While holding binoculars about 30cm from your eyes and looking through the ocular lenses, you can see bright circles in the middle; these are called the exit pupil (see below).
The exit pupil shows how much light makes it to your eyes. The larger the exit pupil, the more light reaches your eyes.
The diameter of the exit pupil does not depend solely on the aperture (the diameter of the objective lens). It varies depending on a combination of the aperture and the magnification (see below).
The diameter of the exit pupil can be calculated as follows:
For example, 7×42 binoculars have a 6mm exit pupil.
Because of this, the lower the magnification, the brighter the image becomes. If the magnification remains the same, a larger aperture will produce a brighter image.
If you want a bright image of a distant object using high magnification, you’ll need large objective lenses.
This is why there are so many different binocular configurations; so we can choose depending on our specific purpose.
How large of an exit pupil do you need?
When you compare binoculars with large and small exit pupils in bright conditions, there’s almost no difference. That’s because the pupils in your eyes narrow to around 2-3mm in daylight. In dark conditions, they can dilate to a maximum of about 7mm (see below).
So, if you only use binoculars in daylight, you don’t need a pair with large exit pupils; smaller ones will do fine. The following are rough ideas on how large an exit pupil you need for each purpose.
- For astronomy
It was once believed that 7×50 were best for stargazing because the exit pupil is around 7mm, which is the maximum dilation of the human pupil. However, this is not necessarily the case.
Recently, binoculars with a 4-5mm exit pupil have become more popular for astronomical use. As a result, 8×42 is often used for stargazing instead of 7×50.
If you’d like to learn more about how to choose binoculars for astronomy, please see the article: How To Choose Binoculars For Astronomy.
- For sports events and indoor concerts
A 3mm exit pupil is large enough for sports events and indoor concerts. You should consider the field of view for these circumstances. Players and musicians move around the field or stage quickly, so binoculars with lower magnifications are more useful.
The Hinode 5×21 is what I usually use for sports events and concerts as they’re compact with a wide field of view (11 degrees). They have a 5mm exit pupil, which is bright enough even in dim light.
Knowing a little about the exit pupil can help you choose binoculars for a given purpose.