Choosing Binoculars

One pair isn't necessarily as good as another. When you're choosing binoculars, make sure it's an informed choice.
By Hugh M. Johnson
September/October 1984
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If you want to make an informed decision when choosing binoculars, it helps to know their parts. This pair uses the porro prism system.
Illustration by MOTHER EARTH NEWS Staff
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Good binoculars are an investment that can magnify your enjoyment of almost all outdoor experiences — and a good many indoor experiences. But there are more than a few points to consider before plunging into what can be a sizable purchase (top-line binocs go for several hundred dollars). Factors such as weight, size, magnification power, image brightness, field of view, focusing, lens construction and coating, and — most important — your own particular needs all deserve careful thought when choosing binoculars.

The Basics

Before we jump into the nitty-gritty of long-range optics, we need to get a few terms and definitions under our belts. The first of these is the most basic and frequently the least understood. I'm referring to the differences between binoculars and field glasses. Though the two terms are often used interchangeably, there is a distinction. Field glasses are the ancestors of binocs, and they're simply two Galilean-type telescopes — each with two in-line lenses hooked together with a common focusing apparatus — and fraught with several optical defects. Not least among the shortcomings of field glasses is a field of view (the area visible through the lenses at a given distance) that decreases dramatically as magnification increases. Consequently, field glasses are useful only for low-magnification applications, usually not to exceed a power of four (4X). But if you can get by with such moderate magnification, field glasses do have the singular advantage of being far less expensive than binoculars — more correctly called prism binoculars — to which we'll turn our attention in a moment.

But first we need to learn two lens-defining terms: Ocular lenses are the small ones on the eye end of binoculars (in fact, they're often called eye lenses, or eyepieces), while objective lenses are the big ones at the front, where light enters.

Binoculars are capable of optically precise magnification far beyond that of field glasses because they employ prisms as well as magnifying lenses. The most common prism arrangement used in binoculars is called the porro system, which utilizes two prisms in each barrel; the first magnifies and inverts the image, and the second turns what you see right side up again. In addition to increased power, the prism setup allows for wider separation of the objective lenses, thus increasing binocular vision.

Uh-oh, there's another term — binocular vision. In simpler words, this means two-eyed vision: seeing the same object through two eyes set a distance apart, thus allowing the brain to compute estimates of depth and distance from the two slightly different images. Well, the greater the separation of those two images, the greater the depth of field (the amount of image that can be sharply focused). OK so far? With field glasses, that separation is minimal, limited to the width of the two parallel telescopes, or about the normal distance between human eyes. But with porro prism binoculars, the reflecting prisms allow the front, or objective lenses, to be mounted farther apart, while the ocular (eyepiece) lenses remain a comfortable distance from each other.

Magnification and Lens Size

All binoculars have a pair of numbers stamped into one of the barrels and separated by an X, like so: 7x35, 8x35, 10x40, and so on. The first of those numbers (7, 8, 110) etc.) tells you the magnification power, while the second gives the diameter of the objective lenses in millimeters. For example, a pair of glasses marked 7x35 provides a magnification of seven and has objective lenses that each measure 35mm.

Lower-powered glasses are more forgiving of movement and thus are the best bet for viewing from a moving vehicle — a car, boat or aircraft — or if you intend to "sweep" the glasses to follow a big buck bounding across a field or a hawk in flight. The dividing line between hand-held glasses and those requiring a tripod is about 8X. Above that, you'll need some method of anchoring things down.

Image Brightness

If you hold a pair of binoculars at arm's length with the objective lenses pointing at a light and then look at the ocular lenses (not through them), you'll see a bright circle of light on each of the lenses. Those circles are the exit pupils, which represent the area or amount of light that exits from the glass.

For purposes of comparison, exit pupil is expressed in millimeters and signifies the diameter of the circle of light leaving the binoculars. But you don't have to measure those little orbs. Instead, there's a formula: Just divide the magnification power into the diameter of the objective lens. For example, the exit pupil of a pair of 7x35's is 5mm (35mm divided by 7). But occasionally the light-emitting capabilities of optics are expressed as a second measurement, or guide number, known as relative brightness. It's important to know what both ratings mean. Relative brightness is nothing more than the exit pupil — in the above example, 5 — squared. (Continuing the same example, it would be 5 times 5, or 25.) Since some binoculars gather and pass much more light than others, the exit pupil's relative brightness can be an important gauge for selecting night-viewing glasses.

But regardless of how much light exits from a given pair of binocs, the amount that can enter the human eye is limited by Mother Nature. Everyone knows that the pupils in our eyes adjust to available light by expanding and contracting, opening under conditions of dim illumination and closing down when exposed to bright light. But there are limits: No matter how bright your surroundings may be, your pupils won't contract below about 2mm. In total darkness they'll open to a maximum of only some 7mm. At dawn and dusk, our pupils are "set" on about 5mm.

What all that means is that any pair of glasses having an exit pupil of 5mm (that's a relative brightness of 25, remember) can be used effectively from dawn to dusk. And for night viewing, if the exit pupil of your binocs is 7mm (relative brightness of 49), you've again reached the usable limit.

Remember those numbers when you go shopping, because most good manufacturers list their products' exit pupils (and/or relative brightness) in their literature. And even if they don't, you can figure it out for yourself, now that you know the formula.

Field of View

As I mentioned before, field of view is simply the area that's visible through a pair of binoculars at a given distance. It's sometimes expressed as an angle, but field of view is generally noted as a linear measurement, in feet, at 1000 yards. And except in special wide-angle glasses, field of view diminishes as power of magnification goes up. In other words, 7X glasses have a larger field of view than 9X ones do. Field of view is another measurement that's usually included in the manufacturer's literature.

Unfortunately, not all manufacturers label their glasses accurately. So the best insurance you have is a reputable brand name. Or you can check relative field of view yourself by taking two pairs of binoculars of the same power and focusing them on a distant object (such as a sizable parking lot) that is bigger than the visual area either set of glasses can cover. Note how much you can see through each pair; the binocs that span the larger area have the wider field of view.

For buyers wanting maximum viewing area, many major binocular manufacturers offer wide-angle glasses in addition to their standard models. For example, one maker offers three 7x35's, with fields of view of 420', 520', and 578' (all measured at 1000 yards).

Lens Construction

Sometimes the makers of binoculars with a wide field of view rob Peter to pay Paul. That is, the field of view may be increased at a sacrifice of clear resolution around the edges of the lenses. So another thing to look for when examining glasses is edge sharpness. If the image fuzzes out around the edges, look for another pair. (Again, buying name brands offers some degree of protection, since quality binocs use lenses that have been honed for clarity right out to the extreme edges.)

To perform an in-store test of the overall image quality of a pair of binoculars, pick out a viewing target with lots of straight lines and angles (a group of buildings, for example). If the edges of the target's lines bulge outward or bend in, the glasses have lens distortion and are best returned to the display case.

Be sure to check that the edges and the centers of the image are clear at the same time. If you focus so that the center of the target image is clear, and the edges become fuzzy — or if you focus on the edges, and the center loses sharpness — hand the long-distance specs back to the clerk and try another pair.

Do you wear corrective eyeglasses? If so, then by all means wear them when you look through a pair of binoculars you're thinking of buying. Some binocs restrict the field of vision of eyeglass-wearers, while others don't. The spectacle-wearer can enjoy a full field of view with binocs equipped with removable rubber eyecups.

A final lens consideration is coating. Whenever an image passes through a lens, it loses some of its brightness. If you wear eyeglasses, for instance, about 8% of the incoming light is lost through reflection before it reaches your eyes. And the same concept applies to binoculars: When light passes through a lens or prism, the glass "eats" between 4% and 6% of the brightness. Since modern binoculars may have as many as 12 lenses, half of the incoming illumination may be lost before it reaches you.

To improve light transmission and reduce reflection, modern optical lenses are coated with a molecular layer of special metallic salt that increases light transmission by as much as 50%. And lens coating, if done properly, has the added ability to eliminate flat and hazy images and produce crisp detail in which dark areas are dark and light areas are light.

And while coating of the exterior lenses is important, it's even more essential to vision quality that the inside optics get the treatment. To be sure a manufacturer hasn't cut corners by coating only the outside lenses, hold the suspect glasses backward in front of you, with a bright light in the background. Now look through the objective lenses (they're on the large end of the binocs, remember) and pivot the glasses until you can see a reflection. Properly coated lenses glint purple or blue, while uncoated interior optics show up as a bright glare.

Image Focus

Though you can buy binoculars that require separate focusing for each barrel, they're largely impractical. Stick to the standard, center-focusing type. To focus center-adjusting glasses, the first step is to set the hinged barrels to fit the distance between your eyes (the $10 term here is "interpupillary distance"). It's easy: Just fold or unfold the two barrels at the hinge until the split images come together to form one crisp picture.

It's always best to focus on a clearly defined target, such as a sign or building. Close your right eye (or cover the right objective lens) and, using only your left eye, turn the focusing wheel until you see the target clearly. Now close your left eye and turn the adjustable eyecup on the right barrel until the target comes into sharp focus. Now your binocs are set, and all further focusing (for distance changes) can be done with only the hinge wheel. (The right-eye adjusting wheel will have marks to either side of its center. By memorizing or making note of your personal setting, you can adjust it quickly — should it get changed somehow — without having to go through the entire procedure again.)

Parting Thoughts

Outdoors folks such as backpackers, climbers, and hunters often must be weight-and bulk-conscious when selecting equipment, thus the recent popularity of ultralight and compact "mini-binocs." Are they as good as their larger kin? Well, they can be, if you're willing to pay for the technology of reduced size quality. Use the procedures outlined earlier, whether testing the big guys or their smaller counterparts.

OK, the subject is complex, and you wish I could boil all those bothersome tests and terms down to a simple "best bet" statement, right? Well, I might be able to help you a little bit. For general use, dawn to dusk, 7x35's are probably a safe compromise, since that particular magnification/brightness combination offers a lot of advantages and no serious imbalances.

But even then, there are lots of 7x35 binoculars out there, and determining which brand, size, and style is best for you is ultimately your decision. I hope you'll use the information I've provided — and this Binocular Selector Chart — here to help you make an educated choice.

The Binocular Selector Chart is excerpted from Binoculars And All-Purpose Telescopes (Copyright© 1980, Dr. Henry E. Paul) and is used by permission of American Photographic Book Publishing (an imprint of WatsonGupill Publications), New York, NY. The chart is self-explanatory and quite helpful in narrowing the binocular choices appropriate for your planned use, but final purchase decisions should be made only after weighing the considerations and employing the tests outlined in the accompanying article. Remember, the expense of modern, high-quality binoculars puts them in the category of a major purchase, an investment worthy of more than casual contemplation.


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