Binocular Basics

“Never mind binoculars”, say many beginners.  “I want a telescope!”  But binoculars are telescopes!  They are simply two small telescopes side by side, with a little extra optics to make the eyepieces close enough so you can comfortable look through both at the same time.  Unlike a larger telescope, binoculars are easy and intuitive to use.  They produce a right-side-up image and a large field of view which make it easy to aim them at an object and find what you’re looking for.  And they don’t need any time to set-up and align.  You just grab them and head outside under the stars. Binoculars are especially useful for seeing large craters on the Moon, the moons of Jupiter, the occasional comet, close groupings of the Moon and planets at sunrise and sunset, and, once you know how to find them, larger star clusters and groupings of stars all over the sky.

All binoculars are marked with two key numbers: magnification and aperture. A pair marked 7×50, for example, magnifies 7 times (or 7x) and has objective lenses 50 mm in diameter. The bigger the lenses, the fainter the objects and the more detail you will see.  A 50 mm lens will collect 50-60 times as much light as your dark-adapted eye, which means you will see objects 50-60 times fainter.


The Pleiades star cluster in the constellation Taurus… an excellent target for binoculars

Binoculars that magnify between 6 and 10 times are extremely useful for stargazing and are usually light enough to hand hold for short periods of time.  Higher power means you’ll see more detail and a darker background sky.  But you’ll also see a narrower field of view, and it’s harder to keep a high-power pair of binoculars steady enough to see fine detail since the slight shaking of your arms is also magnified.  For hand-held use, magnification of 7-8x is optimum, and 10x is maximum.

Because of their larger aperture, a pair of 10×80 binoculars lets you see fainter objects than a 10×50 pair.  The trade-off?  Bigger lenses mean more weight, and lenses larger than 50 mm are heavy which makes it harder to hold such binoculars steady for any length of time.

Binoculars also allow viewing with both eyes.  This is more comfortable and natural, and it helps many observers get a perception of depth, though this is an illusion with objects of such large distance.  You also avoid the distracting effect of blind spots and floaters in one eye or other. Dim objects actually appear brighter when viewed with both eyes.

A pair of 10x50 binoculars

A pair of 10×50 binoculars

Some binoculars are marked with the size of the field of view, either in degrees or “feet at 1000 yards”.  This tells you how wide a scene you’ll see.  For a fixed lens size, higher power means the field of view is smaller.  So you’ll see less sky with 10×50 binoculars, for example, than you will with 7×50 binoculars, all other things being equal.

Another key measure of binoculars is the “exit pupil”, the size of the bright disks of light you see in the eyepieces when you hold the binoculars at arm’s length.  The exit pupil is simply the ratio of aperture to magnification. So a 7×50 pair has an exit pupil of 50/7 = 7 mm (roughly), and a 7×35 pair has a 35/7=5 mm exit pupil.

You want to try to make sure the exit pupil of your binoculars is no larger than the size of your eyes’ pupils when they are adapted to the dark.  Under age 30, most people have a dark-adapted exit pupil of 7 mm. But we lose about 1 mm every 10-15 years. At age 50, for example, it may not make sense to use binoculars with an exit pupil larger than 5-6 mm. So if you’re older, a pair of 7×35’s or 8×42’s (both with a roughly 5 mm exit pupil) might be a better choice than a pair of 7×50’s (with a 7 mm exit pupil).  Such binoculars are, on average, less expensive as well.