What do the numbers on binoculars mean?

BBC Sky at Night Magazine's Scope Doctor solves your practical astronomy ailments.

Opticron Adventurer II WP 10x42 binoculars

What do the numbers on binoculars mean?

Binoculars are excellent for observing the night sky but it is important to buy a pair that you can handle while maintaining a good grasp. High magnification instruments are more difficult to hold steady.

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Binoculars are always marked with two numbers, for example 10×50. This means that the magnification is 10x and the aperture is 50mm.

There is often additional information describing the field of view, for example 80m at 1,000m indicates you’ll be able to see an area 80m across 1,000m away.

You can convert the field width to degrees, a more useful measurement for astronomy, by copy-pasting the following formula into a cell on a spreadsheet:

=DEGREES(ATAN(Field width in metres/2000))*2

Read our list of the top 10 best binoculars for astronomy beginners.

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Email Steve your astronomy queries to scopedoctor@skyatnightmagazine.com.