Bird watching or birding is an activity or sport that is popular among a small niche of people all over the world. National Parks and other undeveloped areas can form a natural bird sanctuary, as can your own back yard. While viewing birds by making them come to you with the aid of a bird feeder can be enjoyable, birders tend to like to observe birds in their wild state.
Because wild birds are often quite wary of people, getting close to them can be difficult. In order to observe birds in their element without disturbing them, it’s often necessary to watch them from a distance with bird binoculars.
While any binoculars can be used for observing birds, the most appropriate optics tend to have similar characteristics. To the uninitiated, binoculars can have a confusing system of numbers, and it can be hard to determine why seemingly similar optics can have such as wide variety in price.
Binoculars are generally classified as one number, an X, and another number, such as 7×35. In this system, the first number is the magnification, and the second is the size of the objective lens in millimeters. Dividing the first number by the second will give the exit pupil in millimeters.
The magnification is obviously the reason the birder is using the binoculars instead of just the naked eye in the first place, so it might seem that higher is always better. However, as magnification increases, so does difficulty in holding the optics steady. A device more powerful than a magnification of 10 is usually too shaky to use without a tripod, and seven or eight power is more standard.
The exit pupil is most significant in low light situations. In bright conditions, the pupil of the average human eye narrows to about 2 to 4 millimeters, but in low light, the eye’s pupil widens to let in as much light as possible. For young, healthy eyes, this can be as wide as 7 millimeters. Because the exit pupil on binoculars is determined by the size of the objective (front) lens, larger binoculars are better for low light. While 8×21 binoculars (with a resulting 2.625 mm exit pupil) would be fine for noon, something with 7×50 (or even larger) has a large enough exit pupil to use when there is little ambient light.
Other factors to consider include coatings and design of prisms. While things like ruby coating are mostly a gimmick, lens surfaces should be coated with magnesium fluoride to increase light permeability. The more surfaces that are coated, the higher the quality (and price) of the optics.
Prisms can be arranged in porro prisms, or roof prisims. Porro prisms are an older design, and while they are easier and cheaper to make, they result in binoculars with eyepieces that are offset from the objective lenses, and are therefore less compact. Roof prisms are more compact, but because they are more difficult to make properly, they will be more expensive if all other factors are equal.