Exit Pupil of Spotting Scopes
The exit pupil is the point at which all the light rays from the object entering the objective lens (aperture) exit through the eyepiece to form a magnified, circular image. It can be seen by holding the spotting scope at arm's length against a diffuse source of bright light, and its diameter can be calculated by dividing the aperture by the magnification. For example, a 60mm scope at 20X would have a 3 mm exit pupil. Most spotting scopes designed primarily for terrestrial use will have exit pupils that will range from 1 to 4 mm in diameter.
An important consideration is how the exit pupil size relates to your eye pupil size under varying conditions of observation. The eye pupil will compensate for the amount of available light by constricting or dilating accordingly; 2 to 3mm on a bright day, 4 to 5 mm at twilight, or 6 to 8mm at night. The best performance of the instrument is obtained when both exit and eye pupil size are the same, so it's important to determine under what conditions you will primarily be using the spotting scope and choose the configuration that gives you the required exit pupil. Usually this means deciding what type of eyepiece you will choose since it is the eyepiece that magnifies the image and therefore will determine the exit pupil for a given aperture.
Thanks to our friends at Eagle Optics for providing this information!
Back to Bird Spotting Scopes