THE CAMERA-TO-SUBJECT DISTANCE. -

The closer the action is to the camera, the faster the shutter speed must be. A car traveling 60 miles per hour across the lens axis at a distance of 100 feet would be "stopped" by a shutter speed of 1/1000" (or perhaps 1/500"). However, if the camera-to-subject distance were increased to 500 feet, the action could be "stopped" with a shutter speed of "1/250" or" 1/125." If the car was a half mile away, 1/60" should be sufficient to stop the movement.

DEPTH OF FIELD

Selection of a f/stop is done mainly for the desired depth of field. "Depth of field" is defined as the distance between the nearest and farthest points of acceptable sharp focus of the scene photographed (fig. 11-22).

Control of the depth of field is a valuable tool in photography. Depth-of-field charts are given in all camera instruction books as well as in photographic reference manuals, but many photographers fail to use them to their own advantage.

Simply stated, depth of field increases as the focal length of the lens decreases (a shorter focal-length lens is used), as the lens aperture decreases (gets smaller in size) and as the distance focused on (focal point) increases, or both. Inversely, depth of field is less for long-focal-length lenses than for short-focal-length

Figure 11-21. - Slowest shutter speeds necessary to stop action.

Figure 11-22. - Depth of field.

lenses, is less for wider apertures and is less for shorter lens-to-subject distances.

A peculiarity of the term depth of field is that it is usually used to define a condition of maximum depth of field; and when the depth of field is shallow or purposely restrictive, the term selective focus is used. Selective focus is merely "selecting" a depth of field that will satisfy a requirement to have the foreground, background, or both, not in sharp focus. The use of selective focus to make the main subject standout in the picture by being "sharp" while the rest of the image area is blurred is a good technique for gaining subject emphasis.

The importance of proper focus and shutter speed cannot be overemphasized. Incorrect focus, subject movement, camera movement, improper use of depth of field, and so forth, can result in a blurred image on the photographic negative that cannot be connected in any way. Film latitude can take care of minor errors in the exposure, but there is no latitude for focus and stop action. An image is either in focus or out of focus; action is either stopped or blurred.