Impact of movement on "dragging the shutter"

I've yet to experiment with this technique, but before I do I was curious how the movement of a model would impact the outcome of the image - let me elaborate.

Say you have a sunset, or a city scape behind a model that you want to expose with a longer shutter speed. Lets also say you are illuminating the main subject with a flash. AFTER the flash, does slight model movement really impact the image, or does the "flash exposed" portion of the photo pretty much over ride any movement? I hope that explanation is sufficient.

Depends on how much ambient is on the subject. If the flash is only to fill in some shadows, it may not be strong enough to "stop" anything. The ambient will start to take over. However if you are lighting the subject mostly with the flash. And there is all but no ambient, then the flash will stop most any movement.

Thanks Mark, intuitively, thats what I thought, and yes, I was considering a fairly dark scene with little ambient in the foreground.

I've made the mistake a few times during weddings in which the flash freezes the subject but the ambient created a ghost image as well. Sometimes that's okay, but this was during the formal shots and it ruined it.

As others have said, it depends on how much ambient you have in the exposure.

To minimize this effect you can change the flash curtain to rear. This causes the flash to fire at the end of the exposure, rather than the beginning. People tend to stay fairly still before the flash fires.

Dragging the shutter is a very effective technique, especially if you are adept at using gels to match flash color temperature with ambient.

There are two caveats:
1) This ain't a 'sports' technique. Dragging works on mostly stationary subjects who would nevertheless be obviously blurred at such low shutter speeds.
2) There are limits to how slow you can go. I typically drag the shutter in ranges from 1/15 to 1/30. If you start trying things like 1 second you will have monster ghosting and blur.

Don't forget, the key to dragging is that you must underexpose at least one stop for ambient light. If you expose ambient too close to correct the flash will not brighten the subject enough to freeze them.

Bear in mind also that there are two "meanings" to the phrase "dragging the shutter".

The one means to have a slow enough shutter speed to let in some ambient light or, as you say background light from maybe a cityscape. You do need to try to avoid much subject movement. You do not want a "really slow shutter speed". A lot of times I just set a shutter speed to a general 1/60 sec. in order to let in "some" ambient but still catch enough "freeze subject" with the flash.

The other meaning of "dragging the shutter" is to actually have a slow enough shutter speed to show subject movement in addition to the "moment" of the flash. This is an "effect" -- it can look cool or it can be, well, not so cool. You also learn to work between first curtain and second curtain flash and the differences in the effects. And, the shutter speeds are quite slow to get the movement in the frame. It's a whole other subject!