Canon released the EOS-20D DSLR camera in 2004. As a continuation of the EOS-D line, it offers 8.2 million pixel shooting, nine-point autofocus, and a continuous shooting rate of five frames per second for up to 23 shots. The EOS-20D accepts a wide range of external flash units, but not TTL flashes.
TTL
A TTL flash unit, designed for a film camera, communicates with the camera through the lens. This means that it takes light readings using the camera's light detection technology, and not as a separate function. Digital EOS models do not support through-the-lens technology.
Alternatives to TTL
Canon developed two types of flash units as alternatives to TTL flashes. A-TTL stands for advanced TTL and E-TTL is evaluative TTL. The EOS 20D uses E-TTL technology. It's compatible with all E-TTL flash units.
E-TTL
Like regular TTL flash metering for a film camera, E-TTL uses the lens to evaluate the light level. E-TTL flashes release a pre-flash to further evaluate the need for flash, which has pros and cons. The pre-flash provides an accurate light reading, but might cause subjects to blink.
Tags: flash units, film camera