Deep focus photography (Citizen Kane, Gregg Toland) keeps foreground and background in sharp focus simultaneously, allowing complex staging in a single shot without cutting.
André Bazin argued this was more "realistic" and morally superior to montage — it preserved the unity of space and time, leaving the viewer free to decide where to look. Eisenstein and the Soviet montage theorists argued the opposite: reality is not self-interpreting, and the editor's job is to construct meaning, not present material.
Both positions assume cinema should be doing something. They disagree about what.