Exit Pupil
Also called: bright circle, exit pupil size
The small circle of light that leaves the eyepiece. Divide the front lens size by the power to find it.
- Type
- Specs & Terms
- Formula
- Exit pupil (mm) = objective size ÷ magnification
- 8x42
- 42 ÷ 8 = 5.25 mm
- 10x42
- 42 ÷ 10 = 4.2 mm
- Human pupil
- About 2 to 3 mm in bright light and up to 5 to 7 mm in the dark, shrinking with age
- Used for
- Hunting, Stargazing, Birding & wildlife, Target shooting
What it is good for
- Judging how bright a view will look in low light.
- Comparing 8x42 with 10x42.
- Comparing compact binoculars with full-size.
Where it falls short
- A bigger exit pupil than your own eye wastes light.
- It is only one factor; coatings and glass quality matter.
- Daytime use does not need a large exit pupil.
Tips
- For dawn and dusk, aim for an exit pupil of about 5 mm or more.
Works well with
- Magnification: the divisor.
- Objective Lens Size: the number divided.
- Compact Binoculars: small exit pupils.
Field notes
If you hold binoculars at arm's length and look at an eyepiece, you can see the exit pupil as a small bright disc.
Related guides
Specs here are typical, not guarantees; models differ, so check the maker's published numbers. Never look at the Sun through any optic without a certified solar filter, and zero or mount a scope with the firearm unloaded and pointed in a safe direction. Rules on optics, such as scopes on certain hunts or lasers and electronics, differ by state; our state resources directory links to each state's official regulations. Want a buying guide? Read Choosing Hunting Optics.