I have noticed that whenever I mount a CPL on my Tamron 28-300mm lens (on a Canon 6D), the focus goes a little haywire. Same thing happens when I mount the UV filter too. While the UV filter is a Hoya, I must admit that the CPL is a cheap ebay purchase.
Before I spend good money on a brand name (e.g. Singh Ray), can someone advise me as to why this happens? Is there a solution with my current combination, or do I have to go in for a brand name filter.
I have read many forums on the web. Many people have the same problem, but no one seems to give a reasonable explanation as to the cause, or offer a good solution.
Any help appreciated.