My Canon HV20 has 43mm threads, and I have a nice wide angle adapter for it. I just purchased a Canon HF S20, with a 58mm barrel, and would love to avoid paying for a new adapter. I realize I may need to use a combo of 2-3 step-down rings to use it. But I have no idea what this means for picture quality. Can anyone enlighten me?

Wouldn't you need to zoom in past the edges of the step-down conversion in that case, thereby eliminating any chance at squeezing a wider view out of your lens?

I guess that's what I'm trying to figure out. I don't know whether filter size correlates to lens size, and I'm not having much luck via Google. Maybe this ought to be moved to a Canon subforum?

Filter size correlates to lens thread size. That's not necessarily the same thing as your lens size. But, I'm pretty sure if you're needing to use a step down ring of any sort in order to put that wide angle adapter on your lens, you're probably defeating your purpose entirely, and then eating additional light and image quality in the process. I think you're much better off getting a properly sized adapter.