Fuji X10 filter
I don't have time at present to perform a test of the nature you described. I can tell you that at 28mm at f2.0 I can detect no vignetting. But, for that matter, I could detect no vignetting in the photo in the thread you referred to and it's author was also somewhat ambivalent on that point. In any event, if vignetting is a problem when using 40mm filters, it will certainly be more obvious with the CPL filter which I assume will be thicker. When the CPL I have ordered arrives, I will try to test both filters in the manner you suggrested and post the results. Based on my experience, however, the protection afforded by the 40mm lens protector would outweigh any vignetting which such test might reveal at the "pixel peeping" level. All to often in these forums I feel that we end up in debates about differences which don't make any difference to me.