Unless you are an optician, how is the average photographer-in-the-street to best decide among the myriad of filters on the market? So, I went for a well-established make and a Pro model on the basis that thinner glass is better (less if any vignetting). What can I say: it screws onto the lens (Canon EF-S 18-200mm IS), without problem; you could screw another filter onto it; the glass does not rattle around, and I've noticed no strangeness with the images produced. I wanted protection for the lens, besides the UV filtering.