Most electronic flash units can be operated in an automatic exposure mode. An automatic flash unit eliminates the need to determine the correct f/stop for each flash-to-subject distance, providing the subject is within the flash distance range of the unit.

On the front of an automatic flash unit, a sensor reads the light reflected from the subject that is produced by the flash. When this sensor is satisfied with the amount of light received, it automatically shuts off the flash. The closer the subject is to the lamp, the quicker the sensor shuts off the light.

Some automatic electronic flash units allow you to select two or more apertures to control depth of field. To determine an f/stop in the automatic mode, you can use the calculator dial, located on the unit that is being used. By matching the indicator to an IS0 film speed number on the dial, you can use the f/stop within a minimum and maximum distance. Once an f/stop is selected and set, it becomes a constant factor regardless of the flash-to-subject distance, providing it is within the flash distance range of the unit. This feature allows the photographer to move closer to or farther away from a subject without having to calculate an f/stop for each change of flash-to-subject distance (fig. 5-25).

Figure 5-25. The automatic operating range, using JSO 100 film at f/2.8, is 1.6 to 50 feet.

Figure 5-26. Manual exposure scale.

When the flash unit is in the manual mode, the f/stop must be changed each time the flash-to-subject distance changes. A scale on the flash unit indicates the proper f/stop to use for the various distances. To determine the flash-to-subject distance for on-camera flash, focus on the subject and read the distance directly from the focusing ring on the camera. Figure 5-26 indicates with IS0 100 film at full power the proper aperture setting at 10 feet is between f/11 and f/16.4 feet is f/32, 40 feet is f/2.8, and so forth.