Macro the 1/f rule and flash duration

Started Sep 8, 2008 | Discussions thread
JimH Forum Pro • Posts: 12,911
I did the same sorts of experiments.

A while back, I did the same kinds of experiments, but I used the MT-24EX and a 580EX.

I used a very high-speed photodiode of the sort optimized for visible light.

I tested the photodiode and measuring system using a purpose-built pulse generator driving an LED. I found that I needed to use a fast FET to drive the LED at a high enough speed to even be able to approach testing the speed of the photodiode.

In any case, I was able to satisfy myself that the photodiode, its circuit, and the rest of the measuring system was orders of magnitude faster than the flash pulses being measured. So the waveforms I was capturing truly represent the actual light output from the flashes I was testing.

The photodiode lived up to the claims of the manufacturer of having a 50 nanosecond response time, which was nice to see

I posted some threads about this, and I'll try to find them. In those threads, I calculated the flash durations using both a full width at half maximum method and a different method that I came up with that I hope better represents the periods of time where visible blurring of an image might occur.

I think what I did was to measure the widths of the pulses down to 3 stops below the peak brightness. In any case, we're on the same track here

Here are the results I got for the MT-24EX and the 580EX:

http://www.flickr.com/photos/98036178@N00/sets/72157603894160312/

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

1. 580EX Flash, Firing at 1/128 power.
2. 580EX Flash, Firing at 1/64 power.
3. 580EX Flash, Firing at 1/32 power.
4. 580EX Flash, Firing at 1/16 power.
5. 580EX Flash, Firing at 1/8 power.
6. 580EX Flash, Firing at 1/4 power.
7. 580EX Flash, Firing at 1/2 power.
8. 580EX Flash, Firing at Full power.

1. 580EX Flash, Firing at 1/32 power, high speed sync mode, 1/1000th shutter speed.

2. 580EX Flash, Firing at 1/16 power, high speed sync mode, 1/1000th shutter speed.

3. 580EX Flash, Firing at 1/16 power, high speed sync mode, 1/2000th shutter peed.

4. 580EX Flash, Firing at 1/16 power, high speed sync mode, 1/4000th shutter speed.

5. 580EX Flash, Firing at 1/16 power, high speed sync mode, 1/8000th shutter speed.

6. 580EX Flash, Firing at 1/16 power, high speed sync mode, 1/4000th shutter speed, partial trace at higher sweep speed to show detail of waveform.

7. 580EX Flash, Firing at 1/16 power, high speed sync mode, 1/4000th shutter speed, partial trace at an even higher sweep speed to show better detail of waveform.

Based on my testing, I'm wondering if perhaps the photodiode you used, or the circuit for it, might not be too slow to see the true waveforms from the flash.

I come up with a similar figure for the full-power flash duration, but at the lower output levels, I see considerably faster pulses.

Then again, I was not testing the MR-14, but instead, the MT-24 in one case, and the 580EX in the other.

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Jim H.

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