Confused about step up rings..

Yes I asked this question quite a few times, but still haven't made the purchase. I'm buying a 72mm CPL for my 35L, but I want it to also work with my 17-40L. Getting a 72mm to 77mm step up ring seems to make sense, but won't I have to worry about heavy vignetting or some type of similar problem?

I don't use them but it's my thinking you want to get a 77mm CPL filter to work with the ring in question. That way it'd work on the 35L. If trying to use a 72mm filter on a 77mm diameter Lens would led to darkening in the corners.

Get the 77mm CPL and a 72-77 step up ring. If you get the 72mm CPL you will need a 77-72 step down ring and would have vignetting at the corners.

Rings are listed as the lens size first and the filter size second.

Steve - Toledo

Right - get the bigger filter size and get a ring to step-up the smaller lens diameter to the larger filter size. Going from a large lens diameter to a small filter is taking a step down.