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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compared to that same object seen with the naked eye. Magnifcation can't answer the question, "How far can I see?" (After all, the sun is 93 million miles away and visible with the naked eye.) What it does answer is, "How small of an object can I see at a particular distance?" With binoculars, we might be able to see a freight liner a few miles out from shore, but that doesn't mean we will be able to see a gull at the same distance. As you increase the magnifcation of binoculars, you have the advantage of making something farther away appear closer. As magnifcation increases, the "power" goes up, but as that number increases, it has other consequences that will make the binocular less userfriendly. Any movement of your body will be exaggerated through the optics, causing a bouncier image and resulting in eye fatigue and a reduction of resolution. This is why we have to mount spotting scopes (which have higher power) on tripods. The conventional wisdom here is that magnifcations beyond 10x become really challenging to hand hold, and any beneft of the added magnifcation is offset by a "bouncier" image. Another point to note about magnifcation is that as you increase power, your depth of feld becomes shallower. This means that as birds come toward you or move away from you, you will have to change the focus wheel more often to maintain a crisp view of them, whereas you tend to have more distances in focus at the same moment with lower magnifcations. Magnifcation has practically no impact on a binocular's dimensions or weight. The second number, however, is indicative of the binocular's physical dimensions. The number following the "x" refers to the diameter, measured in millimeters, of the binocular's objective lenses. This is referred to as the binocular's aperture. The bigger this number is, the larger and heavier you can expect the binocular to be, and the more light the Ocular Lens Diopter Center Focus Fig. 2. The parts of your binocular that Twist-up Eyecup Objective Lens you should know. Image © Eagle Optics. binocular will be able to collect. We love a bright image but also want lightweight binoculars; as with many other aspects of binocular design, it's all about tradeoffs. Binoculars are often lumped into three general size categories: compact, midsized, and full-sized. The term "compact" refers to binoculars with objective lenses smaller than 30mm, "mid-sized" binoculars are those with 30–35mm objectives, and "full-sized" binoculars start at about 40mm and go up to 50mm. A Useful Equation I'm not a big believer in throwing out a lot of numbers to illustrate a technical point, but a binocular's exit pupil is a great concept to be familiar with. Understanding this concept will allow you to see how magnifcation and objective lens size work together to impact your experience using a binocular. The exit pupil is the diameter of the shaft of light (measured in millimeters) exiting the ocular lens. The larger a binocular's exit pupil, the brighter your image will be. Here is a simple equation that will tell you how to determine the size of a binocular's exit pupil: objective lens diameter exit pupil = –––––––––––––––––––––––––– magnifcation December 2013 | Birder's Guide to Gear 13

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