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Experts say you should look for the following when buying binoculars:
* A magnification of 7X or 8X is usually best for general use. Binoculars with a magnification of 10X or more can be difficult to hold and may require a tripod and/or shoulder harness.
* 32mm and 42mm diameter binoculars are the most popular. Compact binoculars fall into the 20mm to 30mm range, midsize around 32mm. Full-size 40mm to 42mm binoculars are generally used for birding and hunting; 50mm and above are most often used for boating and astronomy. A larger diameter makes for a heavier binocular.
* The wider the field of view specification, the more you'll see. Binocular reviews recommend a minimum field of view of 300 feet. Beginning bird watchers need an especially wide field of view, which makes it easier to track birds in flight.
* Experts recommend an exit pupil of 2mm to 4mm for day use, 5mm to 6mm for low light and at least 6.5mm for night use, although this depends partly on how well your own eyes adapt to dimmer light. The exit pupil is just the ratio of the aperture to the magnification, and it is only a rough guide to the image brightness in various conditions since optical quality can affect brightness more.
* Don't settle for less than fully coated lenses at the very lowest prices; multicoated lenses (MC) are better, while fully multicoated lenses (FMC) are best, meaning that all lenses are multicoated on both sides. These coatings improve brightness, contrast and color accuracy.
* Look for a center focus knob with adjustable diopter. The diopter control adapts the focus to accommodate to natural differences between your eyes, so that one central focusing knob controls the focus once the diopter adjustment is made. Individual focus is too slow and awkward except possibly for astronomy binoculars, while auto-focus and fixed-focus binoculars aren't sharp enough.
* Especially if you wear glasses, look for an eye relief of at least 14mm, as well as adjustable twist-up eyecups. Eye relief refers to how far back from the eyepiece your eyes can be and still see the entire field of view. Short eye relief could mean that you'll only see the center part of the image if you wear glasses, and is generally less comfortable even if you don't wear glasses. Twist-up eyecups are better than fold-down eyecups.
* Consider a tripod socket. Some binoculars lack this feature. Others have a socket but require relatively expensive adapters in order to use it.
* Look for Bak-4 rather than Bak-7 glass in prisms. Also, for roof-prism binoculars, look for phase-coated (PC) prisms, recommends the buyer's guide at Birding.com. The coating solves a problem in roof-prism binoculars that isn't present in porro-prism binoculars.
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