Binocular finder

A worked example with 10x42

The chart holds 87 models in 10x42, more than in any other size. Choose only that size and all 87 are a match, so the list is long.

Add a second choice and it shrinks. Eyeglass wearers can ask for 16 mm of eye relief or more, the figure the RSPB gives as the minimum for glasses. Across the whole chart, 337 of 529 binoculars with a printed eye relief reach that mark.

If nothing fits, the finder says that no binocular in the chart matches all of your choices and asks you to loosen one. Drop the least important choice first and the list returns.

How the figures behind each match are worked out

Every figure comes from the makers' documents for each model. The finder only compares them with the choices you make; it does not guess at a number the maker did not print.

Exit pupil is the front lens width in millimeters divided by the magnification. A 10x42 works out to 4.2 mm. ZEISS says the exit pupil should be at least 4 mm for dawn use and that anything under 4 mm is unsuitable as light fades.

The twilight factor is the square root of the magnification times the front lens width. For 10x42 it is 20.5. The size also shows its own weight and view, which are medians across its 87 models: 336 ft of view and 24.9 oz.

Nikon says smaller lens apertures are lighter and more compact, while larger ones gather more light and add weight. The finder's size choice follows that tradeoff, so a bigger front lens usually brings a heavier model.

Sizes in the chart

The table lists seven of the 20 sizes in the chart with the figures the finder uses. Models counts the binoculars you can end up with when you pick only that size.

Audubon says most birders prefer 7x or 8x for brightness and a wide field, and it used 8x42 for its own testing. The 8x42 row shows why: 393 ft of median view and a 5.3 mm exit pupil.

What to do with the matches

Start with the size that suits your main use. The Royal Astronomical Society names 8x40 or 10x50 for starting out in astronomy. Practical Sailor calls 7x50 the traditional marine size, with a 7.1 mm exit pupil that suits low light.

Open a size page to see its models side by side, or the best binoculars compared list for the full field. The exit pupil calculator explains the brightness figures, and the eyeglass checker covers eye relief.

If you have two sizes in mind, the 8x42 against 10x42 comparison sets their view, brightness and weight next to each other. The 10x42 size page lists every model in that size, and the full chart holds all 554.