Hi,
I have the Sigma 70-300mm DG APO Macro and use it on the Pentax K-30.
I don't know the difference between the APO and OS version.
But I know that I am very satisfied with my APO.
I have a dedicated Macro (Sigma 50mm 2.8 EX DG Macro 1:1), but tested the Macro function (1:2) of the 70-300 APO out of curiosity.
It is quite good, but I am sure that I have received an excellent sample of this lens as it is very sharp even at 300mm!
On a Pentax camera you don't need a stabilized lens (you for sure know that), but I understand your thoughts as some stabilized, later released versions can be "better".
But mind that it can also be the other way round - the "Tamron SP 17-50mm f2.8" is an excellent lens (if the copy is decent) and it is presumed to be much sharper than the stabilized version.
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