Originally posted by tim60 Dave, do you use the original hood, the rectangular one? I was lucky when I bought mine that the seller offered both lens and case and the original pouches for each, and at least the original front cap, I forget whether I got the original rear cap with this lens.
Mine also came with the original two-part rectangular hood, which consists of a 58mm-77mm step-up ring, then the hood itself which has a 77mm thread. As Jorgario has discovered, you can't even use a 58mm filter on full frame without vignetting -- you have to use a 77mm filter on the step-up ring.
I'm not sure if a standard any-brand 58-77mm step-up ring plus standard 77mm hood would work. The step-up ring that comes with the original hood is a clever design specially for the job, and I think you might get vignetting with just an off-the-shelf replacement. It would be interesting to know if anyone has tried that approach.
Since I usually use my 20mm on APS-C these days, I just use a round metal 58mm wide angle hood with no problem with vignetting.