Stu_bert said:
Thank you. So why don't we hear much (or do we and I'm just not very observant) about the same issues with APS-C sensors?
Because you get a much tighter framing, here everybody expects to need higher shutter speeds or a tripod.
People keep quoting the 1/f rule of thumb, 1/400s for 400mm and so on. With crop cameras people understood that the focal length has to be the apparent FL to keep the validity so you needed 1/640 under otherwise similar circumstances. Now with the higher res sensor you get the original framing of the lens back, but still require the faster shutter speed from the crop camera - it's the same pixel pitch after all, just with more picture along the borders. That raise from 1/400 to 1/640 would be the one way to get sharp pictures, the other would involve better technique, to get similar improvements at the old shutter speed.
(nota bene: the rule of thumb is just that; between higher resolution and IS it's even less set in stone then in analouge times)