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. FOCAL LENGTH AND IMAGE SIZE.–When you photograph the same object at the same distance, a lens with a long-focal length produces a larger image than one with a short-focal length. In effect, the longer focal-length lens magnifies or brings the subject closer to the camera without changing the camera-to-subject distance (fig. 1-19). For example, a man 6 feet tall stands at a distance of 25 feet from three cameras, one equipped with a 6-inch lens, one with a 12-inch lens, and one with a 24-inch lens. The 6-inch lens produces a 1 1/2-inch image of the man. The 12-inch lens produces an image that is 3 inches high. The 24-inch lens produces a 6-inch image. From this example, it is obvious that the longer the focal length of the lens, the larger the image size of a  given  object  from  a  given  lens-to-subject  distance. FOCAL LENGTH AND SUBJECT COVER- AGE.–Focal length and subject coverage go hand in hand-just as do focal length and image size. But, whereas image size increases with increased focal length, coverage decreases with increased focal length. We can consider coverage as the amount of subject matter included in a given format film size from a given lens-to-subject distance. With two cameras-each with a different focal-length lens-at the same distance from the same subject, the camera with the shortest focal-length lens includes the greatest subject area-the camera with the longest focal-length lens the least subject area (fig.  1-20). Angle of Field.–The focal length of a lens is a determining factor in the coverage of that lens. The maximum coverage at the focal plane of a lens is expressed in degrees as the angle of field. Angle of field is the widest angle at which light entering a lens produces a usable portion of the circle of illumination at the focal plane. Light around the edges of the entire circle  falls  off  in  intensity  before  disappearing completely. The usable portion of this circle is called the circle  of  good  definition. The maximum size of film you can use with a lens depends on the angle of field because any part of the film outside the circle of good definition produces an indistinct  image. Angle of field is a basic optical condition that is approximately  equal  for  all  normal  focal-length  lenses. A normal lens, as it is called, has an angle of field of about 45 degrees to 55 degrees. This angle of field 1-15