What do the numbers on binoculars mean

A worked example with 8x42

The chart holds 75 binoculars sized 8x42, with a median view of 393 ft and a median weight of 24.2 oz. The decoder reads 8x42 as 8x magnification and 42 mm front lenses, so a subject looks 8 times closer than with the bare eye.

The exit pupil works out to 5.3 mm, which is 42 divided by 8. That is at or above the 4 mm ZEISS names as the minimum for dawn. Nikon puts the eye's pupil at about 7 mm in the dark and about 2 mm in daylight, so a 5.3 mm exit pupil passes plenty of light by day.

A 10x42 changes one number. Its exit pupil is 4.2 mm and its twilight factor is 20.5, against 18.3 for the 8x42. The chart holds 87 models of that size, with a median view of 336 ft and a median weight of 24.9 oz.

How each figure is worked out

National Audubon Society reads the first number as magnification and the second as the front lens diameter in millimeters. Nikon adds that this is the inside diameter of the lens frame, and that larger lenses gather more light but add size and weight.

Exit pupil is the front lens diameter divided by the magnification. Nikon, ZEISS, Swarovski Optik and the RSPB all give that formula. Nikon's own examples are 42 divided by 8 for 5.3 mm and 50 divided by 7 for 7.1 mm.

The twilight factor is the square root of the magnification times the front lens diameter. Relative brightness is the exit pupil squared. Both are worked out from the size alone, so they describe the glass in general and not one model.

Hand-holding rests on a simple link. Audubon notes that binoculars magnifying 8x also magnify the small movements of your hand 8 times. The British Astronomical Association gives 10x as the hand-held limit, and Celestron calls a tripod necessary from 15x.

Reference figures for common sizes

Each row comes from the chart's own records. Exit pupil and twilight factor follow from the size, while view and weight are medians across the models of that size.

Eyes change the reading of the exit pupil column. ZEISS says pupils can open to more than 8 mm in children but seldom exceed 4 mm in old age. A 7x50 with a 7.1 mm exit pupil therefore sends more light than an older eye can take in.

What to do with the result

Read the exit pupil first. At 2.5 mm, the 10x25 sits close to the 2 mm Nikon gives for the eye's pupil in daylight, so it is a size for bright conditions. The RSPB describes 25 mm lenses as light, small models for daytime use, and the chart's 10x25 median weight is 11.1 oz.

Then weigh the front lens against what you carry. Audubon says objectives under 30 mm are light and easy to carry but not bright enough for detail in poor light. At 45 oz, the chart's 15x56 median is heavy, and it is a size where a tripod belongs.

Finally, look at eye relief if you wear glasses. The RSPB advises 16 mm, and 337 of 529 binoculars in the chart reach it. The eyeglass check tests one model, and binoculars for glasses wearers lists the long-relief picks.

To go on, open the 8x42 models or the finder to filter all 554 models by size and use. Best binoculars compared puts the whole chart in one list, and the exit pupil calculator works one size in more detail.