Richard, this is the first I've heard of the maximum aperture affecting the focus speed. I know there is the f/2.8 focus sensor for higher precision, but that an f/4 lens would focus half the speed of an f/2.8, or and f/5.6 being quarter speed, I've certainly never heard of that. The reviews for this lens don't mention this level of degraded performance either, and you'd think that would be a major concern to point out.

I'm aware of the other benefits of f/2.8, however the new IS II version of the lens is well out of our price range, and the previous versions don't have the level of sharpness we want.

We can't afford the best of the best, and we know we have to give up something for a reduced price. We've ruled out primes, which leaves slower aperture lenses, unless everyone agrees that it's not reasonable to expect one of these lenses to shoot agility.