Polarizing Filter for 24-70 with a 20D

Is there any reason I should purchase a "slim" circular polarizing filter for my 24-70 2.8L instead of the standard filter (using a 20D)? I was looking at purchasing a B&W filter, and the slim filter costs $25 more.

If the 24-70 isn't wide enough to worry about using a slim filter, at what point would it be a concern? For example, the 17-40L also uses a 77mm filter.

Bob

I think slim filters are more useful if you're stacking filters. I got the "standard" B+W multicoated one for my 70-200, though I haven't actually had time to attach it yet.

Thanks Tim,

I believe that the slim filters are to be used with wide angle lenses so that you don't get any vignetting. To make them slim they don't have threads, so another filter can't be stacked on top. I'm just not sure how wide is wide 24mm doesn't sound very wide to me, so maybe the standard type of filter is ok ... but what if I purchase a 17-40 in the future, would the standard filter still work?

Regards,

Bob

With the 24-70 on the 20D, you won't need a thin filter; if you were using it on a full-frame camera, you might. But the 20D doesn't use the full FoV that the 24-70 has.

Bob, if you are going to get any wider lenses with the same filter thread, it might pay to get a thinner polarizing filter up front. Canon's lens hoods for the big lenses don't screw into the filter, so you don't need threads on the front side of the filter's frame.

Thanks Skip and Jon,

It's an expensive filter, so it would probably pay off in the end to get one that covers all of my bases. It sounds like my best bet would be to get the slim filter since it would work well using my 24-70 either on my 20D or on my Elan II, and it would be the best choice if I end up getting the 17-40.

Bob

I use the B&W 77mm CPL (not thin) on my 16-35 f/2.8 and 24-70 f/2.8 on the 20D and 1DM2 and I never have problems with vignetting.

Thanks Scott!