There are several factors that must be included when you are considering the characteristics of lenses. To perform well as a Navy photographer, you must recognize the effect of these lens characteristics. Realize also that it is the recognition and use of these various lens features and/or qualities that can make the difference between good and poor photography. You must learn to recognize the photographic effect of these characteristics and be able to apply them to produce top quality photography. Finally, you must learn how some of the lens characteristics may limit photographic quality or operational capability.

Lens Focal Length

In photography, lens focal length is the distance between the optical center of a lens and the focal plane (film plane) of the camera when the lens is focused at infinity (fig. 1-17). To understand this definition, you must fully understand the terms focal plane, optical center, and infinity.

Figure 1-17. Focal plane and optical center.

Figure 1-18. Effects of lens-to-subject distance on light rays.

Focal plane-The surface (plane) on which an image transmitted by a lens is brought to sharp focus; the surface or area at the back of the camera occupied by the film.

Optical center-The optical center of a lens is a point, usually (although not always) within a lens, at which the rays of light from two different sources entering the lens are assumed to cross.

Infinity-This term is not easily described. When light is reflected from the point of an object, the closer the point is to the lens, the greater is the angle of the spread of light rays from the object (fig. 1-18). As the object point gets farther away from the lens, the angle of spread becomes less and less until a distance is reached at which the rays from a single point, for all practical purposes, can be considered parallel. This distance is known by the term infinity. For all practical purposes, light rays from a distant object or an object at 600 or more feet away may be considered to be parallel. But this is only for practical purposes. When very long focal-length lenses or telephoto lenses are being considered, the distance of 600 feet may be much less than infinity. In other words, infinity is a distance so far removed from the camera lens that the rays of light reflected to the lens from a point at that distance may be regarded as parallel. Infinity is expressed by the symbol and is a setting on a camera focusing scale.

The manner in which light rays are refracted by a lens determines the focal length. This refraction, in turn, depends on the nature of the glass used in the elements, the curvatures of the element surfaces, and the separation of the elements. The first two factors are fixed quantities once the lens is manufactured, but the third factor may be changed individually in certain lenses.

In zoom lenses the distance separating the lens elements can be changed. In convertible lenses, portions or elements of the lens can be used by themselves. In either method, the focal length of the lens can be changed. When one of these two conditions cannot be met, the focal length is fixed and constant.

Photographic lenses are measured according to their focal length which is normally imprinted somewhere on the lens mounting (usually the front surface of the lens barrel). This focal length information is sometimes given in inches, sometimes in millimeters, and occasionally in both systems. Focal length is frequently used to indicate the size of a lens. Thus, a lens labeled as an 8-inch lens indicates that when it is focused on a point at infinity, the distance from its optical center to the focal plane is 8 inches.

The focal length of a photographic lens dictates the size of the image produced by the lens at a given lens-to-subject distance. Focal length also determines the minimum distance between the lens and the focal plane. The normal focal length of a lens (normal lens) for a camera is approximately equal to the diagonal dimension of the film being used. Since the diagonal dimension of a 4x5 film is 6.4 inches, a lens about 6 inches is a normal lens for such film.

Lenses with a longer than normal focal length may be used on a camera, provided the distance from the lens to the film can be increased sufficiently to accommodate the increase in focal length. Lenses shorter than the normal focal length may also be used, provided they are designed to meet the constraints of the camera and film size.