My canon new f1 came with a Zenitar 16mm 2.8 fisheye lens. it is actually in m42 mount with a canon fd adapter, which is good and bad. Bad because I have to use stop down metering, good because I have 2 other m42 lenses and no m42 camera! So.. how good is this lens? It looks like it can take tiny rear filters.. the one on mine says "c". Can I get more filters for it? Will m42 lenses focus to infiniti with the canon FD adapter? Thanks for the info!