Those binocular “numbers” tell you exactly how strong the view is and how bright it can be.
If you’ve ever stared at “10×42” or “20×70” and wondered what you’re really getting, you’re not alone. Binocular specs are compact, but they encode several practical trade-offs you’ll feel immediately: how shaky the image is, how easy it is to find targets, and how comfortable the view remains at dusk.
This guide decodes the core numbers first, then shows you how to use them to choose binoculars for stargazing, wildlife, travel, and “low light,” without getting lost in marketing terms.
Binocular numbers like 7×35 or 10×50 mean two things: magnification and objective lens size. The first number (7x, 10x, 20x) is how many times closer objects appear. The second number (35, 50, 70) is the front lens diameter in millimeters, which strongly affects brightness, size, and steadiness requirements.
The basic format: magnification × objective diameter
Most binocular labels follow the same pattern: Magnification × Objective lens diameter (mm). The objective lenses are the big front lenses; their diameter is a major driver of light-gathering and overall bulk.
- 7×35: objects appear 7 times closer; 35 mm objectives.
- 10×50: objects appear 10 times closer; 50 mm objectives.
- 20×70: objects appear 20 times closer; 70 mm objectives.
In practice, the first number changes how large details look and how difficult it is to hold the view steady. The second number changes how bright the image can be (especially at dusk or under the stars), plus the binocular’s weight and the size of the prisms and housing needed to support those big lenses.
One important caveat: these numbers don’t tell you optical quality. Coatings, prism type, collimation (alignment), and internal baffling can make a “10×50” look spectacular – or disappointing – regardless of the headline specs.
What magnification really changes: detail, shake, and ease of finding targets
Magnification is tempting: bigger number, closer view. But higher magnification also narrows your usable “window” and makes every hand tremor more obvious.
- 7× to 8×: easiest to hold steady; fast target acquisition; great for scanning wide areas and casual stargazing.
- 10×: a common “do-it-all” level; noticeably more detail, still hand-holdable for many people.
- 12×: detail jumps again, but shake becomes a real limitation for long viewing sessions.
- 15× to 20×: often best with a monopod/tripod or at least braced elbows; excellent for distant terrestrial targets and selective sky objects.
For context, a common 12×50 model like the occer 12×50 is theoretically a strong “reach” option for wildlife or long-range viewing, but you’ll typically get more real-world detail if you can steady it (lean on a car roof, use a trekking pole/monopod, or sit with elbows anchored).
At 20×, like the Aurosports 20×70, the magnification is high enough that support is usually the difference between “wow” and “wobbly.” If you plan handheld-only use, consider whether 10×-12× is a better match for your tolerance for shake.
What objective diameter changes: brightness, size, and “low light” reality
Objective diameter (the second number) is measured in millimeters and is often discussed as “light gathering.” Bigger objectives can deliver a brighter image, but only if the rest of the optical system is efficient and your eye can use that light.
- 35-42 mm: lighter and easier to carry; good daytime brightness; often ideal for travel and hiking.
- 50 mm: classic step up for dusk, dense woods, and stargazing; heavier but still manageable.
- 70 mm: noticeably bulkier; excellent potential brightness for astronomy and low-light scanning when properly supported.
Marketing phrases like “low light vision” are usually shorthand for a combination of objective size, coatings, and exit pupil (covered next). Binoculars do not magically “see in the dark” – they only deliver whatever light is available more efficiently to your eye.
Also note the practical trade: big objectives usually mean bigger prisms, thicker barrels, and more weight. If the binocular is too heavy to hold steady, you can lose the very detail and brightness you thought you were buying.
Exit pupil: the most useful brightness number you can calculate
Exit pupil is a simple calculation that predicts how bright and “easy” a binocular can feel, especially in dim conditions. It’s the diameter of the light beam leaving the eyepiece.
- Exit pupil = objective diameter ÷ magnification
- 7×35: 35 ÷ 7 = 5.0 mm
- 10×50: 50 ÷ 10 = 5.0 mm
- 12×50: 50 ÷ 12 = 4.2 mm
- 20×70: 70 ÷ 20 = 3.5 mm
As a rule of thumb, a larger exit pupil tends to look brighter at dusk and feels more forgiving of small positioning errors (your eye doesn’t have to be centered perfectly). But there’s a limit: your own pupil size constrains what you can use. In bright daylight, your pupil may be ~2-3 mm, so extra exit pupil won’t look “brighter,” though it can still feel more comfortable.
Notice the trade-off in the examples: a 20×70 can have huge objectives yet a smaller exit pupil than a 7×35. That doesn’t mean 20×70 is “dim” – it means the high magnification spreads the available light over a larger image, and eye placement may feel less forgiving.
Field of view, eye relief, and close focus: the numbers that affect comfort
The headline “7×35” style numbers are only the start. A binocular that’s easy to use often beats a higher-spec one that’s frustrating.
- Field of view (FOV): listed as degrees (e.g., 6.5°) or feet/meters at a set distance (e.g., 342 ft at 1000 yd). Wider FOV helps you find birds, boats, and constellations faster.
- Eye relief: the distance your eye can be from the eyepiece and still see the full image. If you wear glasses, you typically want ~15 mm or more (rule of thumb; fit varies).
- Close focus: how near you can focus (often in feet or meters). Important for butterflies, feeders, and trail use.
- Interpupillary distance (IPD): the hinge adjustment that matches the barrels to your eyes. If you can’t set IPD correctly, the view will never feel right.
When you compare something like a 12×50 versus a 20×70, don’t just compare magnification. Check FOV and eye relief: higher magnification models often have a narrower FOV, which makes tracking moving wildlife and hand-held scanning harder.
Putting it together: choosing 7×35 vs 10×50 vs 20×70 for your use
Use the numbers as a fast “fit check” for your main activity, then refine with comfort specs and optical quality.
- General stargazing and casual scanning: 7×35 or 10×50 are classic because they balance brightness and steadiness. A 5 mm exit pupil (like 7×35 and 10×50) is a comfortable target for many people under dark skies.
- All-around nature + some astronomy: 10×50 is often the sweet spot if you want one binocular that can do a lot without demanding a tripod.
- Long-range daylight detail (shorelines, ridgelines): 12×50 can work well if you brace or use a monopod. The occer 12×50 is in this “high power but still portable” category; just be realistic about hand shake.
- Dedicated “big view” astronomy or distant spotting with support: 20×70 can be impressive when stabilized, but it’s less forgiving handheld and typically narrower in view. The Aurosports 20×70 fits the high-magnification, large-objective idea – plan on a tripod adapter or steady resting point to actually benefit from 20×.
If you’re torn between “more magnification” and “more steadiness,” steadiness usually wins for real detail. A slightly lower magnification you can hold stable often reveals more than a higher magnification that jitters.
If you can’t read small text or pick out fine branches at your intended distance while standing and breathing normally, drop magnification or plan on support. The “right” power is the one you can actually hold on target for 30-60 seconds.
Frequently Asked Questions
What does 10×42 mean on binoculars?
It means 10× magnification with 42 mm objective lenses. The 10× affects detail and shake, while 42 mm affects potential brightness and size.
Are higher numbers always better on binoculars?
No. Higher magnification can show more detail, but it also increases hand shake and usually narrows field of view. Bigger objectives can help brightness, but add weight and don’t guarantee better optics.
What do the numbers like 8×42 or 10×50 have to do with brightness?
They determine exit pupil (objective ÷ magnification), which strongly influences perceived brightness and viewing comfort in dim light. Optical coatings and overall transmission also matter a lot.
What is exit pupil, and what number should I look for?
Exit pupil is objective diameter divided by magnification. As a rule of thumb, ~4-5 mm is a comfortable target for dusk and stargazing; smaller can look dimmer and be less forgiving of eye placement.
Is 20×70 good for stargazing?
It can be excellent for certain sky objects because it brings them closer and can be bright, but most people need a tripod or solid bracing at 20×. Handheld use is often frustrating at that power.
What’s better for low light: 12×50 or 20×70?
It depends on exit pupil and stability. A 12×50 has a larger exit pupil (about 4.2 mm) than a 20×70 (about 3.5 mm), and it’s usually easier to hold steady, which can make it effectively more usable in low light.
Do binocular numbers tell me how clear the image will be?
Not by themselves. Clarity depends heavily on optical quality factors like coatings, prism quality, alignment, and internal glare control.
Binocular numbers are a practical shorthand: the first number tells you how close the view appears, and the second tells you how much lens diameter you have to work with. Once you add exit pupil, field of view, and eye relief, you can predict whether a binocular will feel bright, steady, and comfortable for your specific stargazing or daytime use.
Okay super basic question: what do the numbers on binoculars mean? Like is 20×70 automatically “better” than 12×50 for everything, or is it just more zoom and you lose something?
Honestly surprised there’s no Nikon or Vortex mentioned. I’m not saying these are bad, but I’m curious how you decided on just these two—was it based on testing, reviews, or just what’s popular right now?
Do the occer 12×50 pair okay with a cheap tripod adapter? I already have a basic photo tripod and I’m wondering if it’ll actually help, or if these are mostly meant to be handheld. Also, how do binoculars work with glasses on?
A tripod can definitely help with 12x, especially for stargazing or reading distant signs. The catch is the occer 12×50 needs to have a front hinge/center area that accepts a standard binocular tripod adapter (the little L-shaped one). If it has the small threaded socket under a cap on the front of the hinge, you’re good—then your photo tripod will work fine.
With glasses, you’ll want the eyecups fully twisted down and enough eye relief so you can see the full circle without vignetting. If you’re seeing dark crescents on the edges, pull the bins slightly away from your glasses or re-adjust the eyecups.