Wednesday, 19 March 2008

Filling the brolly

I've seen it asked a couple of times now on different forums what "setting" the flashgun should be at for the best use of an umbrella. This question applies equally to reflective umbrellas and shoot through diffusing umbrellas - it's basically a case of how wide an angle you set your flash to in order to optimally fill the umbrella.

I've opted for a "shoot-thru" umbrella in this case because it makes it nice and easy to see what the light is doing. It's a relatively average 32inch white diffuser umbrella from Portaflash.

So firstly, what is happening as you zoom the flash head? Well, you're increasing or decreasing the patch of light that hits the brolly and therefore the area of the brolly which appears to provide diffused light on the subject. The bigger this patch of light is, the softer the light on the subject.

For this example, I set the flash relatively far from the umbrella. I could pull the umbrella in closer to the flash making the patch of light smaller if I wanted a less diffuse light source. In all cases I'm showing though, the flash was in the same position near the end of the umbrella stalk, with just the flash head being zoomed.

So firstly I set the 550EX (equivalent to Canon 580EX MkII) flashgun to 105mm. You see this creates a relatively defined patch of light in the centre of the umbrella.

Zooming out to 50mm, you see the patch is somewhat larger and also has a more pleasant softer edge. You'll also notice if you look carefully that there is a loss of light because the flash is now lighting a larger area of the brolly.

Zooming out again now to 24mm, we see the flashgun is now covering just about all of the brolly. If you want to use the full diffusion of the brolly, this is not far off an optimum setting, because the flash is covering just about all of the brolly and no more.

However, let's go wider. I now put down the diffusion cover on the flash that allows the flash to cover an area of about 17mm. You can see there's an incredibly significant loss of power now. What you can't see is that, although every inch of the brolly is lit now, so also is a large swathe of the room around the brolly, so a lot of light is being wasted. You also have to be careful you're not including this much brighter, undiffused light within your photograph. Suppose you light from the side of a subject, you could easily find that the undiffused light hits the background, and depending on distance, could actually be brighter than the subject. Not only that, but you can also find you end up with the shadow of the brolly appearing on the background, giving a nasty light/dark line up your background.

So what's best? Well the more you zoom out, the more you lose power, so if the idea is to over-power available light while only getting a little diffusion, then for a 32inch umbrella, 105mm is probably a good setting. If you want to really use the soft light of the brolly, then 24mm will probably give a good combination, covering all of the brolly. Going further than this is simply wasting light by spilling it all over the place.

I wouldn't of course recommend you use these values as gospel. Get your camera, flash and brolly, set the flash to low power, and take a few shots like these yourself to see how the light spill falls on the brolly. This will give you a much better feel for how the light is diffusing with your own setup.


blaise said...

Excellent post! I never understood what chocking the umbrella meant. thanks!

Gustavo Santos said...

Very interesting and instructive!
Thanks for the post.

Anonymous said...

Can you quickly just explain the flash itself? I am having a hell of a time finding the basics of basics of flash and am trying to follow this...set 105 mm on the back of your flash? Then,if you set this number manually, then will the flash and camera not speak to each other (because I use the infra-red in the flash, not a pocket wizard)... I can't handle all of the tech at the moment, apologies!

n506 said...

@Anonymous Setting the "zoom" of your flash shouldn't have any effect on the method by which the flash is triggered. By default, a flash not on a camera hotshoe will usually set itself to a relatively wide angle field of view, but this default can be changed without affecting any other settings. This is my experience with Canon EX flashes at least.