ABC’s of Binoculars
A good pair of binoculars is your most important tool in identifying a variety of birds. If you have never used good binoculars for this purpose, you will be amazed at the detail and color you will see. You will be exposed to a whole new world of fascinating observation.
There are several characteristics shared by all binoculars that are well suited for bird watching. Your old pair of opera glasses does not have any of them. (Well, I guess they are light weight.).
We elicited the assistance of Sharon Stitler (aka the Bird Chick) for information on binocular selection.
For ready reference, here are some of the items covered by Sharon.
Most binoculars will have a series of numbers printed on the body of the binocular, usually just below the eye piece. You will see a number like 7 X 35 or 8 x 42 or 10 x 50.
The first number, the 7, 8, or 10, is the "power" or magnification of the binocular. Objects seen through a 7x binocular will appear 7 times closer than they really are. Objects seen through a 10x binocular will appear 10 times closer than they really are.
Beginning birders sometimes think that high powered binoculars (such as 12x or more) sound better, but most bird watchers prefer a 7x or 8x binocular. Lower powers do not provide enough magnification, and higher powers have a narrower field of view (making it harder to locate a bird using the binoculars) and can be difficult to hold steady.
2. Exit pupil - Light gathering capability
The second number is the diameter of the objective lens, which is the lens on the big end of the binocular. This measurement is given millimeters. Thus a 7 x 35 binocular has a magnification of 7 times, and an objective lens with a 35 mm diameter.
The diameter of the objective lens has an effect on the "light gathering" or image brightness of the binoculars. In general, the larger the diameter, the brighter the image will be. More specifically, the diameter of the objective lens divided by the magnification provides a good indication of the potential light gathering capability of the binocular. The higher the number the better, in terms of a bright image.
Using this formula for a 7 x 35 binocular means dividing 35 by 7, yielding a ratio of 5.0, which is considered good. Avoid binoculars that have a ratio much below 5, such as a 7 x 30 or 7 x 25.
3. Field of view
Another number that is usually printed on the body of the binocular is the field of view. A large field of view makes it easier to locate a particular bird. You will usually see the field of view measured either in terms of viewing angle (6 degrees, for example), or the number of feet (such as 400) at 1000 yards. Avoid binoculars that have a field of view under 6 degrees or less than 300 feet at 1000 yards.
4. Close focal distance
Another key factor in selecting the right binocular is the shortest distance at which the binoculars can focus. Some of the less expensive binoculars available in the department and discount stores will only focus down to about 30 feet or more. There will be many occasions when you will want to be able to focus to a much closer distance than this. Some binoculars designed for bird watchers can focus down as close as 5-6 feet. These are also excellent for watching dragonflies and butterflies, something many birders eventually become interested in.
The close focus distance is not usually marked on the binoculars. You'll have to test them yourself, ask the person from whom you are purchasing the binoculars, or consult the detailed product specifications.
5. Eyeglasses relief
The eye relief measurement determines how far your eye can be from the eyepiece while still allowing a full field of view through the binocular. This is important to eyeglass wearers. Look for a high eye relief design if you plan to wear glasses while using your binoculars. Eye relief of 15 mm or greater is recommended to provide you with the widest possible field of view.