Polariser problem

[flatlander]

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

[Don Hoey]

If I have the lens correct then at 135mm the largest aperture is f4.5 and at 400mm its f5.6.
The polariser will cut the light by up to 2 stops so a/f will struggle as its equivalent to using a lens with max aperture of f8 or f11. If using at long range ( infinity ) the best bet is to use manual focus. Easy if your lens has a hard stop at infinity mark.

Don

[petrochemist]

I think the problem may be that you are using a linear polariser instead of a circular one. Many auto focus systems do not work with linear polarisers. A CPL has an extra stage to get round this.