Choosing Binoculars

The Essentials

Welcome to our brief rundown of all things binocular. The ideas that most binocular users will need to understand lie just below—and the more rarified topics, like “ED glass,” are towards the end.

What the “Numbers” Mean (7x35 vs 7x50, etc.)

Above: 7x50 Binoculars (7x magnification and 50mm diameter lenses)

If you’ve looked at (or through) binoculars, you’ve probably noticed the numbers printed somewhere on the binocular body (they’re commonly towards the rear, near the eyepieces). Ordinarily, there’s a notation like “7x35” or “10x50,” and the first number expresses the binoculars’ magnification. The second number is for the diameter of the objective, or “main,” lenses. A classic combination is the 7x35, which means that the view you see is magnified seven times, while the binoculars’ image-forming lenses are each 35mm in diameter. In contrast, 10x50 binos offer 10-power magnification, and the lenses up front are 50mm across.

Lens Diameter (“Aperture”) vs Weight and Bulk

Let’s look at that second number, for lens diameter. Just as with telescopes, bigger lenses up front mean more light captured, so you can better see dim details and magnify more before the image gets too dim. In this respect, bigger is better—but along with their extra cost, big-lensed binos generally weigh more than smaller ones. That means that you might have a harder time holding the bigger binos up to your eyes, especially when we’re aiming them high overhead towards the sky. The weight might also be enough to prevent you from holding the binoculars steady—in which case you’d see a less-sharp image, despite the better optics.

Bottom line: Past a certain point, usually 50mm (or maybe 60mm for some folks), binos get to be too heavy to look through when they’re handheld. At that point, either avoid the larger size for handheld observing, or consider mounting your binoculars to a steady tripod.

So Now, Magnification

For better or worse, most folks’ hands do shake a bit. That normal little tremor we have gets magnified with binoculars, so the higher the power of the binos we’re looking through, the more the shaking blurs our view. For that reason, handheld binoculars for most people usually top out at 10x or less (and maybe 12x for some especially steady folks).

In practice, 7x is already plenty, and for any given design, this lower power also means you get a wider angle of view. This gives you a more panoramic sweep around the area you’re observing and makes it easier to find your target in the first place.

Recommendations

Putting the diameter of binoculars’ lenses together with common magnifications, you get several traditional pairings for handheld use: 7x35, 7x50, and 10x50. The 7x35s are the easiest to handle for most folks, because of both the lower overall weight and the moderate magnification. For a few more bucks and a few extra ounces, the 7x50s make a great choice—they provide a noticeably brighter and sharper image, at the same 7x magnification as the 7x35s. Some people like 10x50s, because they magnify noticeably more than the first two designs, and they’re about as bright as the 7x35s, which is fine. If you like extra magnification and your hands are steady, 10x50s are cool. If not, then either of the 7x magnification combinations above are easier to use.   See Links, below. 

Other Features

If you’re a true binocular enthusiast and want to go beyond just the basics, here are some other aspects of binocular design to consider...

Eye Relief

For a great many folks, a basic pair of binoculars work very well indeed—but if you wear eyeglasses, or you just don’t like to get your eyes way down into the eyepiece lenses, then we should talk about eye relief. This is the distance from the glass surface to your eye—longer values, roughly 16mm-20mm, give you enough room to fit a pair of eyeglasses in between. Even if you don’t wear glasses, better eye relief can also just feel more roomy or spacious than the shorter eye relief distances that are found on conventional binoculars, but improved eye relief usually comes on more expensive binoculars.

Standard vs “Sport” Designs (Types of Prisms)

The incorporation of different prism designs determines how compact your binoculars can be, as well as their performance. Here are the two typical styles:

Porro Prisms

Above: Classic Porro-prism Binoculars

Found in the most common type of binoculars; it’s Porro prisms that give “classic” binoculars their well-known shape. Porro-prism binoculars are generally larger and wider (and perhaps a bit heavier) than binos that use roof prisms (see below), but Porro prisms offer wider views and they’re less expensive than roof-prism binos for the same level of image quality. Also, these binoculars’ widely spaced main lenses increase your impression of depth.

Roof Prisms

Above: Roof-prism Binoculars

Roof prisms allow the design of much more compact binoculars, as seen in “sport” or “travel” models. Much of the compactness comes from the “straight-through” look of their optics—there is no sideways “offset” as you find with traditional binoculars, even though there are prisms within. Roof prisms and their related components are more challenging to design and assemble than porro prisms, which is why this type of binocular tends to be more expensive.

High-End Glass

ED Glass for Reducing Chromatic Aberrations

If you have ever looked through more inexpensive binoculars, you may have noticed a colored “fringe” or “halo” in the images, especially at high-contrast edges like three branches or leaves against a bright blue sky, or the Moon against the night sky. This characteristic is known as “chromatic aberration” and is annoying to some people; it can reduce image sharpness a bit as well. Modern binoculars do a much better job of reducing this problem, but if you want the very best image quality, look for binos that use “ED glass” (ED stands for “extra-low dispersion”). ED glass lenses are harder to make, and so they’re more expensive.

BAK-4 vs BAK-7 Prisms

In the same way that special ED glasses can help control chromatic aberration, the quality of the glass in the prisms can make a difference, too. BAK-7 glass, the more common and less expensive type, does a good job—but BAK-4 is even better, offering better light transmission for brighter, sharper, and clearer images.

What’s the Bottom Line?

The differences between porro prisms and roof prisms, or special types of glass, or the other factors mentioned above, can be important to some folks (especially if you wear glasses or use binoculars a lot). On the other hand, it must be said that these differences, though quite real, can be subtle. If these small improvements aren’t of critical importance to you, you can save a lot by just observing with basic binoculars. If you’re not sure, come on down and see us—we’ll show you some binoculars so you can see for yourself.

Links

For most folks, a good pair of basic 7x35 or 7x50 binoculars will do just fine, and they're less than $50. 

7x35: Celestron UpClose G2 7x35 Porro Binoculars

7x50: Celestron Cometron 7x50 Binoculars

"Sport" binoculars, like the Sirui 10x42 ED, are compact, sharp and clear—and more expensive (a bit over $200), but they are really cool: Sirui Beyond Series 10x42 ED Binoculars

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