Shopping for Binoculars
The December, 1995, newsletter included a short blurb about the quality of binoculars, noting there are inferior products on the market, just as there are inferior telescopes. With decent telescopes being relatively expensive, one good way to become an active amateur astronomer is to begin observing with a binocular. (The correct term for one of these things is a binocular, but habits are hard to break, so you'll forgive my occasionally slipping in the plural binoculars.) Telescopic observers also regularly use binoculars for wide-field locating of objects. Here are some thoughts on being a smart shopper.
Below is a simplified diagram of a "typical" binocular. The prisms perform two functions: they increase the effective distance (baseline) between your eyes and they fold the light path to reduce the size of the binocular. Specifications for binoculars are given with the magnification first, followed by the diameter of the front (objective) lenses in millimeters. For Example, 7x50 means a magnification of 7 power, with objective lenses of 50mm. The larger the lenses the brighter the objects will appear, and the higher the magnification, the more you will be able to see of those objects. But this comes at a price -- namely, increased size and weight. Consider that if you're going to be hand-holding the binocular looking at objects in the sky, your arms are going to get pretty tired. On top of that, you won't be able to hold the image steadily, especially with higher-powered binoculars. Opinions vary, but most seem to agree that about the largest size that's appropriate to be hand-held is a 10x50. Anything much larger really requires tripod mounting for extended use.
Inferior binoculars look pretty much like good ones. Some lower end products even have colored lenses to lead you to believe you're getting special high-transmission coatings. In fact, all you may be getting is a colored coating that does more to detract from the image than to enhance it. One advantage with choosing binoculars is that because of their relatively low power and wide field of view, you can test them before you buy, whereas there's seldom any way to pre-test a telescope under night sky conditions. A good test is to look out the store window at the license plate of a car or a small sign. The farther the target the better. If numbers and letters can be brought to a crisp and clear focus while using both your eyes, then the binoculars are probably okay. If a binocular gives a good terrestrial view, then scanning the night sky will probably also be a satisfying experience. The most demanding requirement of binoculars is that the two barrels must be perfectly aligned so that both your eyes are seeing essentially the same image. Actually, the images aren't identical, especially on nearer objects, because there's parallax caused by the separation of the barrels. This is one advantage of binoculars -- they provide a three dimensional view of near objects akin to the way you normally look at things using both your eyes. If you have to close one eye in order to get the best view, the binocular is a bummer. Also, make sure there is a diopter adjustment; this compensates for the fact that our eyesight is different in each eye.
You would probably do better by avoiding low end brands such as Tasco, Jason, or the like. (Coincidentally, these are the same outfits that peddle inferior telescopes.) Bushnell (Bausch & Lomb) manufactures some fine binoculars, but they also have low end products that leave much to be desired. Celestron makes good binoculars, as do the camera manufacturers (Nikon, Pentax, Canon, Fujinon, etc.). While you don't have to pay $500 to $1000+ for Swarovski, Zeiss, or Leica -- the Rolls Royces of binoculars -- price is a fairly good indicator of quality. But beyond a certain point, the quality isn't apparent except to those who use them under the most demanding circumstances. The more expensive binoculars use high-transmission lens coatings that reduce the amount of light loss. They also employ more distortion-free lenses and higher quality prisms such as the BAK4 type. And they tend to be more durable. When shopping for a pocket-sized binocular, I couldn't get a good image on anything under about $60; expect to spend at least $100 for standard design 7x35's. The problem is that there are exceptions. Manufacturers can keep costs down by stinting on quality control. Some people have gotten pretty good binoculars at very low prices, but another pair of the identical product may give horrible images. Lack of consistent quality is the problem. If you find an inexpensive binocular that gives a good image, insist on buying that one -- don't walk out of the store with the identical product that was in a box on the shelf. Finally, from personal experience, it's probably best to buy your binocular at a camera store or other optical goods retailer -- places where they have a genuine commitment to the quality of the products they sell.Published in the March 1996 issue of the NightTimes