Generally, the range in magnification for hand-held binoculars is from 6 to 10 in power. In a binocular designation (7 X 35, for example) the first number indicates the magnification, or how much larger, or closer, the object will appear than seen with normal vision.
When considering magnification, more is not necessarily better. As magnification increases, brightness and clarity may diminish, depth of field can become shallower and the field of view is usually more restricted. More noticeable and disturbing at higher powers are fine hand tremors and the effects of atmospheric conditions, such as the distortion caused by heat waves.
If your observation is done primarily at close range, such as in woodland areas or in your backyard, then a good 6, 7 or 8 power binocular might be the best choice. This range of magnification generally gives you a larger picture (wider field of view) which is especially important for viewing objects relatively close at hand. Also, binoculars of this magnification usually deliver better performance under conditions of low available light, due either to the time of day, weather conditions, or shadows caused by dense vegetation or other structures. This same type of binocular also works well for fast-moving action like sporting events, since the wide field of view allows the action to remain in the viewing area.
For long distance viewing or where greater detail is required, a higher magnification of 8, 9 or 10 should be considered. For example, the demands of observing in wide open terrain with little cover are best met with a binocular of 9 or 10 power. This generally holds true for situations where there is a need for critical field mark identification, as in observing raptors and shorebirds or when the object or animal is difficult to approach. Magnification, as a binocular parameter, should be considered as it relates to other factors such as aperture size, exit pupil, hand-held stability, atmospheric conditions, available light, optical design and the weight of the binocular.
The second number of a binocular designation refers to the diameter, in millimeters, of the front, or objective lens. The diameters usually range from 20 to 50 millimeters and this number will almost always be directly related to the size of the binocular. So called "giant binoculars", used mainly for astronomical purposes, may have up to 70 or 80mm objectives, while compact models will usually be 20 to 25mm in diameter. The objective lens size, or aperture, determines the amount of light that will enter the optical system. The common assumption that the size of the objective lens will determine the field of view is seldom true as field of view is controlled largely by the optical design of the binocular.
A larger objective lens will gather more light and theoretically provide greater detail and clarity of the image. This is especially true under low light conditions. Since the amount of light that will enter the objective lens will vary by the square of the change in the radius, a small difference in objective lens size will have a greater impact on the light gathering ability than one might first suspect. Once the objective gathers the light into the binocular, other factors determine how much light is transmitted through the optical system and all of these factors, including the aperture, combine to determine the brightness and clarity of the image you actually see. These other factors include magnification, exit pupil size, eye pupil size (controlled largely by the amount of available light), the presence and type of anti-reflection coatings used, and the size and quality of the optical glass and prisms used in the construction of the binocular.
Field of View
When looking through a binocular, the widest dimension of circular viewing area that you can see is described as the field of view. This is usually measured either in terms of linear feet at 1000 yards or in terms of angular degrees. Each degree of field corresponds to 52.5 feet at 1000 yards and binocular fields of view will generally range from 5 degrees (263 feet) to 11 degrees (578 feet). As a general rule, the field of view will decrease as the magnification increases so a 10 power binocular will usually have a smaller field of view than a 7 power. Field of view, however, is mainly determined by optical design of the ocular (eyepiece) and rarely a function of the size of the objective lens.
For observing at close quarters in deep woods, scanning the sky for raptors or large flocks of migratory birds or for picking up fast moving objects, a wide field of view is desirable. Wide-field binoculars, however, tend to be heavier and bulkier than binoculars with standard fields (because they employ larger prisms and eyepieces) and on many models there is a noticeable loss in sharpness at the edge of the field. Also, if you have too large a field (9 to 11 degrees, for instance) the object you want to observe may become lost amid the confusion of its surroundings. Another problem is that people who need to wear eyeglasses when viewing will usually have difficulty seeing the complete field with wide-angle optics. Wide field binoculars are generally the most popular for nature observation but you should consider all these factors in your evaluation of this feature.