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In page Lost film:

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Most film studios routinely had a still photographer with a large-format camera working on the set during production, taking pictures for potential publicity use.[1] The high-quality photographic paper prints that resulted — some produced in quantity for display use by theaters, others in smaller numbers for distribution to newspapers and magazines — have preserved imagery from many otherwise lost films. In some cases, such as London After Midnight, the surviving coverage is so extensive that an entire lost film can be reconstructed scene by scene in the form of still photographs. Stills have been used to stand in for missing footage when making new preservation prints of partially lost films; one example being the Gloria Swanson picture Sadie Thompson (1928).