Upload & Sell: Off
John, I have gone through 5 copies of the tamron lens (long story, may make a post about it some day) and have compared it to 2 copies of 24-105, and one copy of 28-70 2.8L, that I have. I finally have what I consider to be a very good copy of the lens. that much said, here are my findings,
1. QC is simply not that impressive on this lens of the 5, one was terrible (think coke bottle), 2 very significantly decentered, and one was moderetely decentered, and centrally a bit soft at 50 and 70.
the rest are assessment of a good copy of this lens
2. 3/5 copies where very sharp in the center throughout the zoom range. there is a bit of a dip at 70, but for portrait work, I would still rate 70 2.8, centrally as excellent.
3. the lens turns in its most even performance at 24mm and 70mm. at 24 mm, it has good edges and decent corner even wide open, and stopped down, is very good, good enough for landscape.
at 70, even at the edge of aps-c frame, the lens would need to bet stopped down to f 4 to sharpen up some, but good news is, by f8, it has very good and even performance, though not stellar.
4. the lens is weakest at 35, wide open, and stopped down, where I dont feel beyond asp-c circle it get critically sharp. the good new is that the corners are good. the bad news it just never gets completely sharp other than zone A. at 50 its a little better.
5. the lens at 24 is better than 24-105, and 28-70, open, and stopped down, everywhere. at every focal length, at 2.8 and f4, at center its shaper than the other two lenses, but by smaller margin (I have a stellar copy of both of those other lenses though).
6. beyond 24, I would take the 24-105 for landscape photography. stopped down performance is better and much more even with the 24-105. this is also true about 28-70, which is fantastic, except in the very extreme corners. in fact, in zone B (say just beyond the crop circle) at 35, 50, and 70, both 24-105 and 28-70, have a slightly better performance at f4-5.6, than the tamron has at f8.
7. Autofocus on a 5d3, is decided slower than my 24-105, which is no speed demon. the lens struggles tracking moderate speed moving kids. it is also more prone to AF errors even with stationary objects.
Now, owning over 15 lenses, a 5d3, having taken 200K images over the last 5 years, and having tried 5 different copies of the tamron, I am fairly certain that my assessment is neither due to user error, or just having a bum sample. The digital picture, also had about as much luck with this lens, until tamron send him a copy (I am sure one of their best ones) for his test.
Now, I will still keep the lens. for one, I have invested so much time and energy to find a good copy. for two, I would be primarily be using it for shooting people, where the lens performs very well when it focuses properly. it has a vc which is awesome.
All that said, If I were to start over, I would seriously consider the canon 24-70 II