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

Figure 1-21. Angle of field.

closely resembles the central vision coverage of the human eye. Wide-angle lenses have a large or wide angle of field Long focal-length lenses (often called telephotolenses) have a narrow angle of field (fig. 1-21).

Angle of View. Angle of view determines the coverage of a lens with a particular size of film, with the lens-to-film distance remaining unchanged. Angle of view is an angle with the intercept point at the lens and its sides matching the corners of the film.

The angle of view (fig. 1-22) of a normal focal-length lens with a given film size can approach but never exceed the angle of field of the lens. Any lens recording an angle greater than 55 degrees with a given film size has a short-focal length and is called a wide-angle lens. Any lens with an angle of view less than 45 degrees with a given film size has a longer focal length.