Comparing Birding Binoculars

by Michael Porter

(Excerpts from a 1995 article in BirdWatcher's Digest)
How to compare binoculars
Checking for alignnment

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?

Here are suggestions to 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.

Field test results
When we tested all the birding binoculars from the top manufacturers, we were surprised 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.

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.