amfoto1 Loc: San Jose, Calif. USA

No, it wouldn't be equivalent. As far as speed/camera shake prevention is concerned - i.e. shutter speeds usable handheld with reasonable care - the f4 IS would be 2 or 3 stops [u]better[/b] than the f2.8. However, IS with slow shutter speeds cannot stop subject motion. And an f4 aperture cannot render as shallow depth of field as f2.8.

The f4 lens likely will not focus as fast as f2.8, either. Most Canon cameras have AF points that are optimized to give their best performance with f2.8 and faster lenses. They'll work fine with slower lenses, but not quite as quickly and surely. However it's probably not a difference that you would even notice most of the time... might only see it as occasional hunting trying to focus in challenging light. (Note: if using it on a 5DIII specifically, that camera's low light focusing capabilities are better than most Canon, so might offset this to a large degree.)

It's a bit unfair to compare the 24-70/4 IS with the 24-70/2.8 II.... the latter has been called one of the best zooms ever, by a number of reviewers. So a lens that comes in second place to that can still be very, very good.

The main advantage I see with the 24-70/4 is smaller size and lighter weight. It would be a nicer travel camera and walkaround zoom, for that reason.

If you have some fast primes covering two or three key focal lengths, for most people a slower zoom would not be a problem.

JayB Loc: Northeast US

Thank you for that. A nice perspective on this lens. I have the 16-35 and am planning on a 70-200. I could forget about the 24-70 zoom and get the fast 50mm.