What does 8x42 mean on binoculars?

The two numbers on every pair of binoculars are magnification and lens size. In "8x42", the 8 means objects look eight times closer, and the 42 means the front lenses are 42 millimetres across. Everything else you read in a review (brightness, steadiness, field of view) follows from those two figures.

This page explains the numbers in plain language. It is general optics, not a test of any product. To try your own pair of numbers, use the free binocular spec decoder.

The first number: magnification

An 8x pair makes a bird 100 metres away look as if it were about 12.5 metres away (100 ÷ 8). A 10x pair makes it look 10 metres away. More magnification sounds better, but it costs you three things at once: the view gets narrower, hand shake is magnified as much as the bird, and the image gets dimmer unless the front lenses grow too.

The second number: the front lens in millimetres

The objective lens is the big one at the far end. A bigger lens gathers more light, which helps at dawn, dusk and under tree cover. It also makes the binoculars heavier and bulkier. Typical sizes you will see are 25 (pocket), 32 (compact), 42 (full size) and 50 (large and heavy).

What you can work out from the two numbers

The exit pupil is the small disc of light that leaves the eyepiece. Your own pupil is roughly 2 to 3 mm in bright daylight and opens to about 5 to 7 mm in the dark, a little less as you get older. When the exit pupil is at least as big as your pupil, the binoculars pass on all the light they collect, which is why 5 mm or more is the usual advice for dim conditions. A larger exit pupil also makes it easier to keep your eye in the right spot.

The twilight factor is a rough index of how much detail you resolve in poor light. It is only a comparison number, so use it to compare two pairs, not as a measurement of anything real.

The number that is not in the name: field of view

Field of view is how wide a patch of the scene you see, usually printed as feet at 1,000 yards (for example 393 ft) or metres at 1,000 metres. Divide feet by 52.5 to get degrees. A wide field makes it easier to find a moving bird and to follow it. For the same lens quality, a lower magnification gives a wider field, which is one reason 8x is the common advice for beginners.

Which pair of numbers suits what you watch

• Garden and feeder birds, woods, general use: 8x42 is the usual starting point. It is bright, wide and steady enough to hold by hand. Our 8x42 versus 10x42 comparison goes through the choice.
• Open country, shore and water: 10x42 brings distant birds closer, if you can hold it steady and accept a narrower view.
• Light to carry, mostly daytime: 8x32 trades some brightness for weight, and its 4 mm exit pupil is smaller in the evening.
• Skip: "zoom" binoculars such as 8-24x50, 12x and above without a tripod, and very cheap toy pairs. Their optics are usually poor.

Other letters you may see on the barrel

• Roof or porro: two prism layouts. Roof prism pairs have straight barrels and are slimmer; porro prism pairs have the offset "shoulder" shape and often give more for the money.
• Waterproof and fogproof: sealed and filled with dry gas, so they cope with rain and do not fog inside when the temperature changes.
• Multi-coated or fully multi-coated: thin layers on the glass that cut reflections and make the image brighter. Fully multi-coated is the one to look for.
• Eye relief: how far behind the eyepiece your eye can sit and still see the full view. Glasses wearers need roughly 15 mm or more. See binoculars for glasses wearers.

What to do next

Pick the habitat that matches your birding, choose 8x or 10x, then use the decoder to check the exit pupil and field of view of any model you are considering. When your pair arrives, set it up for your eyes so the two barrels and the focus match. If you want specific models to compare, the beginner buying guide lists six starting points and explains how they were chosen.