Budget Binoculars for Stargazing and Outdoors: How to Read the Numbers

This site previously had more than twenty short posts about individual low-cost binoculars, from 8×21 kids’ models and 10×25 pocket folders to 10×50 and 12×26 compacts. Most repeated seller listings. This guide replaces them with one useful page explaining how to read binocular specifications, so you can judge any pair yourself.

What the two numbers mean

Every pair of binoculars is labelled with two numbers, such as 10×25 or 10×50.

• The first number is the magnification. 10x makes objects appear ten times closer.
• The second is the objective lens diameter in millimetres, the size of each front lens. Bigger lenses collect more light.

A 10×50 pair collects four times as much light as a 10×25 pair, because light-gathering depends on the area of the lens, and 50 mm has four times the area of 25 mm. That makes a big difference at night.

Exit pupil: the brightness number

Divide the objective diameter by the magnification to get the exit pupil, the width of the beam of light reaching your eye. For example:

• 10×25: 2.5 mm
• 8×30: 3.75 mm
• 10×50: 5 mm
• 7×50: about 7 mm

In daylight your pupils shrink to about 2 to 3 mm, so small binoculars work fine. In the dark, young eyes open to about 7 mm and older eyes to about 5 mm. A larger exit pupil gives a brighter view at night, which is why 7×50 and 10×50 are popular astronomy binoculars.

Field of view

Field of view is how wide a scene you can see, given either in degrees or as feet at 1,000 yards. One degree equals roughly 52.4 feet at 1,000 yards. For example, one of the 10×25 folding pairs listed earlier on this site quoted 362 ft at 1,000 yards, or about 6.9 degrees. A wide field makes it easier to find birds, follow sports and sweep the Milky Way.

Prisms: BAK4 vs BK7

Binoculars use prisms to fold the light path and turn the image the right way up. Better models use BAK4 glass, which transmits light more evenly to the edge of the beam; BK7 is a cheaper glass that can make the exit pupil look slightly squared-off and dimmer at the edges. You can check by holding binoculars at arm’s length and looking at the bright circles in the eyepieces: round circles suggest BAK4, while grey, squared edges suggest BK7.

Porro vs roof prisms

• Porro prism binoculars have the classic zig-zag shape with the front lenses wider apart than the eyepieces. They give good depth perception and good value.
• Roof prism binoculars are straight and slimmer, common in compact folding pairs.

Coatings

Anti-reflection coatings reduce glare and increase brightness. “Coated” means at least one surface is coated; “fully coated” means all air-to-glass surfaces; “multi-coated” means some surfaces have several layers; and “fully multi-coated” (FMC) means all surfaces have multiple layers, which is usually best. Bright red or “ruby” coatings on cheap binoculars often cut glare in sunlight but reduce brightness and colour accuracy.

Marketing claims to treat with caution

Listings sometimes promise “night vision”, “military grade” or “HD”. Ordinary binoculars cannot see in total darkness. True night vision devices use electronic image intensifiers or infrared sensors. Large magnification claims such as 20x or more on very small lenses usually mean a dim, shaky image. Judge binoculars by their measurable specs: magnification, objective size, exit pupil, field of view, prism type and coatings.

Choosing by purpose

• Astronomy: 7×50 or 10×50, which are bright enough to show Jupiter’s moons, star clusters like the Pleiades, the Orion Nebula and the Andromeda Galaxy as a faint glow. Mount them on a tripod for steadier views.
• Birding and hiking: 8×30 to 8×42 give a bright, wide view and are easy to hold steady.
• Travel, concerts and sports: compact 8×21 to 10×25 folding pairs fit in a pocket.
• Kids: look for low magnification (6x to 8x), small size, rubber armour and an adjustable distance between the eyepieces (interpupillary distance) for small faces.

Comfort features

• Eye relief: the distance your eye can be from the eyepiece and still see the whole view. Glasses wearers typically need about 14 mm or more.
• Twist-up or fold-down eyecups: let you adjust for glasses.
• Centre focus and diopter adjustment: set each eyepiece for your eyes.
• Tripod adapter thread: useful for stargazing with 10x50s.

Safety note

Never look at the Sun through binoculars. Even a brief glance can cause permanent eye damage. Use only certified solar filters designed for binoculars to observe the Sun.