For any given scene, there are usually multiple combination of aperture and shutter speed that will deliver a correct exposure.

Some digital cameras have Program AE mode, indicated by a P on the camera mode dial. When in P mode, the camera selects the shutter speed and aperture automatically, but allows the user switch between those combinations once the exposure has been evaluated. The changes are usually made by half-pressing the shutter button. This is known as Program Shift.

Program shift is useful because it lets the user’s creativity come into play in a simple and reliable way. It makes the selection of various aperture and shutter speed combinations easier for the photographer, while still ensuring accurate exposure.

For instance, a scene can be photographed with an f-number of f/2.8 and a shutter speed of 1/500 seconds. It can also be shot with an f-number of f/5.6 and a shutter speed of 1/125 seconds.

The first settings combination allows for a shallow depth of field and makes it easier to photograph moving subjects. On the other hand, the latter settings ensure that more of the scene is in focus. But both images will be exposed similarly.

Check your manual for camera-specific instructions for using this very powerful, often overlooked, tool for photographers.