Exit Pupil

Drinking Out Of A Fire House.

- Steven K. Ledin

When drinking out of a gushing firehose, you can consume just so much. The rest spills out of your mouth and is wasted. The same principle applies when looking through an exit pupil: You can consume just so much. The rest “spills out” of your eye and is wasted.

The exit pupil of an optic is the column of light transmitted by the ocular lens, appearing as a circle of light visible in the center of the eyepiece when viewed from half an arm’s length away from your face. This exit pupil is measured in millimeters.

The formula to determine exit pupil of an optic is simple:

Objective lens diameter in millimeters divided by magnification

Example: With a 3.5-10x50 scope set at 10 power, divide the 50mm objective lens size by the magnification of 10; the answer is 5mm. Now, lower the magnification to 5x. Using the same formula, the exit pupil is now 10 (50mm objective lens divided by 5x = 10mm).

To continue the metaphor, If you can match the amount of water exiting from a firehose to the maximum amount you can drink, this is the most efficient, and nothing is wasted. You also don’t have to manhandle the impressive power that a fully charged firehose wields.

Likewise, if you can match the amount of light exiting a riflescope or binocular eyepiece to the maximum amount your eye can “swallow,” this is the most efficient, and nothing is wasted. You also don’t have to manhandle the extra weight, size and expense that an optic with a giant exit pupil might wield.

Young pupils dilate (open) a lot larger than older pupils. Various scientific studies reflect that an average maximum pupil size when dark-adjusted is about 7mm for people in their 20s to a maximum of 2mm for someone in their 80s.