I have a few upgrades in mind in the next 6 months, one being a camera body the other being a telephoto lens. I know the benefits of f2.8 vs f4, mainly the DOF and the 1 extra stop of light, but with today's cameras achieving the high ISO that was only a thought 3-4 years ago, are the benefits still there? Sure, on my current XSI, which I cringe if I have to go over ISO 400, the extra stop from 2.8 would be beneficial. But what about on a more recent camera, where shooting 3200 ISO is pretty clean and 6400 is still fine? I'm a Canon shooter so I'm weighing the differences between the 70-200 f/2.8 IS and the 70-200 f/4 IS. The f/4 seems so much lighter and less bulky, has IS, is known to be one of if not the sharpest Canon zoom out there. Does the f/2.8 still have a major leg up on the f/4? A friend of mine and I were discussing this the other day, and with his 5D MkII, he went with the f/4 instead of the f/2.8 mainly due to size and weight.