Binocular size chart: weight, view and brightness by size

What the two numbers mean

The National Audubon Society says the first number is magnification and the second is the objective lens diameter in millimeters, so 8x42 magnifies eight times with 42 mm lenses.

Nikon adds that larger objectives gather more light but add size and weight. It groups objectives under 25 mm as compact and 30 to 49 mm as standard.

Audubon says objectives smaller than 30 millimeters are lightweight and easy to carry. The RSPB describes 25 mm models as small and made for daytime use. Among the 25 mm sizes, 10x25 has a median weight of 11.1 oz and 8x25 has 10.9 oz.

Exit pupil and twilight factor by size

Nikon defines exit pupil as the objective diameter divided by magnification. That works out to 5.3 mm for 8x42 and 4.2 mm for 10x42, and 7.1 mm for 7x50.

Nikon puts the eye's pupil at about 2 mm in daylight and 7 mm in the dark, so any exit pupil above about 2 mm looks bright by day. In the dark, 8x56 at 7 mm fills a young eye's pupil. ZEISS notes children's pupils can open past 8 mm but seldom exceed 4 mm in old age.

Celestron prints twilight factor as the square root of objective size times magnification. For 8x42 it is 18.3 and for 10x42 it is 20.5. The 10x50 reaches 22.4 and the 15x56 reaches 29. ZEISS calls the figure a limited guide and points to exit pupil instead.

Weight and field of view by size

Field of view and weight move in opposite directions across the sizes. The median 8x32 shows 408 ft at 18.5 oz, while the median 8x42 shows 393 ft at 24.2 oz.

At 10x power the view narrows. The median 10x42 shows 336 ft at 24.9 oz, and the median 10x50 shows 315 ft at 29.3 oz. Moving to 12x50 gives 277 ft at 29.2 oz.

The 56 mm sizes are the heaviest. The 8x56 has a median weight of 40.9 oz, 15x56 has 45 oz and 18x56 has 44.8 oz. Their median views are 375 ft, 234 ft and 194 ft. Compare any two in the binocular size comparison tool or read the magnification chart.

Picking a size you can hold steady

ZEISS says free-hand binoculars typically have 7 to 10x magnification. Audubon explains why: binoculars that magnify an image eight times also magnify the small movements of your hand eight times.

Celestron says a tripod is not required at 8x to 12x but becomes necessary at 15x and higher. The 15x56 size page and when binoculars need a tripod cover that step.

For a first pair, 8x42 and 10x42 hold the most models, 75 and 87. The 8x42 vs 10x42 comparison sets them side by side. For a full model list, see best binoculars compared; for pocket sizes, see compact binoculars.