How To Choose Binoculars for Astronomy


Binoculars are ideal instruments for beginner astronomers. They are portable, fairly easy to hold steady, and you can comfortably see the Moon, most planets, stars, galaxies, star clusters and nebulae with them.

Your binoculars will have a pair of numbers associated with them, like 10 x 50, 7 x 50 or 7 x 35. 

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Here’s what this means:

The first number is the magnification. This tells you how zoomed in you are – that is, how much closer the object appears than it is. So in the above case, objects look 10 times closer than they are.

The second number is the aperture (in mm). This tells you how big the lens is. The bigger the lens, the more light it can collect, and the more you can see. The example above has lenses that are 50 mm across.

So the best binoculars would have the highest magnification and the highest objective lens diameter, right?

It’s not that simple. There are some constraints:

1. As magnification increases, shakiness increases and brightness decreases. You can try this with your phone camera – hold it up in the air with your hand, and zoom in as far as it lets you. You’ll notice that the image gets really unsteady, and may get dimmer. This is a problem with high magnifications – anything above 10x will need a tripod to keep steady.

2. As the aperture increases, each lens gets bigger and disproportionately heavier. An aperture of above 50mm makes binoculars significantly heavy to hold up for long periods.

3. The exit pupil of your binoculars matters. This is the number you get when you divide aperture by magnification (number 2 / number 1). Your eyes have an exit pupil too – this is the maximum size your pupils can dilate to in darkness. For you to actually see stuff through your binoculars, the exit pupil of your binoculars needs to roughly match the exit pupil of your eyes. Depending on your age, your exit pupil will fall between 4mm and 7mm. Here’s a short guide:

Under 30: 7-8mm
30 years old : starts shrinking from 7mm
40 years old: 6mm
50 years old: 5mm
65 years old: 4mm
Over 65: 3mm

If you’re too lazy to go into the intricacies of choosing the right exit pupil, a 5mm pair is generally a safe bet for most people.

A good starting pair of binoculars is a 7 x 35 or 10 x 50 pair. You can find these at almost any camera/video equipment store. If you want a smaller lighter pair that you can hold up easily, pick 7 x 35s.If you want more magnifying power and don’t mind the weight, go with a 10 x 50 pair. Initially you don’t need to invest too much money into them – almost any pair will do as long as you can see through them without your eyes hurting and they’re reasonably sturdy.

If you do consider it a worthwhile investment and you know you’ll use them often, Bushnell Falcon 133410 Binoculars with Case (Black, 7×35 mm), Nikon 8248 ACULON A211 10×50 Binocular (Black) or the more expensive Pentax SP 10×50 WP Binoculars (Black) come highly recommended.

TL;DR: Start with a pair of 7 x 35 or 10 x 50 binoculars.

Other Useful Articles:

How To Focus and Aim Your Binoculars

Why You Shouldn’t Buy a Telescope First


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