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.

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Ocular Lens Exit Pupil Glossary you need to be close to the subject, choose a binocular with a low close focus number, ideally six feet or less. Depth of feld. This refers to what you actually see in focus at any one time. It is the area in focus between the foreground and background of the viewing area. Diopter. A focus mechanism on all center-focus binoculars to allow adjustment of one eyepiece separately from the other. This is done to compensate for differences between left and right eyes. Exit pupil. The point at which all of the light rays that entered the objective lens and passed through the binocular exit through the eyepiece to form a magnifed, circular image. If you hold the binocular away from your eyes and look through the ocular lens, you will be able to see the clear circular exit pupil. Eye relief. The distance images are projected from the ocular lens to their focal point. The eye relief of a binocular can vary from 5mm to as much as 23mm. Field of view. The widest dimension of the circular viewing area seen through a binocular. This measurement may be listed on the binocular in either degrees or feet, measured at 1,000 yards. Note: One degree equals 52.36 feet/1,000 yards. Objective lens. This lens is at the end of the binocular opposite the eyepiece. It gathers light into the binocular. Ocular lens. The lens in the eyepiece of the binocular that magnifes the image presented by the objective lens. Aperture. The diameter of the objective lens, measured in millimeters. Close focus. The shortest distance at which a binocular can focus. If, in viewing things like butterfies, Prism. A mirrored piece of glass inside the binocular housing which reorients the image projected to the eye so that it is right side up and correctly oriented from left to right. Eye relief is a fxed distance Fig. 4. Eye relief is the distance the ocular lens should be spaced from the surface of your eye. Image © Eagle Optics. oculars offering 15mm of eye relief or greater. If your eyes are particularly deep set or if your glasses sit toward the front of the bridge of your nose, you may see better with 18mm of eye relief. This fgure generally doesn't exceed 20mm, and, like close focus, it will vary from model to model. In general, compact binoculars often have less eye relief than fullor mid-sized binoculars. Fortunately for the birder who wears glasses, most fulland mid-sized binocular choices on the market today offer ample eye relief, and this specifcation generally isn't a limiting factor in fnding a good binocular. Having the right binocular in your hand isn't necessarily the result of fnding the "perfect" one. I'm convinced that perfection doesn't exist. With an understanding of how binoculars function, you can begin to see the game of give and take when opting to enhance one feature at the expense of another. Some birders approach this dilemma by having multiple binoculars at their disposal and choosing the one to best suit a particular outing. Others choose a binocular for all their birding needs after prioritizing features and realize that it will be the best in most, but not all, situations. While the perfect binocular might not be a reality, a working knowledge of how a binocular's size, style, and features can impact your experience in the feld goes a long way in helping you fnd the binocular that's best for you. Editor's note: For more information on understanding optics, from in-depth articles to videos, visit the educational section of the Eagle Optics website: This topic is also discussed in Rick Wright's book review on p. 46. December 2013 | Birder's Guide to Gear 15

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