Thanks everyone. Especially for those links Les Sarlie,they really cleared up a few things.

Now another more specific question. The links do not cover the 55MM f1.2 SSC lens. I already have a couple 50MM f1.8 but this lens has always caught my eye. Do not know why but I'm interested. KEH has one for $349 in EX condition. Would it be a worthwhile purchase?

It's amazing what good glass can be had for now. Is eBay worth the effort? It seems as if the sellers are getting more and more sketchy as time goes on. That place use to be the online flea market but now not so much.