Paul Jolly Photography Chapter 25

Flash Sync

With a flash there are two main times we can fire the flash, these positions are called first curtain sync and second curtain sync. (Sync is short for synchronization).
These names come from film cameras so let us describe these first:
When the shutter button was pushed, a cover (called a curtain) was moved from over the film starting the exposure. After the set time a second curtain covered the film to end the exposure. With very short exposure times the second curtain would start moving before the first curtain finished exposing the film; this results in a slit moving across the film. If a fast flash is used with short exposure times the film would only be exposed in a vertical stripe on the film. This is why a minimum exposure is required, to allow the film to be fully uncovered and fire the flash before allowing the second curtain to cover the film.
This difference is important when there is other lighting as well as the flash. There is no difference to the result if only a flash is being used.
With the above understanding we can now explain the different types of sync.

Above the tape reel is moving anti-clockwise in both photographs. On the left is first curtain sync, the blur makes it look as if the reel is moving clockwise. With second curtain sync, above right, the blur matches what the eye sees and appears to turn anticlockwise.