But when used in automatic mode, the flash of light can be terminated after a much shorter time. A photocell on the flash unit detects the light reflected from the subject and actuates a quenching circuit when sufficient light has been received for correct exposure. In this mode, durations as short as 30 millionths of a second can be achieved for many flash units. In order to obtain such short durations, the flash unit must be set for automatic exposure and placed as close to the subject as convenient. In such cases, it may be possible to replace the photocell with a variable resistor to allow predictable control of flash duration.
Most flash units are suitable for high-speed photography.
If the unit has a PC cord, this will simplify the electrical connections. When the terminals of the PC cord are short-circuited, the flash discharges. If the unit has its terminals on the flash foot only, connections may have to made there. Flash units dedicated to particular cameras will have several contacts on the flash foot. Typically, the center contact is the positive terminal of the flash trigger circuit, and the contact on the side of the foot is the negative terminal.