Size still matters
In recent posts about diffusion I have been concentrating on small light sources such as speed lights or the "standard" 7-inch metal dish reflectors on studio strobes. But what about larger sources? I was recently gifted a Speedotron 20-inch metal dish reflector. As I am no longer using Speedotrons as my main lights I adapted the reflector to an S-mount for use with my Interfit Photographic lights. This post contains affiliate links and I will be compensated if you make a purchase after clicking on my links.
As I started testing this franken-reflector I noticed that the light was harder than I expected. A look inside the bowl gave some cues as to what was going on. Speedotron lamp heads have large protruding flash tubes and/or flash tube covers. This pushes the light source out in front of the mount on the reflector, allowing the light to completely fill the bowl. The bulbs on the Interfit, though they do protrude, are not quite as long as the Speedo tubes. So most of the light was coming straight out of the flash going forward and not spreading out quickly enough to fill the bowl.
I did some cutting and drilling to change the position of the S-mount to try to push the flash tube a bit further into the bowl. That still wasn't enough. A quick experiment using a 12x12-inch sheet of diffusion gel in the shape of a dome over the flash tube confirmed what I needed. My first thought was to find a Tupperware bowl around the size of the pyrex flash tube cover, but a quick look through kitchen cabinets didn't turn one up. That got me to thinking that people used to call some speed light flash diffusers Tupperware. And I remembered that I had a very old one of those made of a hard plastic that was specific to a speed light I no longer had. I quickly found that diffuser and a hack saw and went to work.
After cutting off the bottom of the diffuser I was relieved to find it to be exactly the right size to fit in the S-mount. A bit of hot glue and clear caulk and it was fixed in place, ready for testing. And some disappointment. The light was still harder than I expected. Even with the diffuser in place, most of the light was concentrated in the middle and not filling the bowl. Then I remembered that there was a second piece to the diffuser, a dome that snaps onto it. I quickly located that and voila, the second test did show a noticeable change in the shadows and more light "wrapping" around to the side of the subject (notice the ear is brighter). Now the diffuser was doing its job--making the light spread wider to fill the bowl of the reflector. The diffuser itself didn't make the light softer, but it did make the light source larger which did soften the light.
The next option was to add a diffusion sock over the entire reflector. This has the effect of spreading out the light across the full 20-inches of the bowl making the light source even larger and more even across the surface. Again the diffuser spread the light and evened it out. The last test was to try the sock with and without the dome inside the reflector. This showed a subtle change. With the dome the light source appeared a bit larger, as seen by more light reaching the ear. But the shadow transition was pretty much the same with and without the dome. It was just that with the dome more of the bowl was lit and the light wasn't as directional.
Let's look at the tests (click on the image for a larger version)...
Here are side views of the interior of the modified 20-inch bowl reflector with and without the dome on the diffuser. I made one more modification, not shown here. I drilled a series of small holes around the edge of the snap on dome to help the fans in the strobe head be more efficient.
IMPORTANT: DO NOT TRY THIS WITH A STROBE THAT USES TUNGSTEN OR QUARTZ MODELING LAMPS!! I was able to do this with the Interfit strobes because they use relatively cool running LED modeling lamps. For comparison, the 60-watt LED in the Honey Badger is brighter than the 250-watt quartz-halogen modeling lamp in my older strobes. But the LED's temperature after being on for hours remains around 85-degrees F while the quartz bulbs reach temperatures over 300-degrees F in a matter of seconds.
So, you might ask, what is the point of all this today. It is all part of trying to dispel myths around diffusion. It is easy to look at the photos and decide that the various diffusers changed the quality of the light. But it still comes back to the fact that the quality of the light is determined by the size of the light. In these examples the diffusion spread the light to better fill the bowl of the reflector. Without any diffusion the 20-inch reflector is bright in the middle and darker around the edges, making it appear smaller than its actual size. As diffusion is added the light spreads out to fill the bowl, making it appear to be its full size.