In normal flash mode the shutter needs to get all the way open then the electronic flash pops and exposes the image. After the flash is complete, which takes 1/1000 of a second or less, the rear curtain closes and the exposure is finished. The fastest or shortest exposure that is possible with your camera that allows the shutter to open fully before it begins to close is called the sync speed. Back in film days amateur cameras usually had a sync speed of 1/60 second. Today even moderately priced DSLRs sync speed is 1/200 or 1/250. Professional cameras maybe 1/500. Very few shutters will create a 1/000 of a second exposure that way. In order to allow photos at shutter speeds greater than the sync speed the rear curtain begins closing before the front curtain is completely open and therefore exposes a narrow slit which travels across the image. If the flash fires during this exposure only a narrow stripe of the image will be exposed. If your camera and your flash support it there is a way to use a flash at speeds faster than the sync speed. We make the flash stay on long enough to let the curtains complete their travel from one side to the other. With this system the flash will expose the film or sensor at any shutter setting during the time it takes the curtains to travel from one side to the other.
As an example using a Nikon D200 and a SB 600, from the camera’s menus choose “Custom Settings Menu”, indicated by the pencil, then choose “e Bracketing/Flash” and then “e1 Flash Sync Speed”. Choose 1/250 s (Auto FP) as in the image to the right.
You can do that as a trick mode with many high-end flashes and cameras. Nikon calls this the “Auto FP High Speed Sync” Canon refers to it as just “High Speed Sync”.
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