EOS Re: Flash sync speed - how important is it? [OT?]

[Henry Posner/B&H Photo-Video]

At 12:22 PM 08/06/2001, you wrote:
>Can you please explain how focal-plane (FP) flash works on the Nikon system?
>What are the differences between Nikon and Canon FP-sync?

While my earlier post was about faster but standard x-sync speeds, as far 
as I know FP flash (which is an entirely different matter) works about the 
same for all brands. With traditional flash, the fastest x-sync speed has 
to be slow enough so the second shutter curtain doesn't start its trip 
across the film plane until the first has completed its journey. With the 
older cameras, like my aforementioned Nikon FTn, this was usually 1/60th 
second because the shutter traveled horizontally. A frame of 35mm film is 
about 1x1.5" so the horizontal trip is the longer one.

A couple of decades ago some bright chap noticed that the vertical 
direction was shorter and that a shutter could make that trip in less time. 
The design problem was what to do with the shutter blades when they're not 
whomping across the film plane. With horizontal shutter curtains, they 
simply sit in waiting, but in the vertical direction, there's no room for a 
full curtain to sit. The result was the multi-bladed shutters we now enjoy, 
a shorter trip from bottom to top and x-sync of 1/250th sec usually.

This is all accomplished with no modification to the flash. A Vivitar 292 
(which was discontinued before the 283 was released) will work at 1/250s 
with a new FM2n and at 1/80s with an F3 and at 1/60s with an FTn today just 
as well as any more modern flash.

To achieve high-speed (a/k/a FP) sync, with shutter speeds faster than 
x-sync, the flash's output has to be tweaked. With a normal flash, the unit 
fires the bulk of it's power at the outset, and then tapers off. FP sync 
mimics the performance of old FP flash bulbs, which took a while to ignite, 
then burned more evenly (but less brightly) for a longer period before 
tapering off rather suddenly. As 
<http://www.chem.helsinki.fi/~toomas/photo/flash-faq.html#working-highspeed> 
describes, "...high-speed synchronization mode ... works by emitting a 
rapid series of short flash pulses... As the slit between the shutter 
curtains moves across the frame, these short pulses illuminate the scene in 
a rapid sequence, eventually resulting in exposing the complete frame.

"The flash unit distributes its light energy to the whole scene in all 
cases. With normal (low-speed) synchronization, all of the light which is 
collected from the scene by the lens, is available to expose the film. In 
high-speed mode, part of the frame is blocked by the shutter at any given 
moment, and therefore part of the flash energy is wasted. As a consequence, 
flash units have lower guide numbers in high-speed mode than in normal mode. "

Klaus Schroiff's page at <http://www.photozone.de/flashtec.htm> has an 
equally nice description of this concept with graphs illustrating the 
difference in the flash burst pattern for normal vs FP modes.

The advantage if FP mode is being able to use flash at very fast shutter 
speeds. The disadvantage is that the quantity of flash available is far 
less than one can have with normal mode.
--
regards,
Henry Posner
Director of Sales and Training
B&H Photo-Video, and Pro-Audio Inc.
http://www.bhphotovideo.com

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