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Tuesday, March 12, 2013

Gadget Bag: Compact Binoculars

Get a different look at your surroundings and spot unique photo opportunities with a pair of lightweight, high-quality binoculars

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Kenko 10x25DH­; Nikon Premier LXL 10x25

Binoculars are often overlooked, yet very useful tools for outdoor photographers, and not just for wildlife specialists. For the latter, of course, it's a lot easier to pick up a pocketable 10-ounce pair of binoculars to scan for subjects than to pull out a two-plus-pound DSLR with a seven-pound 500mm supertelephoto lens. (Of course, if you do scan with your camera, you can just push the button to record an image when you find something interesting.) The binocs provide a sense of depth, due to their two-eye viewing, as well as a generally brighter image that makes it easier to spot near-hidden critters. But the binoculars also give the landscape shooter a different way of seeing scenes than the usual unaided-eye view—the narrow field of view tends to focus your attention on smaller segments of the landscape, as well as letting you see details you'd miss with the eye alone. You'll discover different pictures with binoculars than you would with your naked eye.

Binoculars come in a wide range of sizes and powers, but the 10x25 is a good choice for the outdoor shooter, as it's truly compact, yet provides ample magnification and usable brightness. The first number in a binocular's designation is its magnification; the number indicates how much closer a subject will appear than if viewed with the naked eye. In the case of our 10x25, that's 10 times closer, or 10 times larger, if you prefer to look at it that way.

Celestron Nature DX 10x25
The second number is the diameter of the front lens (the objective), in millimeters. Our 10x25 binoculars provide 10X magnification and have front elements that measure 25mm in diameter. All other things being equal, the size of the front element determines how much light your eyes receive when viewing with the binocs: the larger the objective, the brighter the image. Obviously, a brighter image is better, especially in dimmer light. But large-diameter objectives make for larger binoculars, and when binocs get too big, you tend not to carry them on photo outings. We think 10x25 is a good balance of portability, magnification and brightness (10X, by the way, is about the highest magnification that can be readily handheld).

Binocular specs can list relative brightness in a number of ways, but you can quickly calculate it yourself from the unit's designation. First is exit pupil diameter. If you divide the diameter of the objective (front lens) by the magnification, the result is the diameter of the beam exiting the eyepiece, one measure of the unit's brightness. In the case of our 10x25 example, 25/10 = 2.5. In bright light, the eye's pupils contract to 2.5-3mm, so a unit with an exit pupil of 2.5-3mm will produce an image that looks fine. In dimmer light, the eye's pupils can dilate to 7mm or more, in which case the image of a binocular with an exit pupil of 2.5 will look dimmer than one with a larger exit pupil. But the 2.5mm exit pupil of our suggested 10x25 unit should work well throughout the shooting day.

Swarovski Pocket Traveler 10x25
Another measure of relative brightness is, well, relative brightness. This is determined by squaring the exit pupil. In the case of our 10x25s, 2.5 x 2.5 = a relative brightness of 6.25. Yet another measure is twilight factor, determined by multiplying the magnification by the objective diameter, then taking the square root of the result. For our 10x25 example, 10 x 25 = 250; the square root of 250 (and thus the twilight factor of our unit) is 15.8. Note that all of these brightness measures are just guides. A really good binocular with great optics and coatings will provide a brighter image than a cheap pair with lesser optics and coatings.

Field of view is the width of the area shown by the binoculars at a given distance. Usually, this is given in feet at 1,000 yards or in meters at 1,000 meters. Wider fields make it easier to find subjects when you bring the binocular up to your eye, but wide-field units also generally cost more than normal-field ones.


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