Paul & Rick:

One possible solution is using step-up rings if they're larger-diameter original filters (than the filter-thread on your new lens.) Example: say you have a 58mm filter and now you have a 55mm filter thread on the front of your new lens. A simple metal ring, 55mm on the lens side and 58mm on the filter side, does the trick.

Step-down filters sure won't work if on wide-angle lenses because they'll cause "vignetting" (dark edges), but might work if not too radical a size difference on telephoto lenses.

But of course it never seems to work out that way. My observation is that the diameter of the thread mount of newer lenses is getting larger and larger -- or at least for the new lenses I want. (From absorbing your camera knowledge in past posts, Rick, I'm sure you already know about step-up/down rings -- but others may not be aware that there might be a cheap fix.)

A Tiffen 55-58mm (lens-to-filter) step-up ring is $9.95 at B&H Photo online.

A Tiffen 58mm circular polarizer is $20 at B&H for the basic, $66 for the wide-angle polarizer (no front thread on the filter makes it flatter and less chance to vignette), and $80 for the multi-coated one.

So step-up rings not only save money by having to buy fewer filters, but by buying the filter for the widest thread-mount lens, you save weight by not lugging around duplicate filters.

(Rick, can you explain a little more about these expensive multi-coated polarizers? A regular circular polarizer (a non-linear polarizer) won't do? I'm not getting it.)