What do 8x42 and 10x42 mean on binoculars, and which should you buy?
Short answer
The first number is magnification, the second is the diameter of the front lens in millimetres. Divide the second by the first and you get the exit pupil in millimetres, which is what governs low-light performance — 5.3 mm for an 8x42 against 4.2 mm for a 10x42. For general use buy the 8x42: it is steadier in the hand, has a wider field of view, and most people see more detail through it despite the lower magnification.
Two numbers on a hinge, and almost everyone reads the first one as the important one. It is the second one, divided by the first, that decides whether you can see anything at dusk.
What do 8x42 and 10x42 mean?
The first number is magnification. The second is the diameter of the objective — the front lens — in millimetres.
An 8x42 magnifies eight times through 42 mm front lenses. Nikon gives the clearest illustration of what magnification means in practice: an object 800 metres away through 8x binoculars looks about the size of an object 100 metres away to the naked eye. ZEISS puts the same relation the other way round — at 8x, something 100 m away looks as it would from 12.5 m.
The objective diameter tells you how much light can enter. It does not, by itself, tell you how bright the image is.
Which should you buy?
The 8x42, for general use. Three reasons, and the third is the one people underrate.
Wider field of view. Field of view contracts as magnification rises. On the same model line — Vortex Viper HD — the 8x42 gives 409 feet at 1,000 yards and the 10x42 gives 341. That is a fifth of your view gone.
Bigger exit pupil, which we come to next: 5.3 mm versus 4.2 mm.
A steadier image, which means more detail, not less. This is the counterintuitive one, and ZEISS states it plainly in its own birding guidance: less magnification equals less shake, and most people agree that the more stable image through 8x lets them discern more detail than the slightly larger but shakier 10x image. Magnification magnifies your pulse too.
Buy the 10x when the subject is static and distant and you can brace or mount — glassing a hillside, marine watchkeeping, open-country hunting. Vortex’s own FAQ is refreshingly blunt about the general case: bigger numbers aren’t always better.
| 8x42 | 10x42 | |
|---|---|---|
| Exit pupil | 5.3 mm | 4.2 mm |
| Field of view (Viper HD, ft @ 1000 yd) | 409 | 341 |
| Eye relief (Viper HD) | 18.0 mm | 17.0 mm |
| Handheld steadiness | Better | Noticeably shakier |
| Best for | Everything, dusk, forest, walking | Static distant subjects, open country |
What is exit pupil, and why does it decide low-light performance?
Exit pupil is the diameter of the beam of light leaving the eyepiece. The formula is simple: objective diameter divided by magnification. 42 ÷ 8 = 5.3 mm. 42 ÷ 10 = 4.2 mm.
Why it matters is anatomy. Nikon notes that the human pupil normally opens about 2 mm in daylight and up to 7 mm in the dark.
- In bright daylight, your pupil is smaller than either exit pupil, so brightness will not differ between them at all. The 4.2 mm is not handicapped.
- At dusk, your pupil opens past 4.2 mm and the 10x42 becomes the limiting factor. The 8x42 keeps feeding light your eye can still use.
ZEISS adds two useful refinements. To take full advantage of a binocular’s light, your eye pupil must be at least as large as the exit pupil. And maximum pupil size falls with age — children can exceed 8 mm; in older eyes it seldom exceeds 4 mm. Which means the low-light argument for 8x is strongest for young eyes and weakest for the buyers most likely to be told they need a huge objective.
ZEISS’s rule of thumb for the low-light end: an exit pupil smaller than 4 mm is unsuitable in fading light from the start.
What about twilight factor?
Treat it with suspicion. It is the square root of magnification times objective diameter — 18.3 for an 8x42, 20.5 for a 10x42.
ZEISS, which publishes the figure on its own spec sheets, warns about it in the same breath: it is a mere mathematical parameter, and the genuinely important performance data — image quality, contrast, transmission, colour rendition — are not taken into consideration. Their illustration is memorable: a hypothetical 56x8 would score the same twilight factor as an 8x56 and be totally unusable, even in daylight.
Use exit pupil for the light question and read reviews for the rest.
What must eyeglass wearers check?
Eye relief, and the number to look for is 15 mm or more.
Eye relief is the distance from the eyepiece surface to the point where the exit pupil forms — where your eye has to be to see the full field. Nikon states that at 15 mm or longer, eyeglass wearers can get the full field with no vignetting without removing their glasses. Opticron gives the same threshold independently, which is reassuring for a figure that could easily have been one company’s marketing.
Check it per configuration, not per model. The Vortex Viper HD is 18.0 mm at 8x and 17.0 mm at 10x — both fine, but the pattern of shorter eye relief at higher magnification is common and some lines drop below the threshold at the top end.
What size objective do you actually need?
The 42 mm class exists because it is the compromise that works. Bigger objectives gather more light and weigh more; smaller ones are more portable and give up exit pupil.
A caution Leupold makes in its own myth-busting: a bigger objective is not automatically brighter. What matters is the exit pupil — the combination — not the front lens in isolation. A 10x50 and an 8x42 have almost the same exit pupil, and the 10x50 is heavier.
And do not recompute apparent field of view from published figures. Manufacturers use two incompatible conventions — Nikon uses the ISO 14132-1 formula while ZEISS uses simple multiplication — and the same binocular can be advertised with apparent fields about seven degrees apart depending on which is used. Compare real field of view, in feet at 1,000 yards or metres at 1,000 metres, and only between the same magnification.
What does this leave out?
Everything about how well the glass is made — which is the other half of the purchase and the half that explains why two 8x42s can differ by a factor of ten in price. Coatings, prism type, dispersion glass and manufacturing tolerance are covered in what actually makes binoculars expensive.
For marine use, add one more consideration: at sea, a rocking deck defeats magnification long before your hands do, which is why 7x50 remains the traditional marine specification and why Steiner still builds it. Bigger exit pupil, steadier image, and it does not matter that the subject is a mile away if you cannot hold it.
The short version: buy the 8x42, check the eye relief if you wear glasses, and spend the difference on better glass rather than more magnification.
Quick answers
- What do the two numbers on binoculars mean?
- The first number is magnification and the second is the diameter of the objective lens in millimetres. An 8x42 magnifies eight times through 42-millimetre front lenses. Magnification is the ratio of apparent size compared with the naked eye — Nikon gives the example that an object 800 metres away through 8x binoculars looks about the size of an object 100 metres away seen unaided.
- Which is better for general use, 8x42 or 10x42?
- For most people, 8x42. It gives a larger exit pupil for low light, a wider field of view, and a steadier image. ZEISS's own birding guidance states that most people agree the more stable image through 8x allows them to discern more detail than the slightly larger but shakier 10x image. Choose 10x when you are glassing static, distant subjects, especially with support.
- What is exit pupil and why does it matter?
- Exit pupil is the diameter of the beam of light leaving the eyepiece, calculated as objective diameter divided by magnification. An 8x42 gives 5.3 mm and a 10x42 gives 4.2 mm. It matters because the human pupil opens to roughly 2 mm in daylight and up to about 7 mm in darkness. In bright light the difference is invisible; at dusk, an exit pupil smaller than the pupil of your eye limits how much light actually reaches your retina.
- What eye relief do you need if you wear glasses?
- Around 15 millimetres or more. Nikon states that with eye relief of 15 mm or longer, eyeglass wearers can obtain a full field of view with no vignetting without removing their glasses, and Opticron gives the same threshold independently. Check the published eye relief figure before buying, because it varies within a single model line — the Vortex Viper HD is 18.0 mm at 8x and 17.0 mm at 10x.
- At what magnification do you need a tripod?
- There is no hard threshold, and manufacturers differ. Nikon recommends up to 12x for comfortable handheld viewing by general users. Leupold recommends 8x to 10x for free-hand glassing and says that once you reach 12x to 15x you will need a tripod. Treat 8x to 10x as the comfortable handheld range, 12x as the practical ceiling, and anything above that as wanting support.
Brands in this guide
- Vortex OpticsWisconsin, 2002 — a family bird-feeder shop that became an optics company, and a warranty so broad it turned into the product.
- ZEISSJena, 1846 — owned by a foundation rather than shareholders, and the binocular you buy comes from a division most people don't know exists.
- LeupoldOregon, 1907 — five generations, one factory, and a company that will tell you exactly which of its own products are made there.
- NikonJapan, 1917 — one of the great optical houses, still building excellent binoculars, and out of the riflescope business since around 2019.
- SteinerBayreuth, 1947 — a man building binoculars out of postwar rubble, now a Beretta company, and still the glass the bridge of a ship reaches for.
- Hawke OpticsSuffolk, England — a family optics house that grew out of a garage, sells serious glass at unserious prices, and does not tell you where it's made.