Definitions for "Principal photography"
The filming of the primary script, as opposed to "second unit" shooting with a secondary director of photography and crew that might, for example, pick up location shots from distant cities to provide ambience. Writers' "rights payments" for the screenwriter and owner of the underlying literary material are generally due and payable no later than the first day of principal photography.
The main period of filming for a movie, lasting days, weeks, or months.
the actual filming period.
the phase of production in which all of the moving images are photographed and recorded according to the instructions of the screenplay in preparation for later editorial cutting and assembly.