I appreciate the distinction between a single point with AFMA vs. a number of distances with a USB dock, but there are two different issues at play here:
- Front- or back-focusing AF can be treated with their dock to good effect, and yes, Sigma gives a more comprehensive tool to dial that in as a function of distance.
- Inconsistent AF in which the AF does not consistently hit the same target at the same distance is not dock correctable. Again, see my TDP reference -- no dock on the planet will solve that.
The second one above is the bit people are complaining about.
- A
I think you are right to distinguish between those two potential flaws. I wonder, though, if the first could help cause the last. When you have things hinkey with the autofocus, my experience has been that things act much more inconsistently, especially at apertures <2. I speculate that the camera is attempting to determine a binary status (focused adequately or not) on something that looks fudgy due to back- or front-focus.
A couple of my Art lenses - which are rock-solid now for both focus and consistency - required pretty different adjustments at different subject distances. When new out of the box, they'd appear to be hugely inconsistent, which turned out merely to be a factor of the diversity of the distances I was shooting. Not saying this is necessarily the issue with the OP and others chiming in, but hoping that perhaps some of them might find this is a cheap solution.
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I have not had the chance to return to the dealer, to give them a chance to fix the problem. I doubt that they will though. I have been running both FoCal calibration on multiple cameras (5DSR, 5DIV and 1DX-II) and manual attempts, using a LensAlign rig. I have also tried to do it on different distances from. However, the spread of focus is such that both FoCal and I are unable to determine an AFMA setting. A dock will not change this.
It may well be that I have a poor copy (again), but it behaves exactly as the 35/1.4 and 50/1.4 Art lenses did. So I am not optimistic. However, if they fix it, I will still have a hard time using it for anything critical, because I simply will not trust it. Luckily I have alternatives.