Polyvision was only used for the final reel of Napoleon, to create a climactic finale. Filming the whole story in Polyvision was impractical as Gance wished for a number of innovative shots, each requiring greater flexibility than was allowed by three interlocked cameras.
The use of Polyvision in "Napoleon" was limited to the final sequence, as Gance sought innovative shots that went beyond the limitations of a standard three-camera setup. This approach proved more practical for the climactic finale than filming the entire film with Polyvision.