CNN has an interesting article dealing with the work that went into restoring ‘Seven Samurai’.
Apparently, the original negative of the film is missing, and the criterion DVD restoration was based on an early positive. However, obvious problems remained: “The technical team had to cope with the fact that the positive had shrunk, meaning that light could get in around the edges of the frame; that scenes contained black frames or missing frames, making transitions jarring; even that the original mono soundtrack had to be restored.”
Also mentioned is the fact that restoration decisions were not always easy to make. One example is a scene that Kurosawa seems to have shot with a hair on the lense. The team decided to leave it as it is, since Kurosawa himself had not decided to reshoot it.
All in all, interesting reading.