Can a Contax lens that came off a Contax II camera fit a Contax IIIA?
Not a pre-war 35 Biogon, which has a rear element which will not fit into the the II/IIIA. The lens was redesigned after the war. If it says Opton it will vbe OK.
There may be other exceptions. Which lens are you concerned about ?
"One of the painful things about our time is that those who feel certainty are stupid,
and those with any imagination and understanding are filled with doubt and indecision"
Yes. Which lens is it? 50mm f1.5 sonnar?
It's the 135mm Sonnar
Works fine. Stiil state-of-the-art.