Q) You mentioned the Singh-Ray variable neutral-density filter (Vari-ND) in a previous OP column. If I have my information correct, it appears to be made only in a 77mm mount. I don't know if I can find an adapter to mount it on my lenses with smaller filter sizes, but if I can, will it work properly with such an adapter?
A) Filters are relatively expensive and heavy, so it helps to choose a size that matches the diameter of the largest lens you carry (for many of us, that's 77mm), and use a filter ring adapter ring to fit other lenses, for example, using a 62mm (lens) to 77mm (filter) step-up ring. They're readily available from camera stores or online from sources such as B&H Photo and Video. There can be a problem with using the adapter and filter on extremely wide-angle lenses because the adapter moves the filter away from the front of the lens and may cause slight vignetting.
The Vari-ND is relatively thick and can't be used on extremely wide-angle lenses on full-frame digital or film cameras. I carry a thinner warming polarizer from Singh-Ray that I use on my 17-40mm wide-angle zoom, which causes no vignetting, even at 17mm. This image of a river in Yosemite National Park was taken with the Singh-Ray Vari-ND at 26mm using a 17-40mm zoom lens and a Canon EOS-1Ds MKII. About 24mm is the widest this filter will allow on a full-frame D-SLR.