Originally posted by travis_cooper The other two modes are pre and post curtain. Which to use?
"Pre" makes the flash fire at the beginning of the exposure. This has the advantage of having the flash-exposed portion happen exactly when you tell it to -- you can capture the instant you intended. It has the side effect, though, of causing motion trails from the action that happened
after that -- so, for example, if you take a picture of a car, you get a blur and the trail from the headlights in front, making it look like it's going backwards.
"Post" solves that by firing the flash at the end of the exposure (also called "rear curtain" or "trailing curtain" flash), which makes your motion lines look right, but has the disadvantage of making it hard to sense exactly when the flash will fire unless you have a robotic timing circuit in your brain. So if you're like me, this ends up leaving more to chance.