When Doc and Marty are at the dance midway through the movie, they are posing for a picture in front of a huge clock. Look at the time. Look at the time again after the camera flash. A minute or two have elapsed.
There is one possible explanation, though it is a bit of a stretch. Early cameras required longer exposure time in order to get the image on the film. This could explain why so much time passed as evidenced by the clock. But, I'm no expert.
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