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Birding Binoculars

Part 4

Power and Light

Every pair of binoculars is engraved with a formula, such as "7 x 35" or "10 x 42." The first number in the formula is the power, or how many times the image is enlarged. With hand held binoculars, as with most things in life, there is a practical limit to power beyond which it is not useful. Depending on the individual, as the power increases, hand tremor begins to degrade the image. Binoculars over 10 power usually require tripod mounting.

The second number in the formula is the diameter of the objective lens in millimeters. The bigger the objective, the more light can enter, and the greater the potential resolution of the image.

Low-light performance is largely dependent on the exit pupil. Exit pupils are the small, bright circles you see in the eyepieces when you hold binoculars away from your eyes and up to the light. They are the actual beams of light coming out through the eyepieces. The exit pupil is calculated by dividing the diameter of the objective lens by the power. A 7 x 35 binocular has an exit pupil of 5 millimeters (35 ÷ 7 = 5). A 10 x 32 binocular has an exit pupil of 3.2 millimeters (32 ÷ 10 = 3.2).

At noon, the pupils of your eyes contract to 2 to 4 mm, and at night they may open to 7 mm. If the beam of light exiting the binoculars is wider than the pupil of the eye, the excess doesn't get in: the eye can't see it. During daylight hours things look just as bright through binoculars with 4 mm exit pupils as through those with 7 mm exit pupils. In fact, if they have better coatings, binoculars with 4 mm exit pupils will be brighter.

It's in low light that the larger exit pupil is an advantage. For astronomy, an exit pupil of 7 mm is standard. For birding purposes a 6 mm exit pupil is usually large enough for even the most demanding low-light condition. For daylight viewing, even smaller exit pupils may be more than enough.

As we age, the eye loses its ability to dark adapt. While a 20-year-old person's pupils might open to 7 mm, at 50 years the pupils may open only to 5 mm. Therefore, binoculars with large exit pupils may not help the older birder.

The best birding binoculars are bright as a result of their advanced multi-coatings and top quality optics, which provide brightness you can see all the time, even in daylight.

Will binoculars with a larger exit pupil improve your ability to identify birds at dusk? The best way to tell is to try them, using your eyes.

—Michael and Diane Porter

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