Originally posted by justDIY makes sense ... a popular online auction site has 77mm "thin" circ. polarizers for 1/3 the cost of a hoya or tiffen "thin", might have to give one of them a try, if all else fails, a stepdown ring will mount it on my 72mm 28-200. was also looking for a 'split' nd filter or whatever they're called, where the sky is darkened but not the ground.
An ND filter is really different than a polarizer, and what they call what you were explaining is a graduated filter. But if you don't position the center area correctly, where it blends from one value to the other, it's probably not going to give you exactly what you want. I speak from the film days of yore, and just started with serious DSLR, but many of the principles are the same.
I don't know what kind of shots you have in mind, or your camera, but if you're talking scenics and your camera does HDR--high dynamic range-- that may be exactly what you're looking for. And it's something I want to start experimenting with as well:
You set it for HDR, and the camera takes THREE exposures--based on highlights, mids and shadows--and processes them into one final image. From what I've seen, the results can be SPECTACULAR, and it sure as heck takes a light washed out sky and darkens/saturates it (don't know exactly how to describe it correctly) to make a beautiful image, albeit artificial looking.
Just a suggestion depending on what you're looking to achieve.