in the review by imaging resource they point out that the lens is slightly sharper on a sub frame camera than the full frame one.
This indicates the center is way better than mid-frame to edges.
dpreview made the statement that both jpegs and raws from the 70d were indistinguishable from those taken with the 6d up to about 3200iso.
... if looking at them at the camera lcd, or you're really drunk and everything gets fuzzy :-p ... no, really, for certain pictures with a lot of background blur where nr doesn't have that much of an impact Canon aps-c shots might be ok @100% crop, but iso3200 is where the small sensor really drops the ball. Face it, a aps-c with this Sigma cannot replace a ff+200-400 :-o
the review goes in to detail of center sharpness on both bodies and such. I have researched this lens pretty heavily so i am pretty sure of what its qualities are.
here are the points i am making:
1) 120-300 on a crop body will act in practice as 192-480 on full frame
2) the lens sharpness, build quality are by all accounts in real reviews to be in the same category as the canon primes
3) its a stop faster than the 200-400 the consensus is that ff is 1.5 or 2 stops better than aps-c so you have a one stop equalizer
4) if you are really concerned about the depth of field, they should be very similar wide open
from what i have seen, the canon 300ii and the 200-400 are better on ff but its close, i have read that the sigma is slightly softer than the canon 300ii but not by much.
so as i said before, i think the sigma 120-300 on a crop body is a good alternative to the 200-400 on ff unless money is not a concern and you just want the absolute best that can be had
It's not a 120-300mm, please understand that. It's a 120-280mm and that's quite short. The max focal legth is the same as a 70-200mm with a 1.4x TC. This short coming magnifies with a 1.6x crop to only 450mm not a 480mm.
At close focussing distances, this lens looses a massive amount of focal length. By my measurements is around 240mm at 3m. By MFD it's even lower and not that different to a 70-200 f2.8. So lets look at those figures at close distance, on a full frame at MFD it's a 120-240mm.
Then there's the engineering aspect, it's not particularly durable. Just look back though this thread. There's been a lot of warrenty claims for such a low volume lens. I would question it's long term resell value and durability.
9/10 I was better served using a 70-200 f2.8 LIS II and a 1.4x TC. For the other 1/10 I was better served by other lenses (300mm f2.8 LIS mk I for instance).