This post is only applicable to the Canon range of EOS models that have an integral flash

All of Canon’s past and present DSLR models that have a flash built in into the camera will have this featured enabled by default*. The purpose of this function is to help prevent the problem of red eye when shooting people portraits in low light.

* On the latest EOS models as in EOS 550D & 7D this facility is actually disabled by default

What is red eye and why does it occur?

The iris in our eyes work in a similar fashion to that of the iris blades in a camera lens. Coupled with the metering system of the camera, the lens iris blades open and close depending on the amount of light entering the lens. In bright conditions the camera’s metering system sends communication to the lens telling it to close or stop down. This action prevents over exposure or the images being recorded as too bright. In low light the reverse happens as in the metering system communicates to the lens telling it to open the iris so as allow more light to be received by the camera. In similar conditions the brain sends communication to the iris in the eye telling it to either open or close depending on the ambient lighting conditions.

The direct consequence using flash in low light is that the light from the flash enters the eye and reflects off the red blood cells in the back of the retina hence the term red eye.

The function of the red eye reduction found on the Canon models is to help prevent this phenomenon. Before firing the flash, the camera emits a constant beam of light from an LED lamp that is positioned on the front of the body. The light from this lamp is normally sufficient enough to trick the brain and the lens iris into stopping down, thus preventing the problem.

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