Binocular listings quote a lot of numbers and only three of them predict how a pair will behave on the night sky. This page is those three, and how to read a specification sheet with them in mind.
Exit pupil, which decides brightness
Divide the objective diameter by the magnification. Ten by fifty gives five millimetres, which suits most adult dark adapted eyes. Anything under about three is dim at night whatever the aperture, and anything over about seven wastes light on an older eye. This single division sorts binoculars for astronomy faster than any other check.
True field of view, which decides what fits
Printed in degrees or as feet at a thousand yards. Six degrees is typical for a 10x50 and frames the Pleiades comfortably. Wide angle designs reach seven or eight degrees and are genuinely better for sweeping, which is most of what binocular astronomy is.
Coatings and prism glass
Fully multicoated, meaning every air to glass surface is coated, rather than merely coated or multicoated. BAK4 prisms rather than BK7, which shows as a round rather than squared off exit pupil when you hold the binoculars at arm's length. Both are cheap markers of a serious pair and both are usually printed.
What not to shop by
Magnification, rubber armour, colour, and the number of features listed. High magnification handheld is a liability rather than an asset, and none of the rest changes what reaches your eye on a dark night.
Questions people ask about binoculars
What are the best binoculars for star gazing?
A fully multicoated 10x50 with BAK4 prisms and a six degree field, or a 7x50 for younger eyes and steadier viewing.
What does exit pupil mean?
The width of the light cone leaving the eyepiece, found by dividing aperture by magnification. It predicts how bright the view will be to a dark adapted eye.
Is a bigger binocular always better for astronomy?
Only up to what you can hold or mount. Above 12x or 60mm a tripod becomes necessary, and unmounted large binoculars show less than mounted small ones.