Hi folks , perhaps someone can advise on this question, while out yesterday I was taking some long range shots of some shipping with my Sigma 135-400mm lens and thought I would fit a polarising filter onto the lens to cut down any glare etc, however I could not get the camera/lens to focus sharply, I tried rotating the filter but still to no avail - On removing the filter - the lens autofocus etc worked perfectly
, and I have tried it again to today in more controlled conditions with the same results - has anyone else had this happen to them, - I'm sure I have used this same filter on my 10-20mm with no problems - is this a telephoto thing or a dodgy filter? - hope you can help