A good article Bob but one point, other makes of electronic flashguns can do exactly the same thing.  For example Metz flashguns certainly can.  I believe that studio flash systems cannot.

Ken

It was just a small area photodiode I had lying around. I don't know the specs on it. I dropped the illumination level down to where I was well under saturation by pointing the flash away from the sensor and just having it see some of the bounced light. At full power I also put an ND filter in the path. The signal was never more than about 25% of full photodiode output (i.e. the saturation level), so I suspect it was a faithful representation of the light output. I didn't measure the photdiode response time, but it's clearly faster than 100uS since that's what I measured for the 1/128 power flash. which is probably about right.

This thread got me thinking about another possible way to get high speed synch: slow flash and fast shutter. Studio flash at full power, some at least, can be as slow as 1/750th of a second while shutters of modern 35mm cameras often exceed 1/4000 sec. With first curtain synch, won't a shutter speed of 1/1000 or more, if it coincides with the flash peak power or any reasonable portion of it, give uniform exposure?

I have such a setup and will test when I can, but wanted to get into the theoretical aspects, too.

Frank

I don't get this answer, it seems to refer to a different question: I specified a flash duration of 1/750 that I remember from some spec sheet and a shutter speed that could be much faster than that. You postulate a whole different scenario to refute my speculation. http://www.keradwc.com/articles/studio_flash_duration.html provides a list of flash durations for various studio lights: note that several are below 1/1000 sec, even as low as 1/300sec.

I'll restate my question: Why would there be any difference between a light that's on for a duration longer than the shutter is open, just because it's a flash? If I make an exposure by room light by flipping the switch on, then clicking the shutter at 1/1000 sec, then flipping the switch off, I'm doing the same thing that a slow flash would do: flash comes on at first curtain synch, second curtain travels to end, some time later the flash turns off. The flash was on for the whole duration of shutter operation. It may not be on full-power during that whole time, but that's a different problem, at 1/8000 sec, at least, it probably isn't changing much.

Hi Bob,

I was somewhat with Frank on this one, which means I probably didn't fully understand how the open slit works for shutter speed faster than the sync speed.  You said when an open slit is at work, it always stays open for the duration of the sync speed (1/200s).  So how does this slit achieve shutter speeds such as 1/1000s 1/4000s or even 1/8000s?  

Am I correct to say that for faster-than-sync speeds, for any fixed point on (actually a fixed line across) the film/sensor, it won't be exposed for more than the shutter speed (so the wideness of the slit varies according to the shutter speed), but as far as the entire film/sensor plane is concerned, each curtain still takes the sync speed to move across?

so for 35mm film and sync speed of 1/200s (5ms), the slit needs to be 1/1000s * (36mm/5ms) = 0.72mm for 1/1000s and 0.36mm for 1/2000s and so forth?  (36mm is the width of a 35mm film frame, assuming the shutter goes from side to side, if the shutter goes from top to bottom, then we use 24mm instead because that is the height).

so if the above is true, then when I take a photo of a helicopter, even though I am using 1/4000s or even 1/8000s, the rotor blades may still be distorted if they rotate faster than my sync speed?

[found a mistake in the numbers above, originally I said 0.72mm for 1/1000s and 1.44mm for 1/2000s... bad math...  should be 0.36mm for 1/2000s because it is directly proportional to the time duration]

yes, thank you for finding this picture Bob.  I remember I saw this picture and read something about the distortion in the rotor blades because of the shutter speed a while back when I first started photography, but it didn't make too much sense at that time... but thanks to Bob's article on how the shutter works and Frank's questions, I can now say I understand how shutter works.

I think you answered your own question. Would you buy a DSLR with a maximum shutter speed of 1/500s, plus pay for the shutter to be in every lens? Maybe you could push a leaf shutter to 1/1000s with modern technology, but 1/500s is more commonly the limit.

I think 99.5% of photographers would answer "No". The other 0.5% of the market isn't big enough to make the development and marketing of such a camera a money making proposition. For those who simply must have a leaf shutter, there are still a few cameras out there that they can use.

The fact that most people have never even seen the distortion that can be created by using a fast shutter speed on a focal plane shiutter when photographing a moving object is proof enough that it's not much of a problem!