^ What Zombie said is exactly what I said before but in better words!!!

And in the shots that were originally planed to be optical composites, the generation loss is absolutely minimal, because they used 70mm film to shoot the blue screen elements and then they composited all the elements one by one on one film strip. So only one generation was lost and that was compensated for by using the 70mm film. So it is quite possible that a lot of the original finished composites were just as clean as the live action photography, if not even cleaner due to being handled in lab conditions only.