It is probably the best 50mmF/1.8 lens ever made in Japan for RF. The filter size is odd ∅40mm today, same as LTM 100mm F3.5's. 1956, type II (F1.8-F22), 188g, 4 grp / 6 elm, $100 @2008 Info > http://www.dantestella.com/technical/canoleic.html