To expand just a bit on the "shutter dragging" that Stecyk covered in his post, I would mention that you can precisely determine the amount to "drag the shutter" using a good meter. Essentially you meter the flash on the main subject which gives you the fstop needed. Normally if you were unconcerned about how ambient light might effect the shot, you'd just leave the camera in at its sync speed (usually 1/60th to 1/250th depending on camera). But if you need to include the ambient light you just meter the area lit by the ambient light using the meter set to the fstop already determined for the flash. So it will return the exact shutter speed at that fstop for the best shutter speed to expose the ambient lit area properly.
Cheers,
rfs
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"The map is not the Territory"
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