When I got ahold of the AF-S Nikkor 18-200mm recently, I discovered it took 72mm filters. Great, I though, my 77mm polarizer won’t fit on it. In fact, I had a number of polarizers sitting around (49mm, 52mm, 58mm, 62mm, 67mm, and 77mm), none of which fit the new lens.
I found a cheap 72mm polarizer, but decided to stop the madness and not buy any more polarizers. I poked around on Amazon.com and bought a set of stepping rings, which are also called step-up rings.
The set of seven was only $15, and included incremental sizes from 49mm to 77mm. I might have to stack more than one stepping ring, but I will now be able to get a polarizer on any lens I own.