Being able to draw continuous power at a significant level makes it possible to deliver light that can be diffused, reflected or redirected in all kinds of ways, and still reach the subject at a level that allows good depth of field. A mains flash unit works in the following way. As the power supply is in the form of an alternating current in a relatively low voltage, the first part of the circuitry is a transformer and associated rectifier (or more than one in the case of larger units). The transformer steps up the voltage and the rectifier converts the alternating current to a direct current (in other words, it converts AC to DC). This uni-directional high-voltage source then supplies the capacitor that stores the charge. On command, the high-voltage output in the capacitor is discharged through the flash tube.
Given the light output possible from just a single flash tube-and the fact that it is practically instantaneous-it is not surprising that mains-powered flash is the lighting equipment of choice in studios. Output is measured in watt-seconds, or joules, and typical units are between 200 and 1000 joules. To make use of the extremely high output of the mains flash capacitors, the flash tubes are much larger than in on-camera units. Instead of a short straight tube, the most common design for medium-power units is circular. High-output units may use spiral tubes. One result of the need for high output and larger tube size is that the peak flash duration is longer than that of an on-camera flash-sometimes as slow as a few hundredths of a second.
As with other photographic equipment, such as cameras, there are many competing systems, and these are often not compatible. This is particularly the case with mains flash, in which the power units, connectors, and flash-heads cannot normally be interchanged. Before buying anything, make all the comparisons you can, and anticipate your future needs. You might, for instance, eventually need several identical lamps, or a set of specialized lights-one for still-life main illumination, and others for backgrounds, and son on. Also if you expect to shoot on location as well as at home or in the studio, the weight and transportability of the equipment will be important.
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