For a while now I’ve been struggling with exposure and colour balance problems when shooting roller derby at the Glasgow Caledonian ARC. Every so often a shot will be underexposed and usually of a slightly warmer colour temperature than the shots next to it. At first I thought this was a a problem with the camera’s exposure system, but when shooting on manual I’m getting the same problem – two shots taken seconds apart, with the same camera settings, can have different exposures. It doesn’t happen a lot, but it has resulted in the loss of some good shots.
These two unedited shots were both taken with an exposure of 1/200th of a second at f/2.8, but the second one is about a stop darker than the first and is slightly warmer – look at the colour of the floor and wall.
Sometimes the underexposure is much worse, but this was the best side-by-side example I could find.
It took me a while to figure out what was going on here, but eventually I remembered that when I was shooting with slower shutter speeds I noticed something odd about the lighting. It flickers.
This can be seen best in this detail crop of this panning image, taked with an exposure of 1/40th of a second at f/4.0. Look at the faces in the background – each one is repeated 3 times. So during the exposure the overhead lighting flashed 3 times.
My guess is that the lights are flashing about 100 times a second. Way too fast to be visible to the human eye, but if a camera shutter is set to an exposure shorter than this then there is a danger of catching the lights when they’re at the low point of their cycle, underexposing the image. As dimmer lights tend to be warmer, this would also account for the shift in colour temperature.
Unfortunately the slowest shutter speed that completely freezes the motion is about 1/200th of a second. Dropping this to 1/100th of a second to avoid the exposure issues results in more shots lost to blur than would be lost to underexposure.
Other than using flash, which I don’t really like doing, there’s not a lot that can be done about this other than avoiding shutter speeds faster than 1/200th of a second as this is likely to make the problem worse.
So far I’ve not encountered this problem anywhere else I’ve shot, so it seems to be quite an unusual feature of the ARC lights.