Front Curtain and Rear Curtain Synchronization

This is a term from SLR cameras. Please review the concept of synchronization shutter speed first. Previously, we indicated that a flash discharges right after the front curtain reaches its end and before the rear curtain starts to move. In this way, the whole film frame can be exposed to flash illumination. Since the rear curtain does not move immediately right after the front curtain reaches its destination, there is a brief delay. Hence, there are at least two discharging timing possibilities: (1) immediately after the front curtain reaches its destination, and (2) right before the rear curtain moves.

What is the difference between these two discharging schemes? A big difference, indeed. Suppose we have a subject moving from left to right as shown in the following diagrams. If flash discharge is synchronized with the front curtain, once the film frame is fully exposed to incoming light, the flash discharges to illuminate the scene. In this way, the moving subject will be illuminated properly when it is at the left end of its movement as shown by the red dot in the left diagram below. Since flash duration is generally shorter than the synchronization speed, after the flash discharges, the shutter may still be open for a brief moment. As a result, the film frame is still exposed to incoming light, and may record the movement of the subject. This is shown in the left diagram below in which light color dots indicate the movement of the subject after flash discharge. Consequently, even though the subject is moving to the right, we shall see on the image, from left to right, that the well exposed subject is followed by some of its residue images, giving an impression that the subject is moving to the left.

If flash discharge is synchronized with the rear curtain, it will discharge before the rear curtain moves. As a result, the film frame will first record a dim trail of the subject movement as shown by the light color dots in the right diagram above. Then, the flash discharges and illuminates the moving subject shown in red in the diagram. In this way, we will see on the image, from left to right, that a light trail of the moving subject followed by a properly illuminated subject, giving a "correct" sense of movement.