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35 mm SLR cameras have focal plane shutters.
A focal plane shutter consists of two curtains that move across the film frame. At slow shutter speeds or at the “B” setting on the shutter speed dial, the first curtain moves across to expose the entire frame, then the second curtain moves across to close the shutter. At higher shutter speeds, the first curtain begins to move across the frame, but it is followed rapidly by the second curtain before the first one has completely crossed the frame, like a moving slit. As shutter speeds increase, the second curtain follows the first one even more closely, the effect of which is to make the slit narrower.
Since the entire frame is never completely uncovered at high shutter speeds, using flash at one of these speeds exposes only that portion of film that the slit is open to when the flash goes off.
Therefore, the camera’s shutter must be completely open across the entire frame when the flash reaches its peak intensity. But, flash is so incredibly fast that it can reach peak intensity and shut down again before the shutter has barely begun to open. So, the triggering of the flash must be timed so that it is synchronized with the shutter, and fires only when the shutter is completely open. The triggering of the flash is actually delayed while the shutter opens, and the flash fires only when the shutter’s first curtain reaches the other side but before the second curtain begins to move to close down the shutter.
Since the shutter is only completely open on most 35 mm cameras at speeds of 1/60 sec and slower, flash cannot synchronize properly with faster shutter speeds on these cameras. As we explain in the section entitled Shutter speeds for flash, a number of cameras will synchronize at various higher shutter speeds - 1/80 sec., 1/100 sec. and up to 1/250 sec. - depending on the specifications of individual models.
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