An apparatus for controlling a photogrammetrical camera includes a frosted glass adjacent to which or in an optically conjugate plane thereto a transparent planoparallel sheet of voltage sensitive material is arranged. Spaced electrodes are allocated to the one face of said sheet and a sheet electrode to the other face. Each of the spaced electrodes is connected to respective outputs of an electronic counter, which in dependence of a tunable pulse generator applies voltage pulses to the spaced electrodes and the sheet electrode, both sandwiching said sheet of voltage sensitive material and rendering visible the inbetween portions of said material when submitted to a potential so that lines or a cyclic pattern of lines appear. The voltage sensitive material can be liquid or gaseous or solid and can be sandwiched between two substantially plano-parallel plates which have the respective electrodes or electrode on their inner faces.