The exit pupil in binoculars is one of the underrated specifications. When it comes to choosing binoculars, many of us only look out for specs like magnification, objective lens size, a field of view and among other specs with little to no consideration as to how the exit pupil might affect our viewing under certain conditions.
Understanding Exit Pupil
The exit pupil is that bright circle you can see in the center of the eyepiece of your binoculars when you hold them toward the sky or any bright light about 30cm away from your eyes. The large diameter of the bright circle of that column of light that is coming right through the binocular to your eyes is called the exit pupil of that binocular.
The larger the exit pupil is the brighter the image you are viewing will be especially during low light conditions like stargazing, dusk or dawn hunting, etc.
Exit Pupil Formula- How do you Calculate the Exit Pupil?
You can calculate the exit pupil by dividing the objective lens diameter by the power of the magnification of the binocular.
Exit pupil = The effective diameter of the objective lens ÷ Magnification
With 8×42 binoculars, the formula is 42 ÷ 8 = 5.3.
Therefore, the diameter of the exit pupil is 5.3mm.
This figure indicates the brightness of the image in view.
The larger a binocular’s objective lenses, the larger the exit pupil; but the greater the magnification, the smaller the exit pupil.
The pupil of the human eyes changes depending on the light conditions the person is subjected to. When there is plenty of lights around you, especially the bright daytime the human pupil is around 2-3mm. This is because, in ambient light conditions, part of the column of light coming through the binocular will fall on the iris of the eye instead of the pupil.
That part of the light does not contribute to the brightness of the image. However, in low light conditions like dusk and dawn or night, typically, the human pupil is at 7mm. But this number varies from person to person and it ranges from 5 mm to 9 mm.
The exit pupil of the binocular should correspond with the amount of dilation of your eyes pupil when you are in low light conditions. That way, you can use your device and get bright images when there aren’t enough lights around you.
Exit Pupil and Eye Relief
Eye relief is the distance from your eyes to the outer surface of the eyepiece lens of your binoculars where all the light rays of the exit pupil come to a focus and form the image. Your eyes should be at least 10 mm away from the eyepiece for you to have the entire field of view. Otherwise, you will lose on the exit pupil and not get a brighter image.
For those who wear glasses, it is often recommended you have longer eye relief than those who don’t because the glasses add extra distance. This is to enable you get the best comfort while viewing. Eye relief of 16 mm or more is ideal for eyeglass wearers.
Put your eye so it’s just behind the eyepiece to take advantage of its eye relief. You’ll lose the field of view if you place your eye farther away and may even move your eye out of the beam of light from the eyepiece. Getting too close will prevent you from blinking and may also cause a black ring to appear around the field of view.
Exit Pupil and Age
As you get older the dilation of your pupil gets less in dark conditions. That’s why compact binoculars, such as an 8×32, seem to be just made for an older person. By age 50, the exit pupil may be close to 5mm. For pupils that remain relatively small even in dim conditions, the smaller objective lens and smaller exit pupil provide just as bright an image as a larger, heavier, more expensive binocular.