I've seen it explained in more than one place (but I don't remember where any of those places were, now) that because high-resolution CCDs are more sensitive to the angle of incidence (I assume this is because it results in more chromatic aberration than with film?), lenses faster than f/2.7 or so aren't viable for most CCDs. My ancient Mavica FD-71 had a f/1.8 lens, and took brilliant low-light pictures compared to just about anything I've seen since.