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 All 8x42 binoculars have an exit pupil of 5.25 mm (42/8 = 5.25). When we take a look at a 10x binocular with that same objective lens size, you can see that the exit pupil shrinks to 4.2 mm (42/10 = 4.2), providing less light to the viewer; this is especially noticeable in low-light situations, when our pupils are more dilated. During a bright afternoon when our pupils are generally dilated to 2–3mm, the difference between 4mm and 5mm exit pupils is inconsequential. It's a different story at dawn and dusk, however, when wildlife tends to be more active. For birding purposes, the best binoculars have an exit pupil of at least 4.0 mm. Understanding the exit pupil isn't the whole story regarding a binocular's brightness. Take any two different makes of 8x42 binocular (remember, both have the same exit pupil: 5.25 mm); it's quite possible that the image you see through one will be brighter than the other. The quality of the glass used in the binoculars and the coatings put on lenses also have an impact on brightness. While two 8x42s will collect the same amount of light, the higher-quality binocular will use that light more effciently, with more light reaching your eye and less refected off the glass surfaces to other directions. Price tags usually indicate quality: more expensive binoculars are brighter. Features and Tradeoffs With an understanding of the exit pupil, you can see the tradeoff between a compact, lightweight binocular and a larger one that will perform best in low-light conditions. There are some other binocular specifcations that are affected by size and design. A binocular's feld of view (FOV) is a measurement of how far a user will be able to see, from left to right, when looking through the binocular. A wide FOV is always useful, as it makes it easier for us to scan focks of birds, to draw size and shape comparisons, and to follow fast-moving birds in fight or through the canopy. FOV is often measured as X feet at 1,000 yards. X is the number of feet an observer can see, from left to right, at a distance of 1,000 yards. Another way to measure this is in degrees. You can convert degrees to feet at 1,000 yards by multiplying the degrees by 52.36. This may seem counterintuitive, but a larger objective lens does not mean a binocular will necessarily have a wider feld of view. In fact, as a binocular's objectives lenses get bigger, the barrels often get longer, which tends to decrease a Fig. 3. The size of the exit pupil determines the amount of light the binocular can deliver to your eyes. Image © Eagle Optics. Look for the exit pupil when holding the optics a short distance from your face. 14 Birder's Guide to Gear | December 2013 binocular's FOV. The specifcation which will have the greatest impact on FOV is magnifcation. In general, the lower the power, the wider the feld of view. When comparing roof-prism to Porro-prism models, one tradeoff to be aware of is that Porros will often have a wider FOV. This is because the two objective lenses of the binocular are farther apart. Unlike the measurement for exit pupil, FOV varies from one make and model to the next. Not all 8x42s have the same FOV. Although all binoculars can focus on the distant horizon and the moon, there are limitations as to how closely they can focus. In general, a binocular's close focus won't be affected only by its power, though models with a larger objective lens (and thus a longer barrel) often have a reduced ability to focus closely. When comparing Porro- to roof-prism models, that same feature which helps Porros get a wider FOV—widely spaced objectives—hinders their ability to focus closely. The more closely set barrels of a roof-prism binocular make this an ideal design for close focusing. Eye Relief Particularly for the eyeglasses wearer, it's important to note a binocular's stated eye relief. Eye relief is the optimal distance (measured in millimeters) that the ocular lens should be held from the surface of the eye (Fig. 4). Imagine a movie projector displaying an image on a screen. If the ocular lens is that projector and your eyeball is the movie screen, then the optimal distance from the projector to the screen is the eye relief. The optimal distance from lens to eye is a fxed fgure on a given set of binoculars, regardless of whether you have the eyecups twisted down or fully extended. This is important to consider for glasses wearers because glasses limit the ocular lenses' ability to get close to your pupils. Simply stated, most eyeglass wearers get a more comfortable view from bin-

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