Standard (X-)Flash Synchronization
In std. flash sync. mode the flash fires as soon as
the whole film frame is fully exposed to the incoming light (see picture
below). This time period is obviously limited towards higher shutter speeds
dependent on the speed of the shutter/curtain mechanism. This max. flash
sync. speed is given in the specification of a camera. The current fastest
max. sync. speed is 1/500sec is close to the flash duration of strong
flashlights (around ~1/500sec for a full burst). Consumer grade SLRs can
normally sync. up to 1/180sec.
Despite the short flash duration the chosen flash
sync. speed has certain effects on the final image. The main (flash) subject
itself is quite independent from the chosen flash sync. speed because it
is exposed with the speed of the flash duration. However, this is not true
for the surrounding scene. There're a couple of possible side effects.
So the faster the flash sync. speed ...
...the darker is the scene around your main subject
because the ambient light has less influence on the exposure process. The
effect is obviously dependent on the level of the ambient light.
... with a moving scene (or a static scene with moving
objects) the less blurred are the objects which are not (significantly)
exposed by the flash light. etc.
Many cameras offer the possibility to synchronize either
on the 1st or 2nd curtain. With 1st curtain sync. the flash is fired as
soon as the 1st curtain is in upward position while with 2nd curtain sync.
the flash is fired just before the 2nd curtain moves upwards. Obviously
this means that the natural (ambient) light of a scene exposes the films
either behind or before the flash burst. If you shoot e.g. a fast moving
object with 2nd curtain sync. you'll get a blurred moving effect which
will end in the (flash-) exposed object. With 1st curtain sync. it'll be
the other way round.