Focus on the Right Binoculars

May 22, 2009

It’s a bird! It’s a plane! No, it’s time to get a pair of binoculars! Super-power your vision and you can zoom in to identify that mysterious flying object, get a closer look at the region’s wildlife, or survey a distant landscape. To find the perfect pair, focus on the interconnected elements of size, weight, magnification, image clarity and brightness, and cost.

Power in Numbers Every pair of binoculars is labeled with two numbers, such as 7×35 or 10×25. The first indicates the magnification power, or how many times closer a distant object will appear. The second identifies the diameter (in millimeters) of the objective lens, the larger lens farther from your eyes. When choosing a magnification power, consider your intended use. A higher power (9-10x) is a good option for activities like birding, where identifying small details is important. But keep in mind that higher magnification reduces your field of view, the portion of the surrounding landscape you see through the binoculars. The field of view increases with lower powers (7-8x), making it easier to scan a large area quickly or locate a desired object.

Lighten Up The size and weight of binoculars are directly correlated to the diameter of the objective lenses. The larger they are, the heavier and bulkier the binoculars. Larger lenses capture more light, however, and produce a brighter image that is better for the low-light conditions encountered around dawn or dusk. The size of the lenses also affects the “exit pupil,” or the shaft of light that travels from the binoculars into your eyes. To calculate the exit pupil, divide the objective lens diameter by the magnification power. (For example, a 7×35 pair of binoculars will have a 5 mm exit pupil.) Your pupils vary in size from 2.5mm to 7mm, depending on light conditions. For the binoculars’ image to be as bright as what you’d perceive with your naked eye, the exit pupil must be as large, or larger, than your pupils. Opt for 25mm lenses when size and weight matter, otherwise consider a larger, brighter pair.

The distance between adults’ pupils varies as much as an inch, ranging between 2 and 3 inches depending on the person.

Optical Delusions? Binoculars come in an astonishing range of prices, from less than $50 to well over $1,000 for a top-notch pair. The biggest influence on cost is lens quality. Inexpensive models ($50 or less) use cheap lenses that create prominent distortion around the edges of the image and have poor color transmission, giving the image a distinctly green or bluish hue. There’s a significant jump in quality around the $100 mark; binoculars in this price range tend to be good, all-purpose models that produce true-color images with minor distortion around the edges. In the $200 to $400 range, higher quality lenses produce a clear, crisp image with little to no distortion. These binoculars also feature more lens coatings, which eliminate tiny imperfections on the lens surfaces, reduce the amount of light reflected off the lenses, and create a sharper image. Beyond $400, you’re entering the realm of optical perfection—overkill for all but the most serious users.

Other Things to Look At 
Any pair of decent binoculars will withstand the rain, but waterproof models can survive a full-submersion trip overboard. Binoculars with a rubberized casing provide a better grip in wet and slippery conditions. To increase the odds of retaining your lens caps, look for styles that remain attached to the binoculars even when removed. For up-close viewing, consider the focal distance, or how close you can be to an object and still bring it into focus. Lastly, pay attention to your personal ergonomics. Everybody’s eyes, faces, and hands are different—choose a pair of binoculars that produces an easy-to-locate image and is comfortable in your hands and against your face.


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Dirk Van Susteren

AMC Outdoors, the magazine of the Appalachian Mountain Club, inspires readers to get outside and get engaged. Learn more.