Flash sync speed is the fastest speed that your camera can use when you are shooting with flash. End of lesson. Class dismissed.
But wait. There are a few exceptions to every rule, and this one has two. The first is if you use camera/flash combos that support high speed sync. HSS uses an utrafast stroboscopic sync and acts like continuous lighting - in other words, more than one flash, so that the sensor is lit and the shadow formed by the secondary shutter curtain isn't there. The second is if you turn your camera so that the area shaded by the shutter curtain falls on something that isn't lit by flash.
So what happens if you use a shutter speed that's faster than the sync speed? In other words? What if my camera has a shutter speed of 1/200, and I use 1/400? Will my camera blow up?
No, your camera will not blow up. However, you will see a shadow on parts of your photo lit by flash. See Peng's photos below:
Notice that the shadow only covers up parts of the photo that was being lit by flash. Those areas too far away to be affected by flash aren't affected by the shadow.
Note as well that the flash exposure on Peng is independent of the shutter speed. Let me say that again, Changing the shutter speed from 1/100 to 1/640 had no effect on the lit portions, except that those lit portions had the shadow of the shutter mechanism covering them. On the other hand, the background exposure changes with the shutter speed. When Peng is slowly being engulfed by the shadow. He was scared, and asked to be hugged. I thought about it, and said, "No way. Here, have a fish."
(No Penguins were hurt during the making of this blog post - ed.)
