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Bird Watcher's Digest

Mid-Priced Binoculars Review

For Bird Watcher's Digest, January, 2012

by Michael and Diane Porter

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Michael and Diane Porter wrote the following review, which appears in the January, 2012, Bird Watcher's Digest.

This is page 3 of the review.

Why choose 42mm objective lenses?

More resolution. All other things being equal, the larger the objective lens, the better the resolution in the image. But you pay, with increased weight and size. How much resolution you need depends on how much you’re going to magnify an image.

For 8- or 10-power magnification, a 42mm objective lens provides plenty of resolution. There’s no need to exceed your eye’s ability to see. The reason people might choose larger objectives would not be for increased resolution but for more brightness when night viewing or star gazing.

Brighter sometimes. An 8x42 binocular creates a 5.25 mm exit pupil (42mm ÷ 8 = 5.25 mm). The exit pupil is the diameter of the column of light coming out of the eyepieces. A larger objective lens provides a wider column and more light entering your eyes when your pupils dilate at night.

Looking with 42mmThe average youthful eye pupil dilates only to about 7mm even in total darkness. As we age, our eyes’ ability to dilate gradually diminishes, so a 5.25mm exit pupil probably delivers all the light your eye can use, even in dim-light conditions. Therefore, a 42m m objective is a good, practical choice.

In daylight, when your pupils contract to about 3mm, most of the light coming out of the binocular will fall outside the pupil and never enter the eye at all. Making the exit pupil even larger won’t make the image look any brighter.

Easier to use. However, an exit pupil that is bigger than the pupil of your eye is sometimes useful. It’s easier to position your eyes in the column of light. Helpful if you have shaky hands or you’re on a boat!

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