Exposure compensation works as I expect in daylight, but there’s no apparent effect when I’m using a flash. Why?
Simon Rawlings, Southampton
Brian Says… I don’t think you’re alone in this confusion. The cause is down to the way the EOS camera system has been designed to operate, with completely different automatic exposure metering and compensation controls for ambient light and flash.
While it might seem counter-intuitive – or even slower – to have the two controls unlinked, I now find Canon’s approach is more logical to me. In effect I have a control on the camera body to bias the ambient light exposure and a separate control on the flash or transmitter to adjust the flash’s automatic exposure.
Within the camera’s automated systems, the flash exposure and ambient light exposure are separately metered and adjusted. This makes it possible to mix manual or automatic ambient exposure and manual or automatic flash exposure in any combination and have exposure compensation for either automatic element.
Consider a common situation, where you want to darken the exposure of the background behind your subject, but keep the flash on the subject at the correct level. If you set the camera to Aperture Priority mode, you only need to turn the dial on the rear of the camera to adjust the ambient exposure compensation. The camera applies the exposure compensation only to the ambient element in the picture. If required you can separately adjust the power of the flash in E-TTL mode by rotating the dial on the flashgun or transmitter.
To speed up my work with flash I use one of the flash custom functions, C.FN 13, to allow the rear dial to be simply rotated to adjust the Flash Exposure Compensation on my Speedlite 430EX III-RT, 600EX-RT flashguns and ST-E3-RT transmitter.
Most EOS cameras also have Flash Exposure Compensation controls on the camera body. However, this can be overridden by any compensation amount set on the flash or transmitter.