You do not always want everything in your photographs to be in sharp focus. By using selective focus, you can emphasize the main subject and draw attention to it. Selective focus means the use of a shallow depth of field to isolate or emphasize the subject (fig. 4-11). Selective focus is the control of the zone of sharpness, or depth of field, in your photographs.

Once the lens has been focused on the main subject of the picture, using a progressively larger aperture (f/stop) will reduce the zone in front of and behind the subject that is in focus. Long-focal-length lenses are more effective for selective focusing because of their larger real apertures. Wide-angle or short-focal-length lenses are not as effective for selective focus because of the great depth of field they provide at most apertures. The following factors provide the maximum selective focus control by minimizing depth of field:

Working close-up

Using a wide aperture

Figure 4-12. Iris diaphragm.

Using a long-focal-length lens Focusing on near objects