The original documents of A4 or A3 format are selectively read by a scanning apparatus (2) which is integrated into a system for producing and transmitting images. The originals of A4 format are normally positioned on a fixed exposure surface (5) and the originals of A3 format are introduced into a separate device (7) for transporting documents, which is adjacent to the fixed exposure surface (5). A carriage (22) with lamps (36) is slidable below the exposure surface (5) to illuminate the A4 original while to illuminate the A3 original the carriage (22) is moved into a fixed position (23) corresponding to the separate reading device (7) where the original A3 is slid with respect to the carriage past an exposure zone (E). The reflected images of the originals are passed by means of mirrors (40,42,44) mounted on the carriage (22) to a lens (45) which reflects them on to a photodetector device which converts the light rays into digital electrical signals. To adjust the focal lengths of the lens (45) to the various dimensions of the A3 and A4 formats and to compensate for the different distances (difference D) of the two types of originals from the lens, two of the mirrors (42,44) are mounted on an auxiliary frame which is movable on the carriage. The auxiliary frame is displaced by its own motor (76) and a lead-screw (70) to different positions in relation to the type of original which is to be read.