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Comparing Birding Binoculars

by Michael Porter

(Excerpts from a 1995 article in BirdWatcher's Digest)

Our field testing
Field test results
The questionnaire results
How to compare binoculars
Checking for alignnment
My personal favorite

Birding and binocular technology have evolved together. Before the modern binocular was invented, often the only way to see a bird well enough to identify it was to shoot it. Field ornithologists studied the bird in the hand, because the bird in the bush was too hard to see.

Binoculars changed all that. They made it possible to observe birds alive and wild, without harming them. Now, millions of people are able to enjoy watching birds. All because bright, lightweight, and relatively inexpensive optics have magically extended our ability to see.

Today's birder has more binocular choices than ever before. And while it's wonderful to have so many options, it can also be a little intimidating. Buying a pair of binoculars is an important and expensive decision, and you want to get it right. After all, what other material possession will you depend on so often and carry so close to your heart?

This article will help you choose the binocular that's right for you. The bottom line is that the best birding binocular is a custom fit. Often the most important factors are the personal ones, such as how a binocular fits your hands, how much power you can hand hold, and how much weight you can carry comfortably.

Our field testing
With the help of our local Jefferson County, Iowa, naturalist, Therese Cummiskey, I organized a field test day at the local nature center and invited birders from around the state to participate. I got all the manufacturers to send examples of the binoculars they considered best for birders. The day of the test, I set the binoculars out on tables, buffet style. Outside the windows, the nature center's feeders were similarly stocked with seeds. There were lots of optics to test and plenty of birds to see. The participants filled out questionnaires about their personal reactions to the binoculars they tried.

I also set up testing stations. One station let people measure how close the binoculars would focus. Another was a bean-bag rest where two binoculars could be set side by side and focused on an optical resolution chart. The viewing target was a USAF 1951 Test Pattern chart obtained from Edmund Scientific Company. It shows a series of progressively smaller squares, each made up of three lines. With the binoculars supported on a rest, you focus as sharply as you can and determine the smallest square in which you can see the individual lines. The same pattern occurs numerous places on the chart so that the resolution at the edges of the image can be compared to the resolution at the center.

Field test results
This informal resolution test surprised us with how well all the binoculars performed. When we compared the resolution near the center of the image, called the on-axis resolution, the least expensive binoculars usually resolved just as small a target as the most expensive models of the same magnifying power. Of course, all the binoculars we were testing, including the cheapest, had been selected for their good optical reputations. We were really only comparing the good to the best.

It became clear that we were looking at the resolution limit of our own eyes. This varies from individual to individual and does not improve with age. However, there were some young, teen-age eyes at the field tests, and though they could resolve smaller details, they got the same relative results when comparing two models as did their older comrades. The interesting conclusion is that the on-axis resolution of most decent binoculars exceeds the ability of most eyes to see it.

The resolution at the edges of the image, called off-axis resolution, was a different story. Here resolution differences were more apparent. This is where the top-end optics excel, and it's probably one of the main reasons people consistently preferred the most expensive binoculars.

Of course, resolution is only one factor that affects the perceived image quality. Brightness, contrast, color accuracy, the width of the field of view, how much of the image you can see with your glasses on - all these elements affect the subjective experience. So many factors interact that truly objective comparisons may be impossible. This is why there is a continuing dialog among experienced birders about which binoculars are best. And it's why your personal experience looking though binoculars must be the ultimate test.

The questionnaire results
The questionnaire responses confirmed that ergonomics, the fit and feel, are enormously important. Different birders liked different binoculars. People commented on the weight, how smoothly the focus wheel turned, how well the binoculars worked with their glasses, and so on. Not surprisingly, most participants preferred the more expensive binoculars.

How to compare binoculars
Here is a checklist of questions to ask when comparing binoculars. The relative importance of the questions will differ for each individual.

  • Do they resolve fine details both at the center and at the edge of the image?
  • Do they display a bright, high-contrast image?
  • Do they show colors accurately?
  • Are all the air-to-glass surfaces coated, to cut down on internal reflections?
  • Are they a roof-prism or Porro-prism design?
  • If they're a roof prism design, do they have anti phase-shift coatings?
  • Do they have a relatively flat field of view? The image shouldn't seem to curve or bow.
  • What is their field of view? Do they show a big, eye-filling picture?
  • How easy is it to bring the image into precise focus?
  • How close can they focus?
  • Does the level of magnification match your ability to hand hold them?
  • How likely are you to have them with you when you need them? Do they seem bulky?
  • Can you carry them comfortably for hours at a time?
  • If you wear glasses, do they have long enough eye relief so you can see the whole picture?
  • Is it easy to roll down the eyecups?
  • If you plan to use them at twilight or dusk, how large are the exit pupils?
  • How do they fit your hands?
  • Is the surface pleasant to touch and secure to grip?
  • Is the focus knob located to suit you? Does it turn smoothly? Is there a lot of slop?
  • Where is the diopter adjustment? Is it likely to get accidentally bumped off its setting?
  • How rugged is the internal construction? Can they take heavy use?
  • Are they rubber armored?
  • Are they actually waterproof, able to survive immersion? Or just water resistant?
  • Are they nitrogen filled so they won't fog up internally?
  • How do they mount on a tripod?
  • Do they come with a case?
  • What are the provisions for protecting the lenses when in the field? Are they practical?
  • What's the warranty?

When you're ready to purchase, there is one important rule: try before you buy. The more time you spend handling and looking through the binoculars before you buy them, the likelier that you'll make a decision you'll be happy with.

If you can find a dealer who will let you return or exchange the binoculars, that's ideal. If a friend or someone in your local birding organization owns the model you are considering, talk to that person about the binoculars. He or she might even offer to let you try them out.

Checking for alignment
Each side of a pair of binoculars should be parallel with the other, so the two images perfectly overlap and are seen as one. If the binoculars were not adjusted properly at the factory or have taken a hard bump, the two images may be out of alignment. Your eyes will do their best to pull them back in line. This can cause fatigue or eye strain.

Here's a quick check for mis-alignment that you can do yourself. Look through the binoculars at a horizontal line, such as a telephone wire. Slowly move the binoculars away from your eyes until you see two images instead of one. The horizontal line should stay lined up. If the line appears higher in one circle than the other, the binoculars should be repaired.

The best binoculars are constructed to survive heavy use. They can take a bump or two and stay in alignment. It's a feature worth paying for.

My personal favorite (in 1995) and why
For me the Swarovski SLC Mark III 7 x 42 has all the details right. They show me a big, wide, bright picture even with my glasses on. The optics are impeccable, with fully multi-coated lenses, and phase-corrected roof prisms. They can hold their own with the likes of Zeiss and Leica, but they cost less.

The focus knob is a long knurled cylinder, easy to find and use, even with gloves. It is slightly recessed for protection. The close focus is 13.2 feet, and the adjustment is silky smooth. The diopter adjustment, on the front of the focus knob, is easy to see and use, and it locks in place.

The eyecups don't roll down, but push in or pull out in an instant. At the front of the bridge is a tripod mounting thread, a feature missing from many binoculars.

The design is exceptionally rugged, both inside and out. The soft, polyurethane armored shell is pleasant to touch and gives a non-slip grip. Completely waterproof and nitrogen filled, the binoculars will withstand immersion in water and won't fog up internally in humid weather. The lens protection system is the best-integrated design I have seen. The rain guards sleekly match the shape of the eyepieces and snug down neatly. Small lens covers slip inside the barrels to protect the objective lenses. These attach cleverly so that they pop off instantly but don't get lost. The system is so well thought out these binoculars do not need or even come with a case.

To underscore Swarovski's confidence in their design's ruggedness, they introduced a lifetime warranty program in 1995 that will fix anything that ever happens to them. It's limited to the original owner but other than that seems to cover everything except loss or theft.

Their only drawback is that they weigh 33 oz., which may be too heavy for some people. If you want 10 power, there is an almost identical 10 x 42 model that weighs 30 oz. There is also an 8 x 30 model that weighs only 19 ozs.

Update 2002 Since this article was written, Swarovski has come out with the EL binocular. The EL has all the best qualities of the SLC, but it's light weight, and it focuses close.

--Michael Porter


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