E-TTL II exposures
Adjust the flash output for bolder or more subtle results
canon’s E-TTL II flash metering system takes a lot of the pain out of flash exposures when you need to take shots quickly. Your camera will do all the heavy lifting, calculating how much light from the flash will produce a balanced exposure with the ambient light. It does this by firing a low-power pre-flash before the exposure starts; in some instances, during a slower exposure, you may notice this pre-flash, but it’s usually indistinguishable from the main flash. The light that’s reflected back through the lens from the object bounces off the mirror in front of the camera’s imaging sensor, where its brightness is measured by the camera’s exposure meter. The camera then uses this information to set the power of the main flash.
However, E-TTL II doesn’t always get things right. Bright or reflective objects can cause the flash to be extinguished too early, leaving the flash-lit subject looking too dark, while dark objects can result in the flash not being extinguished quickly enough. The background’s brightness can also affect the exposure in a similar way, especially if the subject is small and off-centre in the picture. If the camera moves and the composition changes, you might find that the flash exposure varies between shots too – sometimes wildly.
You can override the camera’s setting by increasing or decreasing the flash output using flash exposure compensation (FEC), either on the camera or on the flashgun. Canon cameras allow you to apply compensation to the ambient light exposure and the flash exposure independently, so adjusting the flash exposure won’t affect the brightness of the background.