Macro the 1/f rule and flash duration

Started Sep 8, 2008 | Discussions thread
OP fred vachss Senior Member • Posts: 1,292
Re: Macro the 1/f rule and flash duration

Eyvind wrote:

Let's not jump to conclusions, Fred. According to the manual, a total discharge
of the MR14 lasts 1.4ms i.e. about 1/700s. In addition, you can cut off the

blast by 1/64th, at least. So, I would tend to believe that the shortest possible
flash duration with the MR14 should be at least 1/45000s, and with the MT24
even shorter (more powerful flash).

You also said the 1/3000s shot was grossly overexposed, so we do not really
know how long a properly exposed one would last.


As you can see from the scope traces there is some noise in these measurements and some question of how to define the duration. I used Full Width at Half Maximum and got 1.0-1.1 ms in various tries. If you define the width a bit differently I could easily see this being stated as 1.4 ms.

On the short duration end remember that peak intensity as well as duration is being varied to control total exposure. For the shortest duration shots I saw peak values as little as 1/10th those obtained with the max duration. While 1/10th peak times 1/3 duration gets a range of 30:1 not 64:1 that's at least in the ballpark of the expected range. To be fair I didn't accurately control distance from flash head to detector from shot to shot, so I don't really have a quantitative measure of how much peak intensity varied.

The point I was making about overexposure was that I believe I was close enough, at wide enough aperture and high enough ISO that the flash was cutting to its minimum power and still overexposing. I guess I am making the (unverified) assumption that when the exposure is clearly too great for the flash ETTL will limit the flash to the minimum power it can produce. I strongly suspect that f/2.8, ISO 1600 and 2-3 inches subject distance puts one in that regime.

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