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Hi Ron,
I believe you are using a Canon 500D body with your Tamron 18-270mm. Unfortunately there is no back focus adjustment to be made with your camera body. I have found the 18-270mm and my 550D to be much sharper than I expected, beyond 230mm it gets a bit soft and at 270mm it's quite noticeable when "pixel peeping".
Between 50-200mm it's very reasonable, and great around 70-130mm. I use this lens when out and about travelling light. I have better lenses, but I still have a use for the Tamron for it's flexibility. Have you tried testing it for back focus issues? It's worth spending the time if only to prove where the fault lies. If the lens is flawed then you may have good reason for Tamron to fix it for you. If the issue lies with your camera body then you need to know whether investing in another lens will show up the same problem.
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