What the numbers on binoculars mean (8x42, 10x50)

Every pair of binoculars carries two numbers joined by an x: 8x42, 10x50, 12x50. The short version is simple. The first number is the magnification, the second is the width of each front lens in millimetres. Divide the second by the first and you get the exit pupil, the single most useful figure for judging how bright the view will be in poor light.

The rest of the spec sheet, from field of view to eye relief, close focus and prism glass, tells you how the binoculars will feel in use. This guide goes through each figure in the order you will meet it.

The first number: magnification

An 8x pair makes a bird 80 metres away look as if it were 10 metres away. A 10x pair makes the same bird look 8 metres away, and a 12x pair about 6.7 metres away.

More magnification shows more detail, but it has three costs:

• A narrower view. The higher the magnification, the smaller the patch of landscape you see, which makes moving subjects harder to find and follow.
• More visible shake. Binoculars magnify the movement of your hands by the same factor as the image. Most people find 8x and 10x easy to hold, 12x tiring for long looks without support.
• A smaller exit pupil for the same lens size, so a dimmer view as the light fades.

That is why the most popular sizes for birdwatching and walking sit at 8x and 10x. Our guide Best binocular magnification for bird watching compares them in the field.

The second number: objective lens size

The objective lenses are the big lenses at the front. Their diameter decides how much light the binoculars collect. Light gathering grows with the area of the lens, so a 50 mm lens collects about 42% more light than a 42 mm lens (50 x 50 = 2,500 against 42 x 42 = 1,764).

Bigger lenses mean bigger and heavier binoculars. Roughly:

• 20 to 30 mm: compact binoculars that fit a pocket. See compact binoculars.
• 32 mm: mid-size, a common lightweight choice for birdwatching.
• 40 to 45 mm: full size, the most popular all-round size.
• 50 mm and up: for low light, long views and the night sky.

Exit pupil: the brightness figure

Divide the objective size by the magnification and you get the exit pupil in millimetres. It is the diameter of the beam of light that leaves each eyepiece. Hold your binoculars at arm's length, point them at a bright sky and you can see it as a small bright disc floating in each eyepiece.

Why it matters: your own pupil is roughly 3 mm across in daylight and opens to roughly 7 mm in the dark. As long as the exit pupil is at least as big as your pupil, the view looks as bright as it can. In bright sunshine a 10x25 and a 10x50 look almost equally bright. At dusk, when your pupil has opened to 5 mm or more, the 10x50 shows much more. Studies of pupil size also show that the dark-adapted pupil tends to be smaller in older people, so a very large exit pupil brings less extra benefit later in life.

Field of view

Field of view tells you how wide a patch you see. It is given in three ways:

• Degrees, for example 5 degrees or 6.5 degrees. This is the real angle of the view.
• Metres at 1,000 metres, for example 88 m at 1,000 m.
• Feet at 1,000 yards, for example 263 ft at 1,000 yds.

To convert, 1 degree is about 17.5 m at 1,000 m, or about 52.4 ft at 1,000 yds. So 88 m at 1,000 m is 5 degrees, and 102 m at 1,000 m is about 5.8 degrees. Our 12x50 waterproof binoculars are listed at 88 m and 263 ft, which agree.

As a rule, field of view shrinks as magnification rises. For birdwatching, a wide field makes it easier to find a bird and keep it in view. Wide-field binoculars for the night sky, such as our 3x50 stargazing binoculars, go the other way, with a 17 degree field at only 3x.

Eye relief

Eye relief is the distance between the eyepiece and the point where your eye sees the whole field. If you wear glasses, your eye sits further back, so you need longer eye relief. Eye relief of more than about 16 mm is generally enough for glasses wearers, and a little more helps with thick frames. Twist-up or fold-down eyecups let you set the distance: up without glasses, down with them.

Close focus

Close focus is the nearest distance at which the binoculars can be focused. Depending on the design it ranges from about half a metre to 30 metres. For birds at the feeder, butterflies and dragonflies, a close focus of 2 to 3 metres is useful. The 12x50 focuses down to 2.5 m.

Prisms: roof or porro, BaK-4 or BK-7

Binoculars use prisms to turn the image the right way up and to fold the long light path into a short body.

• Porro prisms give the classic stepped shape, with the front lenses set wider apart than the eyepieces, as on our 10x50 porro binoculars.
• Roof prisms fit into straight barrels, which makes a slimmer body that is easier to seal against water. Good roof prisms need special phase-correction coatings; porro prisms do not.

The prism glass is often printed as BaK-4 or BK-7. BaK-4 has a refractive index of about 1.569, BK-7 about 1.517. The higher index means less light escapes at the edges of the prisms, so the exit pupil stays round. With BK-7 prisms in fast binoculars, the edge of the exit pupil often looks squared off, with grey segments. You can check this yourself with the arm's length test above.

Coatings

Every glass surface reflects some light. Anti-reflection coatings reduce that loss and the glare it causes. The terms on the box have specific meanings:

• Coated: at least one surface has a single anti-reflective layer.
• Fully coated: all air-to-glass surfaces have a single layer.
• Multi-coated: at least one surface has several layers.
• Fully multi-coated (FMC): all air-to-glass surfaces have several layers.

FMC is what to look for. Note that a coloured front lens on its own says nothing about quality.

Waterproofing and nitrogen

"Waterproof" on its own means little. Look for a rating such as IPX7, which under the IEC 60529 standard means the binoculars passed temporary immersion at 1 metre for 30 minutes. Binoculars that are nitrogen or argon purged have the air inside replaced with a dry gas, which stops the inside of the lenses from fogging up and protects against tarnish and fungus. Looking after binoculars in wet weather explains what this means in practice.

Numbers that do not add up

Some budget binoculars carry figures such as 30x60, 60x60 or even 90x90. Treat these with suspicion: experienced reviewers report that such labels can hide much lower real magnifications. Real binoculars above 12x to 15x are hard to hold steady and are usually sold for tripod use. Zoom binoculars exist, but they are a compromise, and the top of the zoom range is rarely sharp. If a spec sheet gives a magnification, an objective size and an exit pupil that do not match (objective divided by magnification), be careful.

A quick reference

• 8x42: the all-rounder for birds, walks and woodland. See our 8x42.
• 10x42: more reach for estuaries, reservoirs and hills. See our 10x42.
• 10x50: low light, coast and a first look at the stars. See our 10x50.
• 12x50: long views, best with something to lean on. See our 12x50.
• 10x25 and 8x21: small, light and best in daylight. See our compact binoculars.

If you want one figure to remember, make it the exit pupil. It tells you, in one number, how the binoculars will cope when the light goes.