In article <4djc6o$fqd@senator-bedfellow.MIT.EDU>, ddarmawa@athena.mit.edu says...

>I just bought Sigma 18-35 lens and plan to buy a polarizer filter for >this lens. (...)

I would recommend borrowing one first (and ignor the issue of
vignetting on the borrowed polarizer). You will probably find
that the polarization effect is unacceptably uneven on wide
angle lenses, making the issue of vignetting irrelevant.
Hope This Helps