Sigma 70-200mm f2.8 for low light situations

[gwales]

I am now thoroughly confused!

I was on the verge of sending it back and researched some more online.

Using a focus test chart printed and pinned to a door opened at 45 degrees I can take a pin sharp shot at 200mm f2.8 from just over the min focusing distance (1m) focusing manually. The same on auto focus produces a focus plane centred about 15mm ahead of the point of focus (but not in the view finder?). I can do this consistently.

The weird thing is I have not yet encountered a scenario where I would do this and if you stop back the aperture or zoom then the problem is not evident. Similarly if you are further from the target (as I will most likely be).

Some folk claim that Sigma can calibrate the lens and remove this, others claim that its by design?

I think I will keep it after all since I know when the issue will occur and can switch to manual focus if I can't change settings or composition on those very rare occasions.

I would like to understand more about what is happening though ....