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FORUMS General Gear Talk Flash and Studio Lighting 
Thread started 29 Sep 2011 (Thursday) 17:01
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Flash duration control for high speed photography.

 
mrkgoo
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Sep 29, 2011 17:01 |  #1

Ok, so I want to learn to use my flash to stop action, that is, simulate a super fast shutter by having very low ambient exposure, and flashing the subject with a very brief duration of light.

Are you supposed to use manual mode on the flash? Do you fire the flash at the same time as the shutter, or are you supposed to fire the flash while the shutter is open, on, say, bulb?

I have a 430 Ex II. Do we know the flash duration of the flash? Is it shorter/faster when you use lower power?

Links to information using this technique would be helpful.

Thanks!




  
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emdzey01
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Sep 29, 2011 17:20 |  #2

i've read somewhere (don't remember the source though) that on small flashes, the higher the power, the less the duration. opposite for large strobes.

what sort of action are you looking to stop?


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Enzyme
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Sep 29, 2011 17:38 |  #3

Using manual mode with the flash would be the usual way to go. This way you are in complete control. As the flash power decreases, the motion stopping ability (effective shutter speed) increases.

The exact flash duration figures for canon speedlites have not been published by canon. Lots of people have done tests to measure what the durations are. There is a good round-up of those tests here: http://www.rodandcone.​com …r-canon-580ex-ii-and.html (external link)

I have also had some decent results freezing motion using ETTL mode. (yes, I know you're asking why on earth would he try to freeze motion with ETTL? The reason is that it was shooting dance performances in a theatre with theatre lighting. The theatre lighting would be highly variable and can change constantly through a performance, so I couldn't use manual mode) With ETTL the speedlite still has a flash duration, it is just that you are not explicitly controlling it. But you can still indirectly control it: We know that we want the flash duration to be as short as possible, which means having the lowest power setting possible. ETTL will work out the flash power it needs, but by using multiple speedlites and putting them as close as I could to the subject ETTL would work out that it should use a lower power setting. That worked well, even though I never knew exactly what flash power was.

You can test the results for yourself. Find something that moves (I used a spinning bicycle wheel) and take a few shots with varying the flash power. The improvement in flash duration as the flash power decreases will be as clear as day.




  
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Curtis ­ N
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Sep 29, 2011 17:45 |  #4

emdzey01 wrote in post #13183962 (external link)
i've read somewhere (don't remember the source though) that on small flashes, the higher the power, the less the duration. opposite for large strobes.

It's actually the opposite.

With thyristor-controlled flash units (any hotshoe flash), duration varies more-or-less with power. Less power = shorter duration.

With many (but not all) studio monolights, the shortest duration is at the highest power setting. You need to check the specs for the brand/model you're using.


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Enzyme
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Sep 29, 2011 17:50 |  #5

emdzey01 wrote in post #13183962 (external link)
i've read somewhere (don't remember the source though) that on small flashes, the higher the power, the less the duration. opposite for large strobes.

what sort of action are you looking to stop?

For speedlites, the greater the power, the longer the flash duration, the slower the effective shutter speed, the worse for motion capture. The lower the power, the shorter the flash duration, the better for stopping motion.

For strobes, it depends. Many work the opposite to speedlites: Their motion stopping abilty improves as power improves. For example, Alien Bees AB400s work like this. But other stobes work like the speedlites: Their motion stopping ability improves as power decreases. The Einsteins from the same manufacturer as the Alien Bees work like this.




  
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Mark1
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Sep 29, 2011 22:25 |  #6

mrkgoo wrote in post #13183881 (external link)
Are you supposed to use manual mode on the flash? Do you fire the flash at the same time as the shutter, or are you supposed to fire the flash while the shutter is open, on, say, bulb?

I set the camera to 1-5 second shutter depending on how intricate I have the setup. Aperature as necessary. Flash set as low of power as I can and still be useful. The flash is triggered by the old style www.hiviz.com (external link) control kit.

So, have the ambient as low as you can and not trip over everything. Open the shutter. Set off the event -- that will triger the flash. Then the shutter closes... re set and do it again. adjust as necessary.


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dmward
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Sep 29, 2011 22:54 |  #7

If you want to shot action with a flash you have two options; Speedlites at relatively low power setting or an Einstein Monolight at a relatively low power setting with the light set to action mode.

As mentioned by others, Speedlites, and the Einstein, use a power control technology that cuts of the flash tube based on power setting. The effect is that the flash is much shorter duration with lower power settings.

Depending on what you are wanting to capture, it may be that a longer shutter speed with X sync is all you need. I have shots made at night with shutter speeds approaching 5 seconds that have wave action stopped cold by an Einstein firing at 1/8 power which is about 1/10,000 flash duration.


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mrkgoo
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Sep 30, 2011 00:37 |  #8

This is SUPER helpful!

Thanks so much!

I like the bike wheel idea. Shame about the non-published flash duration, but at least I know where to start!

I want to photograph a balloon popping. Not sure how I'm going to set it up, nor how to trigger the flash correctly. Maybe burst the balloon and fire the flash after triggering the shutter?

Thanks again.

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Mark1
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Sep 30, 2011 12:42 |  #9

Follow my hiviz link and you will find a cheap trigger/control that will do it for you.


www.darkslisemag.com (external link)

  
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Flash duration control for high speed photography.
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