What does diffusion on a speed light do?
I keep seeing photographers talk about diffusing their on-camera flash. Whether it be with a handkerchief, a Stofen diffuser, the built-in pop-out diffuser panel, or some other form of diffusion taped over the light, I am not quite sure what they are trying to accomplish. These various diffusers are used to spread out the light coming out of the flash, basically to allow the flash to be used with a wider angle (shorter) lens. The light spreads out to cover more of the scene at the cost of loss of power.
Quality or character of light?
Some that I talk to say that they use the diffusion to soften the light--to make it look better. I question this. We are talking about diffusion methods that pretty much keep the flash the same size as it is without the diffuser. We should all know that to make a light softer you have to make it larger in relation to the subject it is lighting. A larger light source provides light from more angles allowing it to become its own fill light to soften the edges of shadows. There are devices like the Gary Fong diffusers or the Roque Flash Benders that do make the light larger. I am not talking about those here. Just diffusers like those shown here...
Here are two photos of my favorite mannequin (models are hard to come by in the middle of the night when I get inspired to write these blog posts). One photo was made with the straight flash and the other was made with the 1-stop diffusion material taped over the light. Can you easily tell which is which? All of the photos were made in TTL mode at +/- 0 stops with no post-processin except for white balance.
Two more examples, with a longer lens...
I am not seeing it
My argument is that diffusion right on the speed light, which doesn't make the light source significantly larger, does not soften the light. The only slight exception might be if the wider distribution of the light allows it to bounce off a low ceiling or close by light color walls. But even that will be extremely subtle. To me, these methods of diffusion only do a few things, none of them beneficial. They make the flash have to work harder, putting out twice as much light (1 stop), while increasing recycle time and draining battery power.
Look closely at the shadows in each of the photos. A softer light creates a softer, more gradual transition from the true value of the subject to the underexposed shadow areas. In these photos the shadow edges look the same. There is no softening of the light.
What is your experience with diffusion and speed lights? In the above examples, photos labeled B (on the left) have the diffusion in place. Photos labeled A (on the right) are straight flash. Is there enough difference between A and B to justify the loss of light power and the extra drain on batteries?
But, but, but...
OK, you say. These are close in portraits. What about something more like at an event or reception. Here are six photos taken with an on-camera flash with a 28mm lens about 11 feet from camera to the subject (me!). Straight on flash, diffusion material over the flash, the built-in diffusion panel, a Stofen pointed straight ahead, the Stofen with the flash pointed up at the ceiling, and with a white index card taped to the pointing up flash. This time I did not correct the white balance, these are all "out of camera." All TTL exposures at ISO 400 and f/5.0.
This post is not anti-speed light. I think you can get some very nice light from speed lights--even when mounted on the camera's hot shoe. The following photos were all made using an on-camera flash that was pointed off to the left to bounce off a wall, effectively making the light much larger and thereby softer.