Binocular Calculator: Exit Pupil, Brightness and Field
The two numbers printed on a binocular, such as 10x50, tell you almost everything that matters for stargazing, once you work through what they mean. Enter one pair, or two to compare them side by side, and this works out the exit pupil, twilight factor, how much more light reaches your eye than without them, the real patch of sky you see, and whether the magnification is comfortable to hold by hand. The point most buyers miss: a 7mm exit pupil is wasted on an eye that only opens to 5mm, so for many adults over 50, a 10x50 shows the night sky just as brightly as a 7x50 and magnifies more.
Your binoculars
What each number means
Exit pupil is the width of the beam of light leaving each eyepiece. Hold the binoculars at arm's length towards a bright sky and you can see it as a small bright disc. If it is wider than your own pupil, some of the light you paid for never enters your eye.
Twilight factor is a manufacturers' figure for how much detail you can make out in low light. It rewards magnification as well as aperture, so a 10x50 (22.4) beats a 7x50 (18.7), even though the 7x50 has the bigger exit pupil.
Relative brightness is simply the exit pupil squared. It is useful for comparing extended objects such as nebulae, but only up to the size of your own pupil.
True field is the patch of sky you see, in degrees. The full Moon is about half a degree across, so a 6.5 degree field fits roughly thirteen Moons side by side. Apparent field is how wide that view looks to your eye; above about 60 degrees, binoculars are usually sold as wide angle.
Choosing from the results
For a first pair to use by hand, 7x50, 8x42 and 10x50 are the long-standing favourites, and the calculator shows why: they balance light, field and steadiness. If you are deciding between the two most common astronomy sizes, our 10x50 vs 15x70 comparison goes through it in detail, and anything at 15x or above really wants a tripod and adapter. For our picks, see the best binoculars for stargazing in the UK, and for what to point them at, what you can see with binoculars.
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