[Leica] Flash Sync Question

[reid at mejac.palo-alto.ca.us (Brian Reid)]

There's a very simple way to think about this flash sync issue.
(It only became simple when flash bulbs went the way of the dinosaur and 
"flash" came to mean "X sync" or "strobe" or "electronic flash")

Electronic flash units emit light for a really short amount of time. Small 
numbers of milliseconds. What it means for flash sync to "work" is that at 
the instant the flash fires, the shutter must be entirely open.

Focal-plane shutters like those on Leicas work by having two curtains. When 
you push the button, the first curtain opens. After a period of time 
determined by the shutter speed, the second curtain chases after it, closing 
the hole that was opened by?the first curtain.

If the curtains are made out of some soft material, such as cloth, then they 
can't be moved very fast or the cloth will tear. If the curtains are made 
out of something hard, like titanium, then they can be moved really fast, 
because titanium is strong.

If you could move curtains infinitely fast, then you could sync flash at any 
shutter speed. You open the first curtain: blink; it's open. You fire the 
flash. You close the second curtain: blink; it's closed.

But in a Leica with a cloth curtain, it takes about 1/120 second to move the 
curtain across the film. If your exposure is 1/60, then at the very instant 
that the first curtain finishes opening, the second curtain has to start 
closing. If your exposure is shorter than 1/60, then the second curtain has 
to start before the first curtain is finished. For exposures of 1/500, the 
second curtain starts to close almost immediately after the first one starts 
to open, so that the effect is a narrow 
vertical slit moving across the film.

If you shoot at 1/25, then the first curtain opens, the camera sits there 
for a while saying "ho hum", and then after a nice cup of tea, the second 
curtain closes. As long as the flash fires during that interval, the picture 
will be fine.

In the days of flashbulbs, the camera had to start the flashbulb flashing 
before the curtain even opened, so that the flashbulb's peak brightness 
would have a chance to build up by the time the shutter was wide open. 
Electronic flash just fires when you ask it to, so there's no need for fancy 
tricks about flash timing. Just set the flash-sync dial on "X" and leave it 
there.

Brian Reid
who confesses to have spent a lot of time with oscilloscopes and photocells 
studying focal-plane-shutter behavior when he was a teenager.