I just purchased the Tamron 17-50mm f/2.8. And I am pleased with the results so far, except for the bokeh which is kind of strange sometimes. You can see concentric circles in the out of focus highlights. I`ve tested the Tamron 28-75mm f/2.8 and didn`t have that problem. So here is my question: Is this kind of bokeh normal for that lens or did I just receive a bad copy?