Special Report:What's New In Binoculars? Page 4

Special Report:
What's New In Binoculars? Page 4

Exit Pupil: This is the size of the circle of light that reaches your eye. The larger the exit pupil, the brighter the view and the more effective a model will be in very low light. It can be calculated by dividing the size of the objective lens by the magnification of the binoculars. (For example, a 7x50 pair has an exit pupil of 7.1mm.)

Image Brightness Ratings: In order to determine the actual light gathering ability of any model, check the specs for "Relative Brightness Index." An index around 10 is fine for daytime, but for frequent viewing in low light, you'll want a model with an index of 25 or higher. Some manufacturers provide data on the "Twilight Factor" instead. An 8x25 model may have a twilight factor of 14.14 while the factor for 7x50 binoculars may be 18.71. Binoculars with a factor of 16 or higher are particularly useful in low light.

Optical Quality: Like photographic lenses, some binoculars include aspherical elements for a clear, sharp view with little distortion. A few premium-grade models include a "low dispersion" glass element, useful especially in high-powered binoculars for excellent contrast, clarity, and color fidelity.

Field Of View (FOV): A side to side measurement of the actual area visible through a pair of binoculars, the FOV is stated for a focused distance of 1000 yards. The higher the power, the narrower the FOV, of course. The 8x binoculars intended for general viewing often have an FOV of 300-375 ft. Some manufacturers provide the angle of view, or "angular field," instead, with 5-6Þ being common. Multiply that by 52.5 to get the FOV in feet. Models with an FOV of over 400 ft are particularly useful for scanning a large area or following a moving subject.

Eye Relief: Measured in millimeters, eye relief refers to the maximum distance from the eyepiece that will make the entire field of view visible. With some binoculars, you must hold the eyepiece very close to your eye in order to see the entire field of view. Other models include "long eye relief"--at least 15mm--useful for comfortable observation for anyone who must wear glasses while viewing. Of course, you may not need to wear glasses if you select binoculars with individual diopter adjustment controls for each tube; this feature works well for many near-sighted individuals, though not for those with astigmatism.