I found an old slr at my parents place today. It has a canon 50mm 1.4 lens on it. The filter on it was cracked and I have just managed to get it off, it seems it was cross threaded. I put it back on for the time being lightly screwed in to protect the lense. What kind of filter should I be getting to protect it or do I not need to worry about this? How can I tell if this lense would fit one of the newer Canon DSLR's, though I know it would be manual focus. Any ideas?