I have bought a used 5D3 last month and recently the Tamron 24-70 f/2,8 VC (also used). I just did AFMA for this lens (as I did with my other lenses before) and even with the value set to -20, it's just not enough. It should be about -22 for Wide and about -26 for Tele (these are educated guesses). What can I do?
-I performed a more or less "real life" test at f/4 and it's perfectly ok. Same test at f/2,8 and it seems quite ok.
-I do the following to determine the AFMA values: Tripod, detailed flat object (ie newspaper), camera and object aligned. I defocus manually, and autofocus; then I check the focus using liveview and adjust manually when necessary; I repeat this about 3 times using the middle focus point first and another 3 times for two outer focus points (both left and right). I always defocus first.
-I got very consistent results with my other lenses on this and previous bodies. So I'm confident I do not need FoCal or other packages. It's just this Tamron, which seems too off for calibration but turns out to be ok in real life...