The website for some very interesting new flashguns from MagneFlash mentions more than once that it outputs more light at faster shutter speeds than regular studio lights. Now, conventional wisdom is that a run of the mill studio strobe takes about 1/900th of a second to output its light which is a lot quicker than the length of time the shutter is open below max sync speed (lets say this is 1/200th of s second which will work for most modern SLR cameras), and so as long as the shutter speed is longer than this, you should get all the light right? I posted comment to this effect on LightingRumours. This measurement is to “t.01” or the point at which the output is 1/10th of the peak output. You need some sort of agreed point to make the measurement to compare lights.
However, the light output is a flattening curve and there will be a little bit of light trickling out after this. David Selby, who runs LightinRumours suggested a test which I’ve done using a basic 300 joule studio light See the results below at 1/30th of a second and again at 1/200th of a second. I connected the camera directly to the light using a cable, to minimise any delays caused by radio triggers.
You can see there is a small but noticeable difference between the 2 both in the image and the histogram. It’s not the rear curtain hoving into view either – the image gets darker at the top edge as well as the bottom edge where the rear curtain is about to make its entrance.
It seems on odd feature to sell a light on, as this will only apply within the x-sync range. It’s the action freezing ability of a short flash duration that is much more important to me (minor variations in output I can accept and dial out post capture) and Speedlights are much faster than this already. However high speed lights are only really useful when there is no ambient light. Outside, on location where you are shooting with a combination of flash and ambient light and want to freeze action, you need to use the shutter to do this – and actually, slower flash lights are more useful here as you can use Tail Sync (or Hyper Sync in Pocket Wizard terms, and Super Sync if you use Yongnuo YN622 triggers). See my YN622N tail sync tests here. So selling a portable light on its flash duration? I can see the advantage Magne flash are pointing at – you do get more of the light out (and the difference is more than I expected) at higher shutter speeds (as long as this is below the sync speed) but it’s not a strong argument. Having to fiddle around with an external battery that needs strapping to the stand and connecting to the flash is not a great plan either – the battery is small – why not make it part of the light?