I have some feedback about the Rocketfish Circular Polarizers.... STAY AWAY!
Here is a 100% crop of a sign that I took with my T2i and Canon EF-S 18-200 @ 200mm without the circular polarizer (left side) and then with the polarizer installed (right). No sharpening or other post processing on either crop.
I took about 12 pictures of that sign with the polarizer installed, thinking maybe it was motion blur. Maybe I had missed the focus. I was thinking it was me because the polarizer was VERY clean, and from hand-holding the filter and looking through it, it looked perfectly clear. The results I got from all the polarizer shots are very consistent with what you see here (right).
I took 2 shots without the filter installed, and I got the results you see there (left).
I'm not saying you need to spend $150 on a polarizer to get good shots, but you apparently need to spend more than $30
Even as cheap as this thing was, I was still really surprised at the poor quality of the product.
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