Binocular chooser and spec decoder for birders
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Type the two numbers printed on any binocular (like 8x42) and see what they mean for birding, or answer three questions to get a sensible first spec. Nothing is sent anywhere; it all runs in your browser.
1. Decode the numbers
2. Get a first spec
How the numbers are worked out
Exit pupil is the objective diameter divided by magnification, in millimetres. It is the beam of light reaching your eye. About 4 to 6 mm suits general birding, including dusk and shady woods. A dark-adapted adult pupil opens roughly 5 to 7 mm, a little less with age, so a bigger exit pupil than that gives light your eye cannot use. Twilight factor is the square root of magnification times objective, a rough way to compare detail in low light. Relative brightness is the exit pupil squared, shown against 8x42. Field of view in feet at 1000 yards divides by 52.5 to give degrees, and the width you see at 100 metres is about 1.75 metres per degree. A wide field makes it far easier to find a bird that has just moved.
What the advice rests on
Cornell Lab of Ornithology and Audubon guidance, and many BirdForum discussions, converge on 7x to 10x with 32 to 42 mm lenses for birding; 8x42 is the usual first pair because it is bright, steady and easy to find birds in. Lenses over 42 mm get heavy and under 30 mm struggle in poor light. Glasses wearers should look for long true eye relief, about 15 mm or more, and twist-up eyecups. This tool applies those rules; it is not a lab test. See the buying guide for specific models.