Its for SLR AF.. cause a SLR uses phase detection AF usually.. and a linear polariser will screw it up..
I think its safe to use a linear polariser on a digicam though (non-SLR) cause usually they will use contrast detection AF.. (cheaper solution.. but I don't guarantee it will work.. try it at a shop first.)
I haven't used a linear polariser before, but generally the idea of how a polariser works is that you rotate it such that it cuts out polarisation at that 'angle'
However, this is not exactly how it works for a circular polariser filter, and I don't have a degree on optics, but generally it works the same way..
So, rotating a circular polariser WILL affect the image.. I suggest you try it out first :bigeyes: