Focusing in on the right binocular
William J. Cook
NOT LONG AGO, one of the readers of this magazine wrote to the editors asking for help. She wanted to give her son-an avid birder-a new binocular. But after a visit to a local store, she came home so confused by all of the makes, models, magnifying powers and sizes of the products she saw that she didn't buy anything. Instead, she e-mailed the magazine, looking for guidance. Her problem is not unique.
For people who are looking to get a little closer to nature these days, the choices can be overwhelming. With the recent boom in birding and wildlife viewing, many stores are carrying a dizzying array of binoculars in a wide variety of prices. Indeed, one mail-order retailer alone that specializes in optics for bird-watching and nature viewing carries some 270 different models, ranging in price from $39 to $1,400, made by 18 separate major manufacturers-and that's only a relatively small percentage of the total number available. Which one is the right one for you?
There are a number of things you should consider before buying a binocular. Most importantly: What do you want to do with it? Are your sights set on high-flying hawks, finches at a backyard feeder, humpback whales feeding off the coast of Maine or leopards hunting the African plains at dusk? Determining what you want to be able to see through your binocular is the first step toward making a purchase that will last a lifetime.
Next, consider how much money you can afford to spend. In general, price tends to parallel performance, but there are a growing number of solid binocular options available at the lower end of the price range. The better the quality of the materials used-i.e., lenses, prisms, optical coatings-the higher the cost, but the clearer the image. The following tips may help you find the right product for your budget.
The first thing you will notice when you begin to browse binocular cases is that each binocular bears a set of numbers. For instance, 7x35 or 8x42. The first number is the binocular's power, or magnification. The second number is the aperture-the size of a binocular's front, or objective lenses.
In most binoculars, power ranges from 7x to 10x. The higher the number, the closer the object you are viewing will appear. For example, a 10x50 binocular magnifies images to ten times what would be seen by the naked eye. However, high-powered binoculars are not for everyone, especially first-time buyers. As power increases, several things happen: the field of view becomes smaller, making objects harder to find and keep centered; the brightness of an image decreases, because light is diffused over a greater area; external vibrations, body tremors and imperfections in the objective lenses and prisms are accentuated.
Many naturalists prefer a binocular in the power range of 7x to 8x. But hawk watchers and wildlife enthusiasts with steady hands (or a tripod) often opt for ones with a bit more magnification.
The second number, the aperture, commonly ranges from 20 to 50 millimeters. The greater the aperture, the more light will pass through the binocular, resulting in a brighter image. A large aperture is optimal for viewing wildlife at dawn or dusk when lighting is low. However, the larger the aperture the bigger the binocular.
In addition to binocular aperture, the types of glass, prisms and coatings greatly affect the brightness and clarity of an image, especially at the edge of the field of view.
All binoculars are essentially a pair of telescopes mounted side by side to give you a magnified, stereoscopic view. Each telescope has an objective lens, an eyepiece and prisms. The prisms are located between the objective lens and the eyepiece and act as mirrors that reverse an image and turn it right side up. Without prisms, a goldfinch at a backyard feeder would appear upside down and backward. Prisms also fold the light path, so that the overall length of a binocular can be reduced. The best prisms are made with BaK4 (Barium Light Crown) glass.
Most binoculars today have one of two types of prisms: Porro prisms or roof prisms. Porro prism binoculars have a traditional appearance, with the eyepieces offset from the objective lenses. Though bulkier than roof prism binoculars of the same aperture, these binoculars historically have provided slightly better depth perception.
The sleeker, more modern-looking roof prism binoculars have eyepieces that are inline with the objective lenses, making them more compact. Construction of these binoculars is somewhat more costly, but many buyers feel the reduction of size and weight justifies the cost.
The only way to discover your binocular preference is to try a variety of makes and models before you buy.
When you're ready to test the merchandise, try out binoculars from well-established manufacturers and be sure to pay attention to the following:
Focus: Most binoculars feature a centrally mounted wheel or lever that adjusts both eyepieces at the same time. Make sure all focus mechanisms move firmly but smoothly. Eyepieces that have to be focused individually are not convenient for nature viewing-the animal you're looking at often disappears before you have time to get it in focus.
Beware of "click stop" and lever-action focusing mechanisms. Click stop focusing can be an undesirable feature because it often forces the eyes to strain while trying to focus between the stops. Lever-action focusing mechanisms tend to lose their focus very easily.
No binocular can focus on objects closer than a certain distance. This distance is called a binocular's "close focus." With birding and butterfly watching becoming increasingly popular, manufacturers are continually producing models with the ability to focus ever closer. Today, it is not uncommon to find a binocular that will focus as close as 5 or 6 feet. While this is especially useful for observing small birds, butterflies and other insects, there is a downside. When observers focus up close, they may cross their eyes, making the field of view overlap and giving the impression that the instrument is out of alignment. But it's not. The problem is, in fact, with the observer.
Collimation: This is the alignment of one telescope to the other. If a binocular is not collimated, images will not be as sharp as they should be. Binoculars that are severely out of collimation create a double image and need to be repaired.
Even the best binoculars will lose their alignment over time. But buyer beware: Very inexpensive binoculars can be out of collimation straight out of the box.
Eye relief: If you wear glasses, be certain that the distance from the rear eyelens to the point behind the instrument where the image is formed is sufficient to allow you to see the full field of view. Generally15mm or more of eye relief is sufficient for most eyeglass wearers.
Eyecups: Many years ago, virtually all binocular eyecups were made of hard plastic. Today, most are made of rubber, and accommodate glasses by twisting, pushing or folding down. One style of eyecup is not better than another. It is strictly a matter of personal preference. Chose a binocular with eyecups that feel comfortable-and are soft enough to avoid scratching eyeglass lenses.
Waterproofing: Most water damage comes from moisture being drawn inside a binocular during times of rapidly changing temperatures, not from the instrument being splashed or sprinkled by rain. Therefore, the term "water resistant" has little meaning. Good nitrogen-purged waterproof binoculars will cost a bit more than a similar product that is not waterproof. These instruments are often well worth the price, considering that a thorough internal cleaning and the necessary collimation required to put a binocular back in service may be two or three times the cost of an inexpensive replacement.
This might seem like a lot of information to digest, but buying a binocular doesn't have to be confusing. If you stick with a well-known company that specializes in optics, ask a lot of questions and test the merchandise, you should end up with a binocular that's crisp, clear-and hard to put down!
William J. Cook has worked in almost every aspect of the optical industry, from binocular repair and restoration to serving as an opticalman chief for the U.S. Navy. He is currently the editor-in-chief of an international optical journal.