I really like my Pentax K20 digital camera with the Pentax 18 to 250 mm telephoto zoom lens.   I often use a polarizing lens to help make skies bluer.  When I use the polarizing lens, with the 18 to 250 mm lens I see vignettes in all four corners of my picture.  I’m guessing that this is because the 18 mm setting for the lens “sees” at a wide angle consequently catches the edges of the polarizing lens and that’s what appears in photos.  Here’s an example of what I mean:

Notice the darkened corners. This is know as a vignette.

Step-up rings provide a way to fit a larger diameter filter or accessory onto a smaller lens. The step-up ring screws onto the lenses threading and provides a larger thread which the photographer can then attach other elements including a polarizing filter or more step-up rings.

In my case, my lens has a 62 mm diameter.  I purchased a 62 mm to 72 mm step-up ring.  I also purchased a new 72 mm polarizing filter to screw it into the front of the step-up ring.  It worked great.

No vignette using the step up ring.

The step up ring gave enough diameter space so that the 18 mm wide angle setting did not “see” the edges of the polarizing lens and the vignettes did not appear.