Abstract: A format assembler allows an automatic photographic printer to form any desired size, number, and orientation of images to be juxtaposed on a continuous length of print paper with minimal paper waste. This is done with a carriage 10 that moves along a path parallel with the printing plane and extending in the direction of paper advance between the print paper and interchangeable lens modules with different lens arrays. An interchangeable mask 11 carried on the carriage has a shape that allows positioning the mask along the path to block light from predetermined lenses of any array. The mask shape and positioning allows successive exposures through preselected lenses of different lens arrays of a paper area exposable by any one of the lens arrays without advancing the paper. A control system controls the position of the carriage in programmed relation to movement of the other components of the printer.