How many movies have Christa Ludwig and Humphrey Burton collaborated on?

They have worked on 6 movies together! Their first movie together was Das Lied von der Erde: A Personal Introduction (1972) while their highest rated movie together was Mahler - Symphonies Nos. 1, 2 & 3 (1973). They have not worked together since Candide (1991). They are most known for their music films together.