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Binocular Numbers Explained

Every binocular has two numbers printed on it, such as 8x42 or 10x42. Those numbers tell you most of what you need to know about how the binocular will behave in the field. Here is what they mean in plain English, and how they affect hunting in low light.

The First Number: Magnification

The number before the "x" is magnification. An 8x binocular makes things look eight times closer than your naked eye. A 10x makes them look ten times closer.

More magnification shows more detail at distance, which helps when you are trying to count tines or pick out a bedded animal across a canyon. But it comes with tradeoffs:

• Shake. Higher power magnifies hand movement too. Many hunters find 10x harder to hold steady freehand.
• Field of view. Higher power usually means a narrower view, so finding and following moving game is harder.
• Brightness. With the same objective lens size, higher power gives a smaller exit pupil, covered below.

The Second Number: Objective Lens Size

The number after the "x" is the diameter of each front (objective) lens in millimeters. A 42 means each lens is 42mm across. Bigger lenses gather more light, but they also add size and weight. 42mm is a popular hunting size because it balances light gathering with a binocular you can carry all day.

Exit Pupil: The Brightness Number

Exit pupil is the small circle of light you see in each eyepiece when you hold the binocular at arm's length. You can calculate it easily:

Exit pupil = objective lens diameter divided by magnification.

• 8x42: 42 divided by 8 = 5.25mm
• 10x42: 42 divided by 10 = 4.2mm

Your own pupil opens wider as light fades. In bright daylight it may be only 2 to 3mm, so both binoculars deliver more light than your eye can use. At dawn and dusk, your pupil can open to roughly 5 to 7mm, depending largely on age. Younger eyes tend to open wider, and the maximum shrinks as we get older.

When your pupil is larger than the binocular's exit pupil, the binocular becomes the limit. That is why an 8x42, with its 5.25mm exit pupil, often looks a little brighter than a 10x42 in the last minutes of legal light.

8x42 vs 10x42 in the Field

Choose 8x42 if you:
- Hunt timber, brush, or close cover where shots and sightings are near.
- Want a wider field of view to follow moving animals.
- Glass a lot at first and last light.
- Want the steadiest freehand view.

Choose 10x42 if you:
- Hunt open country where you look at animals far away.
- Plan to glass from a tripod, which removes most shake.
- Value detail over the widest view.

Neither is wrong. Many whitetail hunters lean 8x, and many western hunters lean 10x. For a side by side look, see our 8x42 vs 10x42 comparison. Hunting deer? See what magnification for whitetail.

Other Things That Affect Low Light

Exit pupil is only part of the story. Lens coatings, glass quality, and prism type all affect how much light reaches your eye. A well made binocular with a smaller exit pupil can look better than a poorly made one with a larger exit pupil. Some makers list a "twilight factor," which is the square root of magnification times objective size. It hints at detail in dim light, but it does not measure brightness, so treat it as a rough guide only.

Eye relief also matters if you wear glasses. Look for enough eye relief, and twist up eyecups, so you can see the full field without black edges.

Quick Reference

When you are ready to pick a pair, start with our hunting binoculars guide, or browse by budget with under 500 options.

FAQ

Is 8x42 brighter than 10x42?
In daylight they look about the same to most eyes. In low light, the 8x42's larger 5.25mm exit pupil can look slightly brighter than the 10x42's 4.2mm, if your pupils open wide enough to use it.

Does a bigger objective always mean a better binocular?
No. Bigger lenses gather more light but add weight. Glass and coating quality matter as much as size.

Can I hold 10x steady?
Many people can with good technique, such as bracing elbows on knees. A tripod makes 10x far steadier.

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