease with age. By age 50, the exit pupil may be close to 5mm. An exit pupil larger than your dilation just wastes light from the objective, since the outside of the beam just falls on your iris and doesn't go into your eye.

To calculate the exit pupil, simply divide the size of the objective lens by the magnification of the scope. For example, the exit pupil of an 80mm scope at 24x is 80 / 24 = 3.3mm.

Article Details

Article ID:

2124

Created On:

Nov 26 2008 12:48 PM