Recently I found myself setting up lighting for a headshot session only to find out my flash was stuck on full power. The "on" switch for the flash worked just fine and it fired as normal but all its buttons simply didn't work, presumably because they were worn out. I had several backup flashes back in my truck but I wasn't looking to make the client wait while I walked back to where I parked, so I figured it was a good time to flex those problem solving muscles and make the shoot work with an un-adjustable flash.
While I don't expect many photographers to often find themselves in the same situation as I, there are often a myriad of reasons why you may not be able to control the power of your key light source and you should be prepared to successfully manage those situations without pause.
The examples throughout this article are based on a series of gloriously unscientific tests. To do the tests I set a camera up on a tripod and pointed at a white wall. I set the camera's shutter speed to 1/250 of a second and f/9 which created a black image when fired without flash. When adding in a bare-bulb flash at full power the wall was almost completely white (just a hair under blown out).
Controlling Flash Power With Distance
The most obvious method that can be used to control the power of your flash is by adjusting the distance of flash to subject. The farther the flash is away, the less light will fall on your subject. This method unfortunately comes with a pretty tremendous downside. As the distance increases the sharpness of the light increases. Shadows become harsher and light becomes less flattering. The only way to combat this is by increasing the size of the light source which often can be difficult or impractical. Below you can find examples of a flash being moved back 5 feet at a time without adjusting any settings.
Controlling Flash Power With Diffusion
Most photographers are aware, on a vague level, that diffusion drains intensity from a flash but it isn't often something we consider as a tool for controlling flash output. Rather, we simply use diffusion to soften light and adjust our flash power accordingly. However, diffusion can be an effective tool to adjust the power of a light that you can't dim. This can be a helpful trick when in situations where a powerful studio strobe is still too powerful on its lowest setting. The examples below show light falloff as layers of diffusion are added in front of a light. Note that the size of the softbox also has impact. In this case I used a 20x20 box with no internal diffusion sheet.
Controlling Flash Power With High-Speed Sync
This particular method is slightly more insane than the others in that it is only really useful in a situation when you may find yourself with a flash stuck on full power as I did. Regardless, it is also a fun learning exercise that helps photographers better understand the impact of HSS on their shots. For those that don't know, HSS is a technology that allows a flash to sync to a shutter beyond the normal sync speed of the camera (usually 1/250 of a second). HSS functions by pulsing the flash rather than simply firing in a single burst, which has a diminishing effect on the power of the flash. As your shutter speed increases the amount of power that is lost to HSS also increases. In my particular shoot above I was able to use HSS to harness complete control of my flash output without having to adjust distance, diffusion, or any other factor.
The example below shows the exposure change as shutter speed is increased while all other factors remain the same. Note how quickly the output falls off as the shutter speed increases. The difference between 1/320 and 1/400 is particularly fascinating. I suspect the fall off varies from flash to flash and thus I would suggest testing your flashes in this way to give yourself an idea of how HSS is impacting your exposures. I was using a Nikon SB-700 for this test.
Controlling your light output requires adaptive problem solving as a photographer, especially in situations where there is no opportunity to dim or adjust the raw output of the light being used. This article only skims the surface and is designed to get those wheels churning in your head so that you are prepared for virtually any situation. A few other simple options are leveraging neutral density gels to place on your light or even bouncing your light off a white surface instead of directly at the subject. I'd love to hear about some clever methods you have used to modify the output a light when adjusting its power simply wasn't an option, such as when using non-adjustable hot lights or light sources not designed for photography. Head down to the comments below and tell your story!