Silent-era visual technique: inner thoughts rendered as photographic images overlaid on actor's face, typically via double exposure. Psychological representation before montage.
With thought photography, early filmmakers—notably French cinema of the 1910s—attempted to make the invisible visible: to directly capture a character's visual thoughts onto film. The inner world was literally superimposed onto the ongoing shot, mostly through double exposure or optical overlays, sometimes through pure editing. The effect seems naive today, but at the time it was the only tool to materialize subjectivity cinematically.
Practical Implementation: On set, it was simple: the camera rolled, the actor stared blankly into the distance or touched their temple—universal gestures for "moment of concentration." Then, the same film roll was rewound, and a second exposure was made—ideally a dream, a memory, a face, or a suggestive scene. The two images overlapped in the negative. Exposure times had to be precisely calculated, otherwise the entire shot would become gray and washed out. Light control was crucial: the thought shot had to be significantly brighter to stand out from the main shot. Some cinematographers worked with padded masks in front of the lens to spatially control the overlay—the thought in the upper right, the facial expression in the lower left.
The effect was later superseded by editing, through explicit cuts and dissolves, which was more precise and faster. But thought photography left its mark: it established the visual convention that inner images must appear "softer," "overlapped," "dreamlike"—an aesthetic that still resonates today in modern flashback or memory sequences, only now generated digitally in post-production.
Today, this method is used at most as a stylistic quotation or in experimental cinema. But any DoP who has ever created a seemingly tame double exposure or deliberately diffused the focus in thought montages is still working within the visual logic of these early special effects—without knowing it.