I recently took some shots of a house fly, primarily for use in a simple case study of print sharpness that I posted here. Several of the shots turned out fairly well with flash, however they all seem to have some kind of ghosting going on. I am not really sure if it is actual sensor ghosting, or possibly something else? I'm a bit unsure about what the aberration actually is, as the only part of the scene that appears to be ghosting is the fly itself, and the shape of the "ghost" does not mirror the actual photographed shape...it looks more like the fly moved between the time the shutter opened, the flash fired, and the shutter closed.
Here is an example:
You can see the "ghost" to the left of the fly's head...it looks as though the fly has changed position from the "ghost" position to where it is in the shot.
I checked two things, the exposure length and my flash settings. Exposure length for these ghosted shots was surprisingly long...1/5th of a second through 0.3 seconds. The camera was mounted on a tripod since it was macro, so I didn't really register the exposure times when snapping them. Second, flash sync speed was set to "Auto" in my 450D custom function settings. I'm not exactly what that means (I haven't been able to find any clear documentation on it)...however the alternative setting is 1/200th. Not sure if these factors affect a flash shot much.
I am a bit confused about the nature of the ghosted image. If the fly moved over the duration of the shot, I would have expected the ghost to cover the entire span of motion...however it seems to be frozen at a particular moment. (I can't imagine that was the moment of flash...wouldn't that be the clearly exposed part of the scene?) Anyway...very confusing issue...