Tamron 18 - 270mm wide angle focus problem

I bought the new Tamron 18 - 270mm PZD lens recently for my Nikon d5100.

Recent articles read on the subject of back focusing led me to test this lens. There is a slight back focus from 18 to  around 50mm with AF. This was tested using Live view versus AF. Therafter the lens focuses accurately with AF. This is in a range of 1 - 3 meters, thereafter the DoF at wide angle compensates.

I have the identical problem with an older model lens, the 18-250mm, used on my Pentax KR. The back focus is worse on this lens until around 50mm when it starts to clear up and the AF becomes accurate.

The existence of the same problem on two different models of the lens on two different cameras might suggest that the lens design is at fault, but possibly more likely, the phase based AF systems of the Pentax and Nikon are not fine enough to manage the natural depth of field one gets at wide angle settings. Live view in both cameras uses contrast detect and gets accurate focus.

I have learned to work around by catching focus at around 70mm and then zooming out to the wide angle of choice without releasing focus lock. This can of course affect exposure sometimes.

The misfocus is not so bad that the picture is obviously OOF until you look at the fine detail in the AF vs LV images, when it becomes obvious that AF is soft whereas LV is sharp.

I wonder if other users of this lens have noticed this problem?

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jamesza