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Binocs are named for the configuration of their internal prisms. ROOF-PRISM models have straight tubes, while PORRO-PRISM binoculars have a dogleg shape, with the objective lenses offset from the eyepieces. Most high-end binoculars are roof-prism, mostly due to the more compact size.
1) The DIOPTER—located on the right eyepiece in most models—corrects any vision difference between your eyes. To adjust it, look through the binoculars at an object, close your right eye, and use the focus knob to make it sharp. Then close your left eye and use the DIOPTER ADJUSTMENT to focus for your right eye. Now the focus knob will work perfectly for both.
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2) The best binoculars have COATINGS on both surfaces of all lenses (there may be more than a dozen inside your binocs) to maximize light transmission. Such binocs are "fully multicoated." Better models are also sealed against rain, and the best are NITROGEN-FILLED to eliminate internal fogging.
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3) What SIZE lenses do you need? Compact binoculars have objective lenses in the 18- to 25-millimeter range. Midsize binoculars are 30 to 35 millimeters, an excellent all-round choice. Full-size binos can have 40- to 50-millimeter objectives and offer the brightest views.
The best way to buy binoculars is to look through a pair for as long as possible. The view should feel natural; if you have to consciously readjust your vision when you take your eyes away, the optics are probably not the best. Compare models side by side for BRIGHTNESS and SHARPNESS (viewing a distant street sign is a perfect test for the latter).
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4) Binoculars are classified with two numbers: 8x32, 10x40, etc. The first refers to the MAGNIFICATION—how many times closer they make an object appear. You might think more is better, but higher powers also magnify tremors in your hands. For most of us, seven or eight power is perfect; really serious naturalists may want to go to ten. The second number is the DIAMETER (in millimeters) of the front, or objective, lens. The bigger the objective lens, the more light and detail the binoculars gather, the wider their field of view—and the more they weigh.
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5) If you must wear glasses while using binoculars—only people with astigmatism should need to—look for some with at least 15 millimeters of EYE RELIEF, the distance you can hold the instrument away from your face and still see a full view.
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