The Aging Eye
from "Hands-on field Tests"
in Bird Watcher's Digest, January, 2005
by Michael and Diane Porter
Explaining why you may see just as bright an image through an 8x32 binocular as you do through an 8x42.
THE EYE OF THE BEHOLDER
As we age, it’s normal for the maximum pupil size of our eyes to shrink. Imagine Sonny, Dad, and Grandpa at midday, looking at the ducks on the lake. At age 20, Sonny’s pupils might be as much as 4.7mm in diameter. Dad, age 50, would show smaller pupils of around 3.5mm. And Grandpa, at 80, would have the smallest pupils, perhaps only 2.3mm.
If they stood there until dusk and into night, the differences between their eyes would become even more dramatic. Over the course of an hour, Sonny’s pupils would probably expand until they were 7 or even 8mm, whereas Dad’s would grow only to 5mm. Grandpa’s pupils would barely react to the darker conditions and might get no larger than 2.5mm.
The chart at right shows how the pupil becomes smaller, both in daylight and in darkness, as we age. The numbers represent averages and are only approximate. Not everyone’s eyes change at the same rate.
Binoculars have a pupil size too. It’s called the exit pupil. If you hold your binocular away from your eyes, toward the sky or a bright window, you see the exit pupil as that little circle of light coming out the eyepiece. That’s the light your eye will receive when you look through the binocular.
Midsize binoculars deliver smaller exit pupils, but that may not make any difference. If the pupil of the eye is contracted to a smaller diameter than the exit pupil of the binocular, the light falling outside the area of the pupil cannot be seen. So for most people, in bright daylight a mid-sized binocular will seem just as bright as a full-sized binocular. For an older person, there might not be much difference even at dusk.
When things are jiggling around, however, it can be useful for the exit pupil to be a little larger than the pupil of your eye. If you’re bouncing about in a boat or being buffeted by wind, a binocular with a larger exit pupil helps you keep the binocular positioned where you can see the image. A larger exit pupil can also benefit a person whose hands are unsteady.
The best way to determine whether a mid-sized binocular will give you an optimum birding experience is to try it out in person. Try it in daylight. Try it at dusk and in the darkest conditions you expect to be using it. Remember that it takes at least a half-hour for the eye to become fully darkness adjusted. You can’t just switch out the lights and immediately compare the brightness of two binoculars.
In daylight tests our judges found little or no brightness advantage in the full-sized binoculars over mid-sized ones of comparable quality. In the dim light of very early dawn, with eyes fully darkness adjusted, we found that most of the full-sized binoculars did give us a brighter and more detailed image than the mid-sized ones.
But don’t assume that a larger binocular will necessarily give you a brighter picture. The advantages of a smaller, lighter binocular are impressive enough to make it worthy of serious consideration. Another approach is to own two binoculars, a mid-sized binocular for daytime birding and a full-sized one for dusk or night excursions.
Before you leap, let's talk about something that can make or break your enjoyment of any binocular--eye relief.
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