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The Digital Picture has the best review that compares all four, and I wouldn't consider any lens a significant upgrade from the 24-105L other than the 24-70/2.8L II. The Tamron is nice, of course, but not nice enough unless you're in a pinch; and the 24-105L is quite cheap in a kit.
A VERY brief peek at it makes me think:
24-70 II easily the best overall then a decent ways back indeed the 24-70 f/4 IS then a bit back the Tamron 28-75 2.8 then a bit back the 24-105L. I don't really think the 24-105L can touch the 24-70 f/4 IS at 24-28mm at all, although I have suspicion it might be able to outdo it at 50mm, especially anywhere beyond middle of the frame. I mostly looked at the wider end when I looked at 24-105Ls though. My very rough feeling is that the 24-70 f/4 IS can give pretty decent real world edge to edge by f/5 at the wider end maybe you need f/7.1 for tricky scenarios) and by f/8 at the 50mm and trouble zones, although below f/7.1 at the edges in real world complex scenes might get dicey once you start getting away from the wide end. My feeling is that the 24-105L doesn't give a really nice edge to edge at the wider end at ANY aperture from the three copies I've seen (although a very few copies may deliver better results). So the 24-70 f/4 IS has it beat there, but as I said it is certainly possible the 24-105 gives as good or better over maybe 45-70mm?? Still the wide end is the critical end for some, it was long hard to find anything delivering critical edge to edge near 24mm for FF while a cheap 50 1.8 could give a perfect 50mm on FF and so could most of the 70-200/300L lenses.
The 24-70 II has amazing resistance to PF/LoCA and any sort of halation or glow or fuzzing effects or weird higher order CA and so on, easily the best at all of that. My impression is that the 24-70 f/4 IS is definitely the second best at that near 24mm (and the 24-105L the worst) although it could well be worse at 50mm in those regards than the old 24-105L.
The IS actually works a lot better than I'd expect at the wide lengths, somehow getting around mirror slap and such and I even got a few 1/3s 24mm shots critically sharp at 100% and many such at 1/8th and almost all at 1/15th. Walking through a forest and not wanting to stop for a tripod all the time for reasons of not wanting to hold up others with you or for simply wanting to see the whole trail before it gets dark sort of situations I believe it would be the lens to take over the 24-70 II and I could see some times where I recently would've easily done better with it than the fabled 24-70 II. In situations with enough light or when you have time to tripod everything then I think it's pretty clear you just stick with your 24-70 II though. The IS unit in the 24-70 f/4 IS is crazy silent, spooky silent, never heard a more silent IS, you'd swear it's broken, only but for the images coming out crisp at shutter speeds when they normally wouldn't and the VF being just a bit more stable looking.
The macro mode could probably be pretty cool in a pinch, not everyone carries along a whole collection on every walk. You probably need to watch for focus shift though and maybe dial in a new MFA? It could be a handy thing though indeed.
There definitely seems to be copy variation with 24-70 II and 24-70 f/4 IS going by combo of personal experience and reading lots of user reports and reviews. My 24-70 II is almost perfect but there is a slight tilt in plane of focus where lower left doesn't like stuff to be really close in and upper right stuff super far away in scenes where you need to grab a super wide DOF all at once, otherwise it's crazy superb (especially when you compare it head to head with other lenses then you really see how good it is and that whatever troubles it may have are mostly the same or far worse with anything else and how the micro-contrast and sharp edges tend to really pop out next to same shot with something else, other a bit on far lower left corner at times on mine (if nothing is close then it's crazy crisp to lower left corner even at 24mm f/3.5 on mine).
Haven't touched a Tamron 24-70 VC as of yet.
Edited on Jul 23, 2013 at 10:05 PM · View previous versions