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Sorry if I'm not answering your question. I'm in a discussion with JimH over at DPR about his tests. The problem with his measurements is that it gives people a false sense of how well a flash can freeze motion because he measure a flash hitting a photo diode and not how fast the flash fired at a 13% gray card, and it doesn't take into account the light lost from diffusing the flash...
Edited by Dalantech on Sep 09, 2008 at 12:27 PM GMT
Glad to see this thread started here, too.
You make a good point about the conclusions that people should draw from looking at my waveform tests.
Regardless of the diffusion or the scene set-up, the flash pulse widths that I've measured would be the same *in my tests* because I set the flashes to "manual" mode and directly entered the flash power settings. Doing that eliminates any consideration of the scene reflectivity, diffuser losses, etc.
It also spared me from the requirement to trigger on the pre-flash and then use delayed sweep to try to "find" the real flash pulses. I have thought about building a high-speed logic circuit to place between the photodetector and the oscilloscope's external trigger input so that I could set it to ignore the first pulse, but pass the second pulse through.
But in any case, I really only wanted to establish what the pulse widths were for any given flash power output level. And thus, using the manual mode was actually more suitable because it's repeatable and gives me clearly defined flash power levels that we can all compare.
So what you see presented in those oscillograms are simply the flash pulse shapes and timing for a selected range of absolute flash power levels.
The thing that is important to keep in mind (and this is your point, of course) is that the use of a "lossy" diffuser will require you to set the flash's power level higher than it would need to be without that diffuser, and that, in turn, will result in longer flash pulses.
I'm very interested to find out more about your new diffuser material. I'd prefer to get the least loss possible from any diffuser that I build or use, so when you can, I'd love to hear about what you're testing
If anyone is interested in the flash pulse widths they might actually be getting in their "real world" shots, you can determine the power level that you're using by setting the flash to manual and then seeing what setting it takes to get the exposure you want for that particular scene and lens settings.
I often shoot in manual mode anymore for macros because the 40D seems to expose erratically in ETTL mode with the MT-24 anyhow. Manual is more predictable and faster to use for me with my 40D. With my 20D, I often use ETTL for flash macro shooting.
But you must also take into account the two heads of the MT-24EX.
In most of my tests, I enabled only one head to keep things simple. But normally, you'd be using both heads. Since using both heads at once increases the brightness, it follows that you'll get faster pulses for any given exposure when you do use them both. But again, you can determine the actual power levels you're using by playing with the manual mode and adjusting until you get the same thing you've been getting in ETTL mode.
I thought one of the interesting things was the look of the waveforms that I captured when using both heads. I purposely set one head to produce a longer pulse than the other for those tests, and you can see how the intensity rises to a high level while both heads are firing, and then suddenly drops to about 1/2 that level as the head set to the lower level suddenly "switches off" in mid-pulse, leaving only the light from the head that was set to the higher power level.
Edited on Sep 09, 2008 at 07:55 PM