What Do the Numbers on Binoculars Mean? (10x50 Explained)
The two numbers on a pair of binoculars — like 10x50 — tell you the magnification and the objective lens size. The first number (10x) is how many times closer objects appear, and the second number (50) is the diameter of the front lenses in millimeters, which controls how much light the binoculars gather. So 10x50 binoculars magnify 10 times and have 50mm front lenses for bright, clear views.
Those little numbers stamped on every pair of binoculars aren't marketing fluff — they're the two most important specs you can know. Once you understand them, you can walk into any listing and immediately picture what you'll see and how bright it'll be. Let's decode them.
What do the two numbers on binoculars mean?
Every pair of binoculars is described by two numbers separated by an "x," such as 8x42, 10x50, or 15x70. They always mean the same thing, in the same order.
The first number: magnification
The first number is the magnification, or "power." A 10x binocular makes objects appear ten times larger — or ten times closer — than they look with your naked eye. A bird 100 yards away looks like it's only 10 yards off. Higher magnification sounds better, but there's a catch: the more you magnify, the more every tiny hand tremor is magnified too. Most people can hold 7x to 10x binoculars steady by hand, while anything above about 12x usually needs a tripod to avoid a shaky, blurry view.
The second number: objective lens diameter
The second number is the diameter of the objective (front) lenses in millimeters. In 10x50, that's 50mm. Bigger objective lenses gather more light, which means brighter images — a huge advantage in low light and for stargazing, where every photon counts. The trade-off is size and weight: 50mm binoculars are noticeably bigger and heavier than compact 21mm or 25mm models. This second number is essentially the binocular's "aperture," and it works just like it does on a telescope — a concept we cover in depth in our guide to telescope aperture and why it matters most.
So what does 10x50 mean, exactly?
Putting it together: a 10x50 binocular magnifies 10 times and has 50mm objective lenses. That combination is a beloved all-rounder for beginner astronomers because it strikes a great balance — enough power to reveal craters on the Moon, the moons of Jupiter, and rich star fields, with lenses big enough to keep those views bright, yet still light enough to hold by hand for a while. It's exactly why a pair like the Explore Scientific G400 Series 10x50 binoculars is such a popular first pair for the night sky.
What is exit pupil, and why does it matter for stargazing?
Here's a bonus number you can calculate from the two on the box: the exit pupil. Divide the objective size by the magnification, and you get the width (in millimeters) of the beam of light leaving each eyepiece.
Exit pupil = objective diameter ÷ magnification
For 10x50 binoculars, that's 50 ÷ 10 = a 5mm exit pupil. Why care? Because your eye's pupil widens to roughly 5–7mm in the dark. An exit pupil in that range means your eye receives the full, bright cone of light the binoculars deliver — which is why larger exit pupils make such a difference under a dark sky.
| Binocular | Magnification | Objective | Exit pupil | Best for |
|---|---|---|---|---|
| 7x50 | 7x | 50mm | 7.1mm | Maximum brightness, very steady views |
| 10x50 | 10x | 50mm | 5.0mm | The all-round astronomy sweet spot |
| 8x42 | 8x | 42mm | 5.25mm | Versatile day-and-night use |
| 10x42 | 10x | 42mm | 4.2mm | Lighter, more compact, great by day |
| 15x70 | 15x | 70mm | 4.7mm | Deep-sky objects (tripod recommended) |
Do bigger numbers mean better binoculars?
Not necessarily — bigger just means different. Cranking up the magnification narrows your field of view (the width of the scene you see) and amplifies shake, while bigger objectives add brightness at the cost of weight. A compact, lower-power pair is easier to carry and steadier to hold; a big high-power pair shows fainter detail but may need a tripod. The "best" numbers depend entirely on how you'll use them.
For example, an 8x42 pair like the Explore Scientific G400 Series 8x42 binoculars is a fantastic do-everything choice — light enough for all-day hikes and birding, bright enough for casual stargazing. A slightly more compact National Geographic 10x42 waterproof pair leans toward daytime use and travel. There's no single right answer, only the right fit for your nights out.
What binocular numbers are best for astronomy?
For stargazing specifically, aim for a big exit pupil and enough aperture to soak up faint light. The classic beginner recommendations are 7x50 and 10x50 — both bright, both handholdable, and both wide enough to sweep the Milky Way. Go up to 15x70 or larger and you'll see more deep-sky detail, but plan on a tripod. If you want a deeper walkthrough of matching binoculars to the night sky, our best binoculars for stargazing guide breaks it all down, and you can also see how magnification behaves in telescopes in our magnification explainer.
Ready to browse? Explore our full range of binoculars and sport optics collections, where every pair lists its magnification and objective size right up front.
Frequently asked questions
What do the numbers on binoculars mean?
The first number is the magnification (how many times larger objects appear) and the second is the diameter of the objective lenses in millimeters (how much light they gather). So 10x50 means 10x magnification and 50mm front lenses.
Is 10x50 good for astronomy?
Yes — 10x50 is one of the most recommended beginner astronomy configurations. It offers a bright 5mm exit pupil, enough magnification to show lunar craters and Jupiter's moons, and a wide field, all while staying light enough to hold by hand.
What does the second number on binoculars mean?
The second number is the objective (front) lens diameter in millimeters. A larger objective gathers more light for brighter images, which is especially helpful at night and for stargazing, but it also makes the binoculars bigger and heavier.
Are higher magnification binoculars always better?
No. Higher magnification narrows your field of view and makes hand shake much more noticeable. Above about 12x, most people need a tripod for steady views. For handheld use, 7x to 10x is the practical sweet spot.
What is exit pupil on binoculars?
Exit pupil is the objective diameter divided by the magnification, measured in millimeters. It represents the width of the light beam reaching your eye. For night viewing, an exit pupil of about 5–7mm matches your dark-adapted pupil for the brightest possible image.
Ready to pick your pair?
Now that you can read the numbers at a glance, find your match in our binoculars collection — clear skies!