I've been looking into getting one also.


I just picked up the Canon 300 f4L and will be shooting some motorcycle racing, which normally has plenty of light.
My concern is that with the reduced light and in conjunction of having the 1.4x converter on that I might start to have some issues.


Also, having not used a CPL before, what is the turning part of the filter? What does it change and how do you know where to have it rotated to? Trial and error?