What do the numbers on binoculars mean?
Quick answer: the two headline numbers are magnification × objective lens diameter in millimeters. On a 10x42, the 10x makes objects look ten times closer and the 42 is the width of the front lenses in mm, which sets how much light the binocular gathers. Higher magnification brings things closer but narrows the view, dims the image and shakes more; a bigger objective is brighter but heavier. 8x42 and 10x42 are the all-round sweet spot.
The first number: magnification
Magnification is how many times closer the subject appears. A 10x binocular makes a deer at 300 yards look like it is at 30. More is not automatically better: as magnification climbs, the field of view narrows, the image dims, and every tremor in your hands is magnified too. That is why 8x and 10x are the handheld sweet spot, and why 12x, 15x and higher models are usually steadied on a tripod. For a fuller comparison of the two most popular sizes, see 8x42 vs 10x42.
The second number: objective lens diameter
The second number is the diameter of the front (objective) lenses in millimeters. Bigger objectives gather more light, which means a brighter image, especially at dawn and dusk, but they also add size and weight. 42 mm is the all-round full-size standard; 50 mm and 56 mm pull in more light for low-light and long-range glassing at a real weight penalty; 25 mm to 32 mm keeps things pocketable for travel.
Exit pupil: the number that predicts low-light brightness
Divide the objective by the magnification and you get the exit pupil in millimeters, the width of the beam of light leaving each eyepiece. A 10x42 has a 4.2 mm exit pupil; an 8x42 has 5.25 mm. In bright daylight your pupil is around 2 to 3 mm, so both look bright. But at dusk your pupil dilates to roughly 5 to 7 mm, and the binocular with the larger exit pupil delivers the noticeably brighter image. This is the single best number for predicting low-light performance.
Field of view and eye relief
Two more specs decide how a binocular feels in use. Field of view (quoted as feet at 1000 yards, or as an angle in degrees) is how wide an area you see; a wider field makes it easier to find and track moving subjects, and it shrinks as magnification rises. Eye relief is how far your eye can sit from the lens and still see the whole image; if you wear eyeglasses, look for about 16 to 17 mm so the view does not black out at the edges.
The numbers at a glance
| Spec | What it is | Why it matters |
|---|---|---|
| First number (10x) | Magnification | How many times closer objects appear. 8x and 10x are ideal handheld; 12x+ wants a tripod. |
| Second number (42) | Objective lens diameter (mm) | How much light it gathers. Bigger = brighter but heavier: 42 mm is the all-round standard. |
| Exit pupil | Objective / magnification | 10x42 = 4.2 mm. Bigger is brighter in low light (your eye dilates to 5 to 7 mm at dusk). |
| Field of view | ft at 1000 yds, or degrees | How wide you see. Wider is easier for moving subjects; it shrinks as magnification rises. |
| Eye relief | mm | How far your eye can sit back and still see the full image. 16 to 17 mm is best for eyeglass wearers. |
So which numbers should you choose?
- 10x42 for all-round hunting and general use: the default for a reason. See our best binoculars for hunting.
- 8x42 for a wider, brighter, steadier handheld view (birding, low light): 8x42 vs 10x42.
- 12x50 or 15x56 for open-country and astronomy reach, best on a tripod.
- 8x25 or 10x25 compacts for travel and packing light.
FAQ
What does 10x42 mean on binoculars?
10x42 means 10x magnification through a 42 mm objective (front) lens. The 10x makes objects look ten times closer than the naked eye; the 42 mm sets how much light the binocular gathers. It is the most popular all-round size because it balances reach, brightness and weight, which is why 10x42 models like the Vortex Diamondback HD are the default for hunting and general use.
Is a higher magnification binocular always better?
No. Higher magnification narrows the field of view, dims the image and magnifies hand shake, so above about 10x to 12x you often need a tripod. For handheld use, 8x and 10x are the sweet spot. Jump to 12x50 or 15x56 only when you glass wide-open country or the night sky, ideally off a tripod.
What is the exit pupil and why does it matter?
The exit pupil is the objective diameter divided by the magnification (for a 10x42, 42 divided by 10 = 4.2 mm). It is the width of the beam of light leaving the eyepiece. A bigger exit pupil gives a brighter image in low light, because your pupil dilates to around 5 to 7 mm at dusk. That is why 8x42 (5.25 mm) looks brighter at last light than 10x42 (4.2 mm).
What do the other numbers on binoculars mean, like 330 ft or 6 degrees?
Those describe the field of view: how wide an area you see. It is written either as feet at 1000 yards (for example 330 ft) or as an angle in degrees (for example 6.3 degrees). A wider field of view makes it easier to find and follow moving subjects, and it generally shrinks as magnification rises.
Related: 8x42 vs 10x42 · roof prism vs porro prism · how to choose binoculars · how to focus binoculars · when were binoculars invented · Vortex Diamondback HD 10x42 review · best binoculars for hunting 2026.