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Lighting and electronics

Exit pupil

Also called exit pupil diameter

What is exit pupil on binoculars?

Exit pupil is the width of the beam of light a binocular sends to your eye, found by dividing the objective lens size by the magnification. An 8x25 has a 3.1mm exit pupil and an 8x32 has 4mm; the bigger number looks brighter in dim light.

Exit pupil at a glance

Formula
Objective (mm) divided by magnification
8x25
3.1mm
8x32
4mm
10x25
2.5mm

How do you calculate exit pupil?

Divide the second number on a binocular by the first. An 8x25 gives 25 divided by 8, or about 3.1mm. An 8x30 gives about 3.8mm, an 8x32 gives 4mm, and a 10x25 gives 2.5mm. Hold a binocular at arm's length toward the sky and you can see it: the small bright disc floating in each eyepiece is the exit pupil.

Why does exit pupil matter for brightness?

Your own pupil sets the ceiling. In bright daylight it shrinks to around 2 to 3mm, so any binocular with an exit pupil at least that wide looks equally bright and a 3.1mm pocket model gives up nothing. At dusk or under dense forest your pupil opens to 5mm or more, and a binocular that only delivers a 2.5mm or 3.1mm beam starves it. That's when a compact 8x30 or 8x32 looks clearly brighter than a pocket 8x25.

What exit pupil do hikers need?

For daylight hiking, anything from about 2.5mm up is fine, which is why pocket binoculars work so well on the trail. If you hike at dawn and dusk to see wildlife, 3.5 to 4mm is a real improvement, and OutdoorGearLab puts 5mm or more as the target for true low-light use. The catch is weight, since a bigger exit pupil at the same magnification needs bigger lenses. Glass and coatings affect brightness too, so two binoculars with the same exit pupil can still look different.

What this means when you're choosing

  • About 2.5mm or more for daylight hiking, which every pocket binocular clears
  • 3.5 to 4mm if you often hike at dawn, at dusk or under thick canopy
  • The weight that comes with it, since a wider exit pupil means bigger lenses

The common mistake

Paying for a big exit pupil you only use at noon

In daylight your own pupil is smaller than a pocket binocular's exit pupil, so the extra brightness of an 8x42 is invisible and the extra pound is not. Buy exit pupil for the light you actually hike in.

Common questions

Is a bigger exit pupil always better?

Only in dim light. In daylight your eye can't use more than about 3mm, so a larger exit pupil adds weight without adding brightness you can see.

What is the exit pupil of 10x25 binoculars?

2.5mm, which is 25 divided by 10. It's fine in daylight and one of the first to dim as the light goes.

Where this is defined

Where this comes up

Guides on Tom's Hiking where exit pupil changes which product is right for you.

More in lighting and electronics

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GPX file

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Eye relief

Eye relief is how far behind a binocular's eyepiece your eye can sit and still see the whole field of view, measured in millimeters. Glasses hold your eye farther back, so hikers who wear them need about 15mm or more to avoid a view ringed in black.

ED glass

ED glass, short for extra-low dispersion, is lens glass that bends red, green and blue light by nearly the same amount. In binoculars it cuts the purple and green fringes around dark edges against a bright sky, so the image looks sharper and truer in color.

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