Mid-sized Binocular Roundup

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The Mid-sized Binocular Roundup by Michael and Diane Porter is featured in the November/December 2010 issue of Bird Watcher's Digest. An excerpt is provided below. BWD/eBWD subscribers may read the full article online.

Sometimes small is beautiful and less is more. Smaller binoculars may let you see more because you're more likely to have them with you when you need them. And when you're traveling and may not always want high-value optics dangling from your neck, it's nice to have binoculars that will tuck into a purse or cargo pocket. You never know when a birding opportunity will arise.

Mid-sized binoculars—especially those that are 8x32, meaning 8 power with 32-mm objective lenses—are becoming increasingly popular, and there are good arguments in their favor. Yes, it's true that larger objectives can theoretically deliver higher resolution images. You can see this easily when comparing 60x spotting scopes. But in 8x binoculars, it's hard to detect a qualitative difference between 42-mm and 32-mm objectives. At 8x, the quality of the optics matters much more than the size of the lenses.

Brightness

While 42-mm objective binoculars can gather more light, they won't necessarily look any brighter than 32-mm binoculars. Here's why.

If you hold your binoculars away from your eyes and point them at a window or other light source, you'll see a small, bright circle in the center of each eyepiece. That's the exit pupil, the column of light coming out of the eyepiece. In ordinary daylight conditions, your eye's pupil contracts and may be smaller than the binoculars' exit pupil. Much of the light column does not enter the eye and thus cannot contribute to the brightness of the image. You pay more for the larger, 42-mm binoculars, but as far as brightness is concerned, they're just extra weight. Larger binoculars might be brighter when it's very dark, but only if your eye's pupil can dilate larger than the binoculars' exit pupil. Because as we age our pupils lose some of their ability to dilate, somewhere in midlife you may find that you don't gain anything from carrying full-sized binoculars, even at dusk. You may see just as well with a mid-size model.

A legitimate concern with smaller-exit-pupil binoculars is that it takes more precision to position them exactly in front of your pupils. Full-sized binoculars are more forgiving because of their larger exit pupils. They're easier to use on a boat, or by a person whose hands are unsteady. Other than that, mid-sized binoculars may be an ideal choice.

A Wider View

Another advantage of mid-sized binoculars is that 8x32s often have a wider field of view than the 8x42 version of the same model. For example, the Zeiss Victory 8x42 FLs have a 405-foot field of view. The 8x32s have a 420-foot field of view. A wider field of view makes it easier to spot a bird and provides a more immersive, big-picture experience. Surveying the Field To give an idea of what's available today in mid-sized binoculars, we tested 35 models, with 30- to 36-mm objective lenses, from 19 manufacturers. They included both roof- and Porro-prism binoculars and spanned the price range from high to low.

How We Tested

We set up the binoculars on tables overlooking bird feeders, a birdbath, trees and shrubs, open prairie, and a large pond. There were birds and other wildlife aplenty, close and far.

We could take our time picking and comparing among all the binoculars, steadying our elbows on the table and studying the images for a long time. We switched back and forth, comparing one model with another, discussing our impressions, disagreeing, debating, and testing again. Our conditions were much more satisfactory than the few minutes one usually gets when checking out optics in a store.

Resolution Testing

To test optical resolution, we set up a stable tripod with a beanbag rest. Across the room we posted a standard U.S. Air Force 1951 optics resolution chart that let us translate the level of detail we could see into a numeric score for each pair of binoculars. We also posted a dollar bill, the fine detail and high-quality printing of which made it an ideal graphics target. A ceiling-mounted halogen floodlight provided consistent lighting. We mounted a 2x eyepiece doubler behind the binoculars so we could see the resolution not just at 8x but at 16x. The doubler effectively gave us super eyesight and helped us distinguish fine differences that might be difficult to detect at 8x magnification. Below the chart we mounted a 4-foot ruler to compare field of view. We tested each model of binoculars many times over an interval of more than a month, and we averaged the scores. To minimize the effects of temporary eye fatigue, we took frequent breaks from testing and always had a pair of reference binoculars handy for comparison.

Team Test Day

To help us get a wider perspective, we opened up our "laboratory" to local birders ("Team Iowa") for a test day. We gave them access to the same viewing conditions and all the tools we had set up to compare binoculars. Their opinions, comments, and scores are integrated into the chart and individual reviews.

A Few Caveats

Even resolution scores, which we measured carefully, can have a margin of error. The difference between 5.0 and 4.8 may loom large on a chart, but such scores are actually not far apart. In the field, your eyes may not see any difference. Another caution is that we had only a single sample of each binocular model to compare, and it might have been atypical. Variation in quality is less likely with expensive optics, as stricter manufacturing tolerances are part of why they cost so much. But you never know.

BWD/eBWD subscribers, see the roundup results in our November/December 2010 digital issue >>

More features from this issue >>