Flash Exposure

I have been told a lot of rubbish in my time of using studios, but the one that always annoyed me most was being told to adjust my shutter speed when the exposure was not quite right. In a studio environment this is largely dribble!

The flash sync speed on your camera is the recommended maximum shutter speed when using a flash. It can sit anywhere from around 1/125 - 1/250th of a second (there are things called leaf shutters that are an exception). When you go above the flash sync speed you enter into the realms of high speed sync which is a story for another day. Your shutter speed has no real bearing on your flash duration. Depending on the overall power of your flash and the output power you choose, the flash duration could be down around your flash sync speed but equally when using a lower output power like 1/32, 1/64 or 1/128th power is could be much shorter like 1/10,000th of second.

So why does the shutter speed not have much bearing on the the exposure in a studio? Normally in a studio setup the first important shot to take is one where you do not use any flash. Ideally the resulting image should be completely black. This means that the ambient light in the room is not affecting your overall exposure and you know that any light in the final image will be controlled, by you, using the power settings on your strobes. It also means you will only catch an image for the duration strobes are firing. The duration of your strobe effectively becomes your shutter speed.

Consider a fairly typical aperture and flash sync speed setting of f8 at 1/250th. Our strobe can be a relatively low powered one and, when we fire it at full power, it also has a duration of around 1/250th. First we apply our test shot and make sure that at f8 and 1/250th, with no flash, we do not record anything on our image. Next we take a shot with flash and we will record an image but only during that 1/250th of flash. The resulting image maybe underexposed so you may be tempted to change the shutter speed down to 1/60th (remember 1/60th is a longer duration than 1/250th -2 stops extra) after all, outdoors, shooting landscapes for example, a slower shutter speed will increase you overall exposure. The problem is the flash is still going to fire at 1/250th and we only record an image whilst the flash is firing. The net effect is the change in shutter speed makes no difference at all!

The same is true going the other way. So as not to break our flash sync speed let's say we take our test shot with f8 at 1/60th . Then, when we add our flash, the image is overexposed. Running our strobe at full power the flash duration is still 1/250th. Increasing our shutter speed to 1/125th or even 1/250th will have no real effect on the overall exposure because the flash duration is still shorter or around equal to the those shutter speeds.

If you now allow for running the strobes at lower power settings their duration becomes even shorter, say 1/5000th, making longer shutter speeds even more redundant when it comes to controlling exposure.

Shutter speed does become relevant when we want to control the ambient light (i.e. light not from the strobe). Remember in our test shot we made sure there was no other light affecting the overall image but what if we did want some ambient light like, for example, a candle or a table lamp? By using longer shutter durations we allow the ambient light in the room to continue to expose the final image even after the flash has fired. There is, therefore, a simple rule -

Aperture controls flash exposure / Shutter speed control ambient exposure

As with all things photography, balancing these two variables determines our overall image mood. The image below was shot using two small strobes with gels for the background, and a larger strobe in a soft box for the key light. A small disco globe provided the blue streaks at the bottom but in order to get them to be seen I had to use a shutter speed of 1 second. The shot was actually hand held but because most of the image is captured by the flash it was still frozen with no camera shake. You can see a slight ghosting on the hands. This is because I was not shooting alone and another troublesome photographer obviously triggered the flash just after (or before) I took my shot and caused some movement to be recorded.

I have said this is mainly to do with studio lighting but the same rules apply outdoors. The main difference is that outdoors there is likely to be much more ambient light and therefore more difficult to control. You can use the rules though to change the mood of an image. In the shot of Kamie below, I have deliberately increased my shutter duration from the normal sync speed of 1/250th to 1/125th to bring in more of the sky and sunset light. At 1/250th the background would have appeared much darker.

So there you go. If you are ever in a studio and they tell you to change your shutter speed to adjust your exposure you know what to tell them! Alternatively come to mine and we can get it right first time.