Macro the 1/f rule and flash duration

Started Sep 8, 2008 | Discussions thread
JimH Forum Pro • Posts: 12,911
Do you have another flash to test with?

You might try it with a different flash, and using manual mode with the MR-14, too.

I have not tried it in ETTL mode, myself yet. But I may. In fact, that has been requested by a few people

The thing is, in ETTL mode, I'll have to trigger on the pre-flash and then use the 'scope's delayed trigger mode to find the "real" flash pulse.

And that brings something up:

Are you sure that you're not just seeing the pre-flash that is emitted when shooting in ETTL mode? Avoiding that is the main reason that I did my tests using the manual mode of the flash. But I also wanted to be able to explicitly dial in the exact flash power repeatedly, so it helped me in both of those ways.

I will try to do a search to find the threads where I posted about this before. In those threads (somewhere), I have the numbers I came up with for the flash durations.

It really is an interesting question as to what is the best or appropriate way to characterize the flash pulse widths. If they were nice rectangular pulses, it'd be easy.

But the rising edge is a typical cap-charge kind of waveform, and the falling edge is sharp (when the flash terminates the pulse to lower the output power). And for long pulses, we see a very long cap discharge curve for the trailing edge.

So we're forced to try to assign a "pulse width" figure to an odd waveform that is not of a constant shape.

So that's why I tried to come up with an arbitrary value for relative brightness above which, we get visible image creation (thus adding to blur) and below which, things will be insignificant. So that's sort of a judgment call, I guess.

In any case, though, I do think that this is important because it IS so hard to get really sharp images when shooting macros hand-held.

I've often wondered if camera shake is a big part of problems people have with the MP-E (or other high magnification set-ups) even when they are using flash. And when we use diffusers, and such, that cut down on the flash power, we might pay a penalty of increased susceptibility to blur due to the longer flash pulses that must result.

It'd be neat to design a very high speed flash system using multiple flash tubes, each of which emits only a very brief pulse. We could make up for flash duration with large numbers of flash tubes

One of the contributors to the FM "macro world" forum has built some very high-speed LED illuminators. With those, you can precisely control the flash durations.

Anyhow, this is a good subject to explore, I think!

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Jim H.

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