The Flash Unit: Brightness and Duration
Most flash units that one can buy at photo stores are suitable for high-speed photography. Modern flash units can provide a bright burst of light in a brief interval of time. While these two characteristics, high intensity and short duration, are needed to capture high-speed events on film, they work against each other. The shorter the duration, the less intense the flash. Fortunately, the intensity at the shortest duration, typically 1/30,000 second, is sufficient when the flash can be placed within a few feet of the subject. (With the use of high-speed films and large camera apertures, greater distances are possible.) The problem for the high-speed photographer is to know the flash unit well enough in order to obtain the short duration that is required. These units are designed for snapshot photography rather than for high-speed work. One typically sets the flash controls so that a particular camera aperture can be used, given the distance to the subject. There is no readout for flash duration. A discussion of how to make the flash unit work to best advantage for high-speed photography follows. For additional information, see Activities 2 and 3 of the Electronic Guidebook for High-Speed Flash Photography found on this site.
When a flash unit is discharged in its manual mode, it converts most of its stored electrical energy to light (and heat) in a time span of several thousandths of a second. While this may seem short, the durations needed for high-speed photography are much less. The feature of modern flash units that allow for short exposure times--for example, 50 millionths of a second--is the automatic exposure control circuit. A photocell on the flash unit detects the light reflected by the subject even as the flash unit is discharging. When sufficient light has been received for correct exposure with a given film and aperture, a quenching circuit automatically terminates the discharge. The high-speed photographer exploits this feature by arranging conditions so that the discharge is quenched almost as soon as it begins.
While the typical automatic flash unit doesn't have a control for selecting flash duration, indirect techniques can be used to fool the flash unit into ending the discharge quickly. The most effective is the selection of the automatic mode. If the flash unit has more than one automatic mode, the shortest duration will be obtained using the mode that calls for the largest aperture (smallest f-stop). (For an explanation, again see the Guidebook). The important point to realize at this point is that automatic exposure control is the key to shortening the flash duration. Some older model flash units are strictly manual and are therefore not suitable.
Another technique for shortening flash duration is to move the flash unit closer to the subject, thereby increasing the light intensity reflected to the photocell. Such adjustments can be made independent of the camera position, since the flash unit need not be used on the camera. A disadvantage of this method is that it depends on the reflectivity of the subject and background, and considerable trial and error testing may be needed in order to obtain the effect that one wants.
With flash units having variable-power control, more precise control of the flash duration is possible than what is described above. This feature allows the photographer to select the fraction of a complete discharge at which the flash is quenched (for example, 1/2 power, 1/4 power, etc.) For high-speed work, one would select a low power setting.
On some flash units, the photocell is housed in a removable module. In such cases, it may be possible to replace the photocell with a variable resistor to allow predictable control of flash duration. This is discussed in more detail in Activity 2.
Related topic: Accessories that modify the light output of a flash unit