<p>I recently acquired a FD 300mm f4 and Rodney's filter fix sounds interesting. I just purchased a "cheapy" Vivitar 77mm 3 filter set and I'm thinking another option would be to carefully wrap the filter threads with masking tape and do a friction mount. It probably wouldn't provide much front element protection (since the filter could fall out at any moment) but it would make it possible to quickly attach a filter when you need it for a shot.</p>