The T90 was the first Canon camera to support through-the-lens (TTL) flash metering. This measured the actual light levels reaching the film by measuring reflected light off the film (OTF), shutting down the flash unit once the film is sufficiently exposed. The measurement is calculated using the average reflectivity of color negative film. This system was also used on Canon's new EOS system, making the T90 the only non-EOS Canon body compatible with TTL Canon flashes.
Hmm. I never really new how the flash ttl worked, I guess the different reflectivity of B&W film changes how the film is exposed with black and white? I haven't had alot of experience with flash use, but I plan on it.