Most of the time, when you are working with flash, you need to keep your shutter speed below about 1/250 second. This is because that is the highest shutter speed at which both shutter curtains are fully open. Go beyond that, and you will be rewarded with a dark band on your image, representing the shadow cast by the second shutter curtain. In this example, the shutter speed was 1/500 second.
Unless, of course, you turn on the High Speed Synch (HSS) function of your camera and flash. HSS enables you to shoot at shutter speeds higher than 1/250 by allowing the flash to put out a series of flashes, instead of one short burst of light. In effect, your flash becomes a continuous light source, albeit for a very short period of time.
Why would you need HSS? Well, if you are shooting outside, especially in sunlight, and need a wide aperture to blur a busy background, your shutter speed may need to be very short for a correct exposure.
In this first image, 1/50 sec at f/11 with flash correctly exposes the subject, but the large depth of field clearly reveals the car in the background.
In order to blur the background, I wanted to use an aperture of f/2.0 with my Nikon 135 mm f/2 lens. This required a shutter speed of 1/1000 second, which is only possible if I enable the HSS mode in my Nikon camera (Canon users must enable HSS on the flash body).
Now we have both a correctly exposed subject and a nicely out-of-focus background courtesy of the HSS mode. This begs the question - why don't we just use the HSS mode all the time? The answer lies in the fact that HSS is an inefficient use of the available flash power, and as a result, the light output drops dramatically.
To illustrate this, I photographed a plain white wall with a Nikon SB800, set to manual flash at full power, and positioned 10 feet away from the wall. I took a series of photographs at the same aperture (f/2.8) and ISO (200), but at decreasing shutter speed.
The images are shown here overlaid with the histogram (from Lightroom) and the shutter speed used. Obviously, the effective flash power starts to drop immediately the standard synch speed of 1/250 second is exceeded. This clearly illustrates the trade-off in using HSS. In other words, the flash would either have to be brought progressively closer to my subject (since it was already at full power), I would need more flashes, or I would need a more powerful light.
As in most things, there is no free lunch . . . .
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