JimH.
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Eyvind Ness wrote:
Funny, no response 
And in the mean time, Fred Vachs at DPReview, the original flash-duration experiment designer posted this:
http://forums.dpreview.com/forums/read.asp?forum=1029&message=29276877
Let me quote:
"So it looks like my conclusion was wrong (at least partially). The MR-14 operated in ETTL mode apparently can deliver flash pulses short enough to freeze motion even using the modified 1/f rule at high magnification. The question really is how fast is fast enough. The 1/f rule suggests that even at 5:1 a duration of 1/2000 - 1/5000 or so ought to be fast enough and these speeds can be obtained by bumping the ISO up a bit (and refraining from using f/16 which is pretty useless at such high magnifications in any case)."
:-)
...Show more →
Note that we are still missing the vital component here:
We do NOT know what the actual flash pulse duration was for *your* shot. But it had to be longer (due to the distance from flash-heads to subject) than it would have been had you shot with the flash heads closer to the subject. It had to be because we suffer from the inverse square law here.
The light reaching your subject will drop off as the square of the distance. And thus, the flash pulse widths required to achieve proper illumination will go up as the square of that distance.
Only at very close distances do we get short flash pulses. At longer distances, the power level has to be increased, of course.
Look at all of the test shots I posted. See what the pulse widths are at the higher power levels? We rapidly get into territory where motion blur will be a serious problem.
I'm not sure why people are seizing upon the very fastest pulses I've shown (those at 1/64th power). Those sorts of pulse durations will ONLY happen when you've got the flash heads close to the subject.
I will try to dig up my old gray card from the darkroom at my mom and dad's house when I go over there this afternoon. Then I can make some objective tests of what flash power levels are required at various magnification settings on the MP-E. And while I'm at it, I'll try some tests from longer distances too because this is another interesting aspect to all of this, I think.
That way, I can put some reasonable flash pulse durations to some typical shooting situations.
As it is, as John has pointed out, we don't really know how to correlate my power-level versus pulse width figures against our real-world shooting scenarios. And people seem to be taking my 1/64th power pulse width measurement as being, somehow, universally applicable, or proof that we're freezing movement for all of our shots.
Without any way to know what the power levels are for real shooting situations, we don't know which of the test shots I did is showing us what we want to know for any given "real world" shot.
But here's a test you can do right now:
Set up that same rig - (the 150mm lens along with the 2X extender). Get the same distance from a gray card as you were from your wasp in that shot above. Using the same camera at the same ISO and same f/stop setting, and with the MT-24EX mounted the same way you did before, set it to manual mode. Now shoot test shots at various manual flash power levels until you get the "spike" centered up in the on-camera histogram.
Then tell us what power level was required.
We can then refer to this composite image to find an approximate flash pulse width for your shot:
http://farm3.static.flickr.com/2208/2267782190_589c831e1f_o.jpg
1. MT-24EX Flash, One head only firing at 1/64 power.
2. MT-24EX Flash, One head only firing at 1/32 power.
3. MT-24EX Flash, One head only firing at 1/16 power.
4. MT-24EX Flash, One head only firing at 1/8 power.
5. MT-24EX Flash, One head only firing at 1/4 power.
6. MT-24EX Flash, One head only firing at 1/2 power.
7. MT-24EX Flash, One head only firing at full power.
8. MT-24EX Flash, both heads firing. A = 1/16 power, B = 1/8 power
9. MT-24EX Flash, both heads firing. A = 1/16 power, B = 1/4 power
I do need to do some testing to try to nail down approximately what we can be expecting with "real" shots.
I asked this in the thread over at dpreview, but I'll ask it again:
Wouldn't it be great if the camera would show us what power levels were used for each flash head after the shot, when we're in ETTL mode? And as John pointed out, it'd be even better if it got placed into the EXIF so we could look it up later.
As it is, we really have no way of knowing what power level was required for any given shot other than to follow up with manual mode tests, finding what power level is required to achieve the same exposure under the same circumstances. Bummer!
Edited by JimH. on Sep 11, 2008 at 11:14 AM GMT
Edited on Sep 11, 2008 at 01:14 PM
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