I did a few OCF experiments over the weekend just trying to wrap my head around the basics of it, especially for macros since most of the information readily available on the web is more directed at portraits. This was actually my second experiment after looking at the results of my first but it's easier to explain, and well I am short on time tonight.
I set up my 430 EX on my flash stand, with my pocket wizards and shoot through umbrella. I kept all my settings the same except the focal length setting on the flash, so the power was the same, the distance (6" from center of flower to tip of umbrella), the angle etc. The ambient light was the same (none since it was night time, yes I used a flash light to adjust the settings), as were my camera settings, ISO, aperture, SS, lens, focus, I had the camera on a tripod and used my cable release to trigger the shutter. The flash was directly to the camera right side of the flower.
You can scroll through the examples in this flickr set (along with my other experiments that I haven't written up yet) in the flickr set here I took a photo at each focal length setting for the flash.
Just to show to show a more drastic difference, here is the longest setting:
and the shortest:
If I brought the exposure up in the 24mm one to match the 105mm one, it was almost a full stop underexposed. When I did my first experiment I wondered why the flash power wasn't falling off as quickly as I expected and I think this was why, I had it set to 105mm so I think it was making a more concentrated beam of light.
All these new aspects to work with are kind of blowing my mind.