Exit Pupil of Binoculars

The exit pupil is the magnified image in the eyepiece as it leaves the binocular to enter your eye and its diameter, measured in millimeters, is determined by dividing the aperture by the magnification. Exit pupil diameters will generally vary from 2.5mm to 7.1mm and, as an example, a 7 X 35 binocular will have an exit pupil of 5mm.

You can see the exit pupil as a circular beam of light in the eyepiece when you hold the binocular at arm's length. The main importance of the exit pupil is how it relates to the pupil size of your eye. The eye pupil is controlled by the iris, which acts like a variable aperture for the retina and will allow the pupil to change in size from about 2mm up to 8mm, usually depending on the brightness of the available light. When the exit pupil of the binocular is larger than the eye pupil, some of the light coming from the binocular will fall on the iris and is undetected by the observer. When the exit pupil is smaller than the eye pupil, then the amount of light falling on the retina will be less than that collected with your normal vision at that particular time, and so the object observed will appear dim. Resolution and contrast are affected adversely, resulting in loss of clarity of the observed image.

On a bright day, an observed image through a compact 8 X 20 binocular (exit pupil 2.5mm) will appear just as bright as a 7 X 50 (exit pupil 7.1mm) since the daylight-adapted pupil (2 to 3mm diameter) is the limiting factor in observed brightness under these conditions. At twilight or in other situations of low available light, the eye pupil size will vary between 4 and 5mm. For the best performance under these conditions, a minimum exit pupil of 4mm is essential, such as that provided by a 8 X 32 or a 10 X 40. At night, when the eye pupil size is at its maximum dilation (6 to 8mm), only a binocular like a 7 X 50 or 10 X 70 will meet the requirements for the optimum in observed image brightness. So you can see that it's important to consider the exit pupil of a binocular in the context of eye pupil size and the viewing time or conditions of the observer.

There are some other factors to consider when discussing exit pupil. As we grow older, the maximum dilation of our eye pupil diameter slowly decreases in size from about 7 or 8mm at age 20 to about 5mm at age 50. Of course, there will be some individual variation in these figures due to hereditary or environmental factors, but it's worth noting this physiological phenomenon as it relates to exit pupil size and in choosing the binocular that best meets your needs. Another consideration, is whether or not there will be any observing done under conditions that would not allow you to hold the binoculars steady, such as on the deck of a boat. Here is a situation where a large exit pupil is very useful, even for the daylight-adapted eye, because it is much easier to keep the smaller eye pupil centered in the larger exit pupil when the binoculars are difficult to hold steady.

Thanks to our friends at Eagle Optics for providing this information!

See also:
Spotting Scopes—Tips for choosing the right spotting scope
Birding Tripods—Tips for choosing the right tripod for you binoculars and scopes

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