Craigget,
Here's some text extracted from a post I write to the Canon Talk
forum a while back about the limitations of high speed sync:
Hard as it is to believe, high speed sync is actually a bad idea
for freezing very fast motion. Here's why:
Normally, a flash's pulse of light is VERY fast - plenty fast to
freeze motion all on it's own.
Cameras with focal plane shutters (almost all SLRs and DSLRs) can
only flash sync up to a fairly slow shutterspeed, because the
shutter needs to be fully open during the instant that the flash
fires. At shutterspeeds higher than the camera's max sync speed,
the shutter is never fully open. Instead, the shutter opening is a
slit that moves across the frame. If you try to use flash at too
high a speed, the flash fires so rapidly that it's light only
exposes the part of the sensor that is exposed by the slit opening
in the shutter. The rest of the frame is not exposed to the light
of the flash at all.
In high speed sync mode, the flash and camera work in close
concert, and the flash actually stays "lit" for longer in this mode
than in normal flash sync mode. The flash stays lit for the whole
time it takes for the slit opening of the shutter to travel across
the film plane. Thus you get less motion-stopping ability from FP
(high speed) sync mode than you do with normal flash sync.
You can also get odd distortions, since very fast-moving subjects
actually move at the same time the slit opening in the shutter is
moving. To understand this, Imagine a car speeding side-to-side
across the frame, with a vertical shutter. As the slit opening in
the shutter moves from the top to the bottom of the shutter, the
car moves from side to side across the frame. thus, in an extreme
case, the top of the car might be on one side of the frame, and the
bottom of the car might be stretched over to the other side of the
frame, with an odd blurring effect. This would occur even though
the shutterspeed SHOULD HAVE BEEN fast enough to freeze the motion
of the car.
The best way to get good motion-stopping ability with flash is to
set your camera to it's highest NORMAL flash sync speed (which
varies from camera to camera, but is commonly 1/180 - 1/250) and
control the amount of ambient light so the scene looks black
without the light from the flash. Then the very rapid pulse of
light is the only meaningful light on the scene, and it's so fast
that it does a fine job freezing subject motion for all but the
most fast-moving subjects.
Battery operated electronic flashes like Canon (and Nikon)
speedlights control their light output by shutting off the light
very suddenly once they decide that enough light has been emitted.
That means that the lower the flash power, the shorter the pulse of
light. If you fire your flash at full power, the light from the
flash reaches maximum brightness very quickly, then drops down to a
fraction of it's max brightness after a very short interval. The
light then dies away to nothing fairly slowly. Turing a flash to
even 1/3 stop from full power makes the pulse of light much shorter
and "cleaner" than full power, because the flash shuts off before
that relatively long "fade-away period."
Here's a link to a page with some practical examples of the limits
of high speed (FP) sync for motion stopping:
http://webs.lanset.com/rcochran/flash/hss.html
Another problem with high speed sync is that it reduces the guide
number of the flash quite a bit. This is because the flash creates
a light that isn't as bright, but keeps it lit for the whole time
the slit opening is travelling across the image sensor. Each
portion of the image sensor only gets light very briefly, and the
rest of the light from the flash is wasted against the closed parts
of the shutter.
Mind you, high speed sync is very useful, but more for exposure
control than for motion stopping. Don't be afraid to use it, but be
aware that it had limitations for motion stopping.
Duncan C
--
dpreview and PBase supporter.
http://www.pbase.com/duncanc
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