Binoculars for Stargazing: 10x50, 15x70 and 25x70 Explained

For stargazing, start with binoculars between 7x35 and 10x50, the range NASA’s Night Sky Network recommends for a first pair. The first number is magnification and the second is the lens diameter in millimeters. Larger models such as 15x70 and 25x70 gather more light but weigh three pounds or more and are best on a tripod.

What do the numbers on binoculars mean?

Every pair carries two numbers, such as 10x50 or 15x70. Celestron’s knowledge base defines them this way:

• The number before the x is magnification. A 10x pair makes an object look ten times larger than it does to the unaided eye.
• The number after the x is the diameter of each front (objective) lens in millimeters. Larger lenses collect more light. Celestron notes that doubling the lens diameter quadruples the light gathered.

By that rule a 70 mm lens collects close to twice the light of a 50 mm lens, which is the case for stepping up in size.

More magnification is not free. Celestron says raising it reduces both the field of view and the eye relief (how far your eye can sit from the eyepiece), and that lower magnification gives a brighter image.

What is exit pupil?

Exit pupil is the width of the beam of light that leaves each eyepiece. Celestron gives the formula: divide the objective lens size by the magnification.

Why it matters: your own pupil sets the limit. Celestron puts the human eye’s maximum between 5 mm and 7 mm. Sky-Watcher’s telescope manual gives about 7 mm for an average fully dilated pupil and adds that the figure varies from person to person and decreases with age.

Celestron’s rule is that a larger exit pupil means a brighter image in low light. A 10x50 sends out a 5 mm beam, a good fit for most dark-adapted eyes. A 25x70 sends out 2.8 mm, so the view is dimmer even though the lenses are bigger. A 7x50 sends out 7.1 mm, more than some older eyes can take in.

The same math applies to telescope eyepieces, which the eyepieces and filters guide covers.

How much can you hold by hand?

NASA’s advice for a first pair is direct: “try not to get anything much more powerful than a 10x50”. Its reasons are that larger binoculars with more power have narrower fields of view, weigh more, and are harder to hold steady. They “jiggle” unless you pay for image stabilization or use a tripod.

Weight shows the jump. Celestron’s own figures for its SkyMaster line:

The 15x70 is 3 pounds. The 20x80 is over 4.5 pounds, and Celestron builds a mounting rod into it. Holding either at arm’s length toward the sky for minutes at a time is tiring, and every tremor is magnified 15 or 20 times.

Celestron’s SkyMaster listing has no plain 10x50; the series starts at 12x60 and also includes a 7x50 in its Pro ED line. Plenty of other makers sell 10x50 pairs, and the numbers mean the same on any brand.

10x50 vs 15x70 vs 25x70: which size for what?

10x50 or smaller, hand-held. This is the grab-and-go choice and the one NASA points beginners to. The wide view makes it easy to find your way between constellations.

15x70, on a tripod. About twice the light of a 50 mm pair with a 4.4 degree field. Sky-Watcher’s manual puts the Moon at about half a degree across, so that field is close to nine Moon-widths. The 18 mm eye relief is the longest in the table, which helps people who observe with glasses on.

25x70, tripod only. The same 70 mm lenses at higher power. The field narrows to 2.7 degrees and eye relief drops to 13 mm. It suits the Moon and tighter star clusters more than sweeping the Milky Way.

20x80, tripod only. The most light of the four with a 4 mm exit pupil, at the cost of 75 oz.

What can you see with binoculars?

NASA’s list for a pair in the 7x35 to 10x50 range: the Moon, large open clusters such as the Pleiades (M45), and from dark skies the Andromeda Galaxy (M31). With practice it adds the Orion Nebula, Jupiter’s moons, some globular clusters and fainter nebulae from dark sites.

The Astronomical League runs a Binocular Messier Observing Program that gives a sense of the ceiling. It asks observers to find 50 or more of the 110 Messier objects using only binoculars, and recommends lenses between 20 mm and 80 mm. With 56 to 100 mm binoculars, all 50 can come from its “easy” list. With 20 to 50 mm, 42 objects are classed as easy.

Astronomy binoculars: 10x50, 15x70, 20x80 and 25x70

Shop the range

Browse current binoculars for stargazing on eBay.

Shop binoculars for stargazing→

Opens eBay in a new tab. Check condition, seller feedback and returns on the listing.

• 10x50 binocularsShop on eBay →
• 15x70 binocularsShop on eBay →
• 25x70 binocularsShop on eBay →
• Binocular tripod adaptersShop on eBay →

Tripod adapters for binoculars

A tripod adapter is a small bracket that joins the binoculars to a camera tripod. Celestron describes its Binocular Tripod Adapter as a way to mount a porro or roof binocular to a tripod “for added stability and convenience”.

Before buying one, check three things:

1. Is the pair tripod adaptable? All four SkyMaster models in the table are, according to Celestron’s spec pages.
2. Is an adapter already in the box? The 15x70 and 25x70 include one. The 12x60 does not. The 20x80 has its own rod and needs no bracket.
3. Can the tripod carry the weight and reach high enough? A 75 oz binocular pointed near the zenith asks a lot of a light photo tripod. Look for the tripod maker’s stated load rating.

All four SkyMaster models in the table use BaK-4 prism glass and multi-coated optics, according to Celestron, so compare other brands on those two terms as well.

When to move up to a telescope

Binoculars show where things are. A telescope shows detail: NASA’s Night Sky Network suggests a small reflector on a tabletop or Dobsonian mount, with a mirror between 4.5 and 8 inches, as the step after binoculars. The Dobsonian telescope guide compares the 6-inch and 8-inch sizes by weight and what each shows.

If you would like to see the other options first, telescopes for beginners sorts them by budget and sky. Binoculars also make an easy present for someone new to the hobby, and the stargazing gifts guide matches sizes to people. The stargazing gear overview lays out the whole path.