A somewhat arcane and technical question has been perplexing me for a while. What is the proper zoom setting for a flash when it is going to be used in a softbox? My Canon Speedlite 430 EX IIs have 7 zoom settings to match (as closely as possible) the focal length of the lens that is being used by the photographer. The standard logic goes that if you have a 50mm lens on your camera then you should set the zoom on your flash to 50mm so that the light it gives off covers the area of view of the lens.
This gets jumbled up when you take the flash off of the camera (which many of us prefer to do for reasons that are either apparent or beyond the scope of this discussion). When the flash is off-camera then the zoom function allows you to spread or narrow the coverage of the light it produces. This is all fine and good if you are simply shooting the flash toward your subject or bouncing it off of some reflective surface. When the flash is shrouded in a softbox (or used with a diffuser umbrella, for that matter) the question of what is the appropriate zoom setting gets more complicated.
A softbox diffuses and spreads out light all by itself and the orientation of the flash within the softbox has the flash pressed pretty close to the interior, reflective surface of the softbox. So, I was wondering, what effect would zoom have in this situation where the light didn’t have much distance to travel (and thus spread out via the zoom setting) before it was spread out by the characteristics of the softbox?
In other words, what was the ideal zoom setting when a flash was inside a softbox? In order to answer this question I decided to run a test. My methodology was to set up a softbox with a flash inside against a white backdrop and dial the zoom setting up with all other variables (f-stop, shutter speed, ISO, focal length) being the same. In order to do this I set the flash to manual and at 1/4th power and then shot an image at 24mm, 28mm, 35mm, 50mm, 70mm, 80mm and 105mm respectively.
Now, I think this test is limited and in order to be truly complete one would need to shoot the same variations at a subject, straight-on or at a consistent angle. I chose to photograph this test against a backdrop and from the side because I thought it would illustrate how quickly the light dropped off against a neutral surface and if there was any large variation between the zoom settings then that variation would be apparent, at least in part.
What I found was that the zoom setting seemed to make little difference once the flash was inside a softbox. It seems that the histogram and the photos themselves reveal this. So, I guess that we can all just plop our flashes into our softboxes and fire away without regard to zoom setting, for the most part. If anyone has other thoughts, please say so in the comments.
For those who are interested in how it was set up, I set up my Canon 5D MKII with a Canon EF 50mm f/1.4 USM at ISO 100, f/5 and 1/80th with a Canon Speedlite 430 EX II set at 1/4th power in a Westcott Apollo 28” softbox. The whole thing was tied together with a Pocket Wizard Mini TT1 and FlexTT5. I was about 15 feet from the backdrop and shooting from a tripod. The images are straight out-of-camera, just standard RAW conversation was applied then the histograms were pasted in with SketchBook Pro (which unfortunately stripped out the EXIF data). Here is what I found.