Flash Sync Effects
There’s an interesting discussion in this DPReview thread, where an apparent Photoshop hack job — a cut-and-paste of a skater jumping over a fence — turned out to be a real shot. The give away weird shadowing — why would there be a shadow against the background sky? — turns out to be what you get when a front- or rear-sync’d flash is fired to stop a fast moving, silhouetted object against a relatively bright background.
Basically, the flash is illuminating the object for only a portion of the time the shutter is open. For the rest of the time the shutter is open, the CCD/film is capturing a dark object moving against the bright background. The shadow in the sky is actually the motion blur of the silhouette.
The shadow will appear before or after the object, depending on the direction of motion and whether the flash is fired at the front or rear of when the shutter is open. Obviously, in front of the object with the front sync, and behind with rear sync.
This is just a lesson in counter-intuitive results from the camera.
Note that rear sync flash is recommended for fill flashing rear illuminated subjects, e.g., a person in the foreground, and a brightly lit Empire State Building in the background. The shutter has to be open a relatively long time to properly capture the background (the exposure is metered against it), and the flash should go off at the end of this period, because people take the flash as a cue that the picture has been taken, and may start to move. A front sync may therefore result in a blurry subject because he started to move before everything was done.