Birder's Guide

DEC 2013

Birder's Guide is the American Birding Association's newest publication. Each issue focuses on a key subject, providing tips from experienced birders on a wide variety of topics like Travel, Listing & Taxonomy, Gear, and Conservation & Community.

Issue link: http://bg.aba.org/i/216642

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Binocular 101 | •– Porro Prism Fig. 1 The roof-prism style of binocular can be distinguished from the classic Porro-prism style by its two straight barrels. Image © Eagle Optics. | •– Roof Prism • Objective lenses. These are the big lenses at the "far end" of the binocular, whose function is to collect light. • Ocular lenses. These are the lenses you put up to your eyes; they magnify the information collected by the objective lenses. • Focus wheel. Turning this wheel focuses both barrels of the binocular in unison. Some binoculars have separate adjustments for each barrel; for birding, I recommend avoiding binoculars with eyepieces that focus individually. • Diopter. This secondary focus mechanism allows you to calibrate one side of the binocular to accommodate differences in focus length between your left and right eyes. Almost always, the diopter will focus the right eye inde- pendently from the left. Adjusting the diopter is simple and—provided you don't lend your binocular to someone else, or the diopter doesn't slip—it needs to be readjusted only as your eyes change. Generally, the diopter will be located somewhere on the right side of the binocular, just below the eyecup. Some binoculars have locking diopters built into the center focus mechanism. • Eyecups. The primary function of the eyecups on your binocular is to prevent your eyes from getting too close to the ocular lenses. Eyecups can also help block out lateral light. If you wear eyeglasses when using binoculars, you'll want to be sure that the eyecup is folded back (if it's of a soft rubber design) or twisted down (if it's a mechanical eyecup, which most modern binoculars employ). Why do this? Your eyeglasses will provide a barrier that spaces your pupil farther from the ocular lens, thus negating the need for the eyecup to serve this function. Otherwise, it's like peering into a keyhole. What the Numbers Mean Now that we have an understanding of the parts of a binocular, let's look at some essential concepts. When we talk about the size of a binocular, we refer to it in a confguration, such as 8x42, 10x25, 7x50, etc. The frst number (which will always be the smaller of the two) refers to the binocular's magnifcation: how much larger it will present an object to the user How to Adjust the Diopter on Your Binocular Find a stationary object to focus on at an intermediate distance, and, while closing the right eye, use the center focus wheel to get a sharp image in the left eye. Once this is done, close the left eye and check the right barrel for image sharpness. If it isn't as clear as the left side, use the diopter to focus the right side so that it matches the left. Once you've done this, both eyes should see a sharp image as you use the center focus wheel to watch birds both near and far. Unless your eyes change or the diopter is moved, you shouldn't have to repeat this exercise. 12 Birder's Guide to Gear | December 2013

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