Flash duration - what's it all about?

[Garry Edwards]

After reading through this thread from someone who just wanted to understand what was wrong with his studio shots, I thought I would finish off a basic tutorial that I started writing over a year ago.

Flash duration is simple enough, but it's clear from that thread that although there are plenty of people who understand the process very well, there are plenty of others who just don't seem to understand the basics, and seem to think that it's really about shutter speed
And frequent new releases of flash equipment with increasingly ridiculous claims doesn't help either.

So, I've done a tutorial, explaining it. http://www.lencarta.com/studio-lighting-blog/01/flash-duration-whats-it-all-about/#.UsXhnLS0NIU

 

[Faldrax]

Thanks Gary, I had the basics covered from reading the thread you refereed to (and mentally filtering the posts to those who I had a degree of confidence in), but the tutorial fills in the gaps nicely.

 

[Phil Young]

Good article Gary, thanks for your help before- I went and tried a few things myself with my flash guns and concluded with some great new found knowledge that I can't wait to use on some high speed shoots!

 

[Tunbridge]

Good write up Garry, should be a good resource for clearing up and misunderstandings.

 

[kestral]

Good article. Is there not a point at which flash duration is reduced to such a level in order to get the shortest pulse possible that it is overcome by the ambient light? because when the duration is reduced in those circumstances the ISO has to be increased and/or the aperture widened to compensate for the reduced power of the flash, letting in more of the ambient light which will start to show the movement. If so where do the lines cross on the graph?

 

[Phil Young]

Good article. Is there not a point at which flash duration is reduced to such a level in order to get the shortest pulse possible that it is overcome by the ambient light? because when the duration is reduced in those circumstances the ISO has to be increased and/or the aperture widened to compensate for the reduced power of the flash, letting in more of the ambient light which will start to show the movement. If so where do the lines cross on the graph?

I believe you need to increase the amount of flashe units to get the power up.

Right Gary??

 

[Garry Edwards]

Good article. Is there not a point at which flash duration is reduced to such a level in order to get the shortest pulse possible that it is overcome by the ambient light? because when the duration is reduced in those circumstances the ISO has to be increased and/or the aperture widened to compensate for the reduced power of the flash, letting in more of the ambient light which will start to show the movement. If so where do the lines cross on the graph?

Yes, that's right. But plotting a collision point on a graph would be difficult - the ambient contribution is of course related to the shutter speed, which can be a variable (although not a particularly serious one, as most FP shutters can manage something like 1/200th - 1/250th with a synch cord or a decent radio trigger) and of course it's subjective anyway, one photographer might consider anything more than 10% ambient contribution to be a problem, another may think that much more is OK. This is where some of the really fast (Bron or Profoto) conventional flash heads can come into their own, although they are nowhere near as fast as IGBT and don't have the advantages of very short flash durations or fast recycling, at least they perform their best results at full power.

I believe you need to increase the amount of flashe units to get the power up.

Right Gary??

Yes, but that can bring its own problems, assuming of course that you actually have the extra flash units, because you can't have more than one in the same place and this will affect the lighting effect, although of course this won't necessarily be a negative thing.
That's why there can be a very substantial advantage in having a poweful IGBT flash head, rather than a number of low powered hotshoe flashes.