When I was looking into starting out I read lots of items saying filters were essential so I bought a bunch and stuck them on all my lenses. When I eventually upgraded to a 7D and Canon 100-400 L the IQ was awful. Soft focus, dull and insipid photos seemed to be all I got. I took off the filter one day and tried another shoot and the difference was amazing. Bearing in mind I'd bought good quality filters and spent over £200 on 5 filters I was very disappointed. Now I only use a CPL filter occasionally and only ever use a UV filter if the weather is really adverse.