Abstract: In an aerial reconnaissance photographic camera, the exposure aperture 15c of revolvable focal plane shutter blind is varied by varying the relative positions of an outer part 15a and an inner part 15b of the blind. Two helical driving gears 22 and 23, of opposite hand to each other and fixed on a common shaft 24, respectively drive helical gears 19 and 21 (also of opposite hand to each other) the gear 19 driving outer blind part 15a through one pulley and belt system, the gear 21 driving inner blind part 15b through another pulley and belt system. Axial displacement of shaft 24 produces relative rotation between gears 19 and 21 and hence variation of aperture 15c.