Binoculars, 8x42 or 10x42: How to Read the Numbers Before You Buy

What the two numbers on binoculars mean

In the dark, the pupil of a 20-year-old’s eye can open to more than 7 mm; by around 60, it may struggle to reach 5 mm [1]. That small detail of anatomy matters more in the choice between 8x42 and 10x42 binoculars than plenty of shiny sales pitches. Good news: two numbers and one division are all it takes to see why.

The first number is the magnificationHow many times closer the object looks than it does to the naked eye.: through 10x binoculars, a bird perched 330 feet (100 m) away looks as close as if it were 33 feet (10 m) away [2]. The second is the diameter of each objective lens, the big lens at the front, in millimeters. The bigger it is, the more light it lets in, and the brighter the image [2, 3].

That diameter also drives the weight: the size and heft of binoculars depend mostly on it, far more than on magnification, and the Swiss Ornithological Institute sees 30 to 50 mm as the sweet spot between brightness and weight [3]. It also warns that the numbers don’t tell the whole story: cheap 10x50s with poor glass and coatings can give a darker, less sharp image than good-quality 10x32s [3].

A third number often comes with the first two, on the body or on the spec sheet: the field of view, given as an angle or as a width at a set distance. We’ll get to it right after the one hiding in the division of the first two.

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The exit pupil, the number that decides things at dusk

Divide the diameter by the magnification and you get the exit pupilThe diameter, in millimeters, of the beam of light that leaves each eyepiece and enters your eye. [4]. 8x42s give 5.25 mm, 10x42s 4.2 mm. It tells you how much light the binoculars can deliver to your eye.

Your eye still has to take it in. Its pupil measures 2 to 3 mm in bright daylight and opens up to 7 mm in the dark [4]. In daylight, then, an exit pupil of 3 mm or more is enough to fill yours: 8x42s and 10x42s look every bit as bright as each other. The gap only shows up when the light fades and your pupil opens wider than the binoculars’ exit pupil. For dawn and dusk, Zeiss recommends an exit pupil as large as possible, and never smaller than 4 mm [5]; the LPO, France’s bird conservation league, also advises at least 4 mm [6].

The detail that changes everything is your age. The pupil opens less and less over the years: more than 8 mm in a child, rarely more than 4 mm in old age, according to Zeiss [5]. At 25, at dusk, the 5.25 mm of 8x42s make a real difference against the 4.2 mm of 10x42s. If your pupil barely opens past 5 mm anymore, as can happen after 60 [1], the gap narrows sharply; and it vanishes altogether once it no longer opens past 4.2 mm, because both pairs then give you all the light your eye can take.

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You can actually see this exit pupil, as in the photo above. Hold the binoculars about a foot (30 cm) away, with the objective lenses pointed at a bright light: a small bright disk appears in the center of each eyepiece [7]. On good binoculars, it’s perfectly round, with crisp edges [3].

Field of view, in feet at 1,000 yards or as an angle

The field of view is the width of landscape you can see without moving the binoculars. Makers usually give it in feet at 1,000 yards [9], sometimes in meters at 1,000 m, as in “150 m at 1,000 m” [2], or as an angle, like the 7.5° in the photo above. To turn an angle into feet at 1,000 yards, multiply it by 52.5 [9]; to turn meters at 1,000 m into an angle, divide them by 17 [1]. A pair with a 7° field thus covers about 367 feet at 1,000 yards (122 m at 1,000 m).

The wider the field, the easier it is to find a bird and follow it when it takes off [7, 9]. But magnification narrows it. Take two pairs with eyepieces of the same design and an apparent field of viewThe angle at which your eye sees the image in the eyepiece; it depends on how the eyepieces are designed. of 60°: according to the formula Nikon uses [7], 8x binoculars show about 433 feet at 1,000 yards, 10x binoculars about 346 feet (145 m and 115 m at 1,000 m). The field loses 20% of its width and more than a third of its area. You feel the difference most in the woods, as a kinglet flits from branch to branch, and far less on a lake where ducks doze in the middle of the water.

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If you wear glasses, check the eye relief

With glasses on, your eye sits farther from the eyepiece. If the image forms too close to the lens, all you see is a shrunken disk ringed with black. The number to look for is the eye reliefThe distance, in millimeters, between the eyepiece lens and the point where your eye sees the whole field of view.: Nikon recommends at least 15 mm to keep your glasses on without losing any of the field [7], and binoculars designed for birding generally offer 15 to 20 mm [9].

The eyecups do the rest: twisted down with glasses, up without [6]. Before you buy, make sure you can see the whole field through your own lenses [9]. Many eyeglass wearers can also take them off and let the focus correct their vision; with astigmatism, that’s not really an option [1].

Close focus, weight, and hand shake: the numbers for comfort

The close focus distanceThe shortest distance at which the binoculars give a sharp image. runs about 3 to 6.5 feet (1 to 2 m) on most binoculars [7]. It matters if you also enjoy butterflies, dragonflies, or the backyard bird feeder: the Swiss Ornithological Institute then advises a pair that focuses down to about 6.5 feet (2 m), and points out that 8x binoculars focus closer than 10x [3].

On weight, the National Audubon Society reckons that beyond 42 mm objectives, binoculars get too big and heavy for most people to carry all day; below 30 mm, they aren’t bright enough when the light fades [9]. Nikon puts the general limit for handheld use at around 50 mm [4].

Then there’s hand shake. Binoculars that magnify 10 times also magnify the small movements of your hands 10 times [9]. Nikon recommends 6x to 10x for general use and considers the image unstable above 12x [4]; for the RSPB, Britain’s bird conservation charity, anything stronger than 10x needs a tripod for a steady view [10]. Strong magnification also amplifies the heat shimmer over a field or a marsh [9]. Image-stabilized binoculars do exist, but nearly all of them run on batteries, and use a lot [3].

To steady the view, hold them closer to the objective end than in the middle, sit down with your arms tucked against your body, or brace against a fence or a wall [1].

Roof or Porro prisms, coatings, and waterproofing

In binoculars, prisms flip the image right side up; otherwise the objective lens would deliver it upside down [11]. Two families share the shelves, and their shape is enough to tell them apart. Porro prisms, invented in the mid-19th century, send light along a Z-shaped path: the eyepieces sit closer together than the objective lenses [1], and the body is wider, but these prisms lose no light and are easy to make [11]. Roof prisms keep a straight path: two parallel barrels, slimmer and more compact [11].

The roof prism makes two demands, which spec sheets turn into jargon. One of its faces doesn’t reflect all the light, so it gets a metal coating, aluminum or silver, or a multilayer “dielectric” coating that reflects more than 99%. Its roof edge also shifts the light slightly and costs some sharpness, even on a perfectly made prism: that’s what “phase-correction coating” fixes [12]. Today, most high-end binoculars use roof prisms, but both families make good binoculars [10]; for the night sky, where light is precious, the British Astronomical Association recommends Porro prisms instead [1].

Anti-reflective coatings cut light loss and reflections on every glass surface [2]. On the spec sheet, look for “fully coated” optics rather than merely “coated” ones, which may only be coated on their outer surfaces [1].

Finally, waterproofing. At Nikon, waterproof binoculars have airtight barrels filled with nitrogen, which keeps fog and mold from forming inside; that doesn’t mean they can go underwater [7]. Rubber armor protects against bumps, not water [3].

Birds, mountains, night sky: which pair of numbers to pick

No pair of binoculars is perfect for everything, only trade-offs worth making. Here are the ones the experts quoted here recommend, use by use.

For birding, the RSPB sums up the dilemma: in open country or across big stretches of water, 10x gives you extra detail; in the woods, at close range, or when the light is poorer, at dawn as at dusk, you’re better off with 8x [10]. The LPO recommends staying between 7x and 10x, with 42 mm objectives [6], and many birders go for 10x, notes the Swiss Ornithological Institute [3]. For backyard bird counts, a few yards from the window, field of view and close focus matter more than magnification.

In the mountains, every ounce gets carried for hours. Binoculars of 30 to 35 mm lose a little detail when the light fades, but make up for it in size and weight [9]. To pick out bighorn sheep on an open slope, 10x makes a good case; for a pair that never leaves your backpack, 8x is more forgiving of hands still shaky from the climb.

For the night sky, Nikon advises 40 to 50 mm, an exit pupil of 5 mm or more, never less than 3 mm, and a magnification of 7x to 10x held firmly; beyond that, or above 50 mm, a tripod [13]. A former section director of the British Astronomical Association says he uses his 10x50s more than any of his other binoculars: light and compact, they show a wealth of objects under a dark sky [1]. One practical catch: a camera tripod quickly becomes useless near the zenith, where a reclining lawn chair works better [1]. For meteors, though, leave them in the case: they can show up anywhere, and the naked eye, which takes in the whole sky, catches more of them (our meteor shower calendar).

Finally, give your eyes 10 to 30 minutes to adjust to the dark, and cover your flashlight with red to read your star chart without starting over [13]: a headlamp with a red light does the job. These are the same habits as for watching for the northern lights.

That leaves the simplest and safest step: try before you buy, comparing several models [3], ideally in the light you plan to use them in [10]. A late winter afternoon at an optics counter will tell you more than any spec sheet. And then one morning, by a pond still gray with dawn, an egret will land right in the middle of the circle, and you’ll know you picked the right numbers.

Worth watching

The RSPB, Britain’s leading bird conservation charity, explains what the numbers on binoculars mean, then lens sizes, coatings, and the features to look for in a birding pair.

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Nolwenn Bastide

Trail running, snow, water, nature and destinations

Nolwenn covers trail running, snow, water, nature and destinations for Outfarer. Curious about everything, from meteor showers to night-train schedules, she has a knack for digging up the one detail that changes everything, and nailing it in a single sentence.

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