Abstract: The separation between two plates or electrodes--between which plates a developer solution is applied to develop a latent electrostatic charge image on a photoconductive medium within an electrophotographic imager--is varied in proportion to a detected concentration of toner within the developer solution. Resultantly to this control of the separation of the two plates, the proximity of the medium's photoconductive surface to a one plate, and the strength of the electric field between that plate and the photoconductive surface, is also controlled. By the control the thickness and density of the applied toner film is maintained constant, and a high quality image is maintained, despite depletion of toner from the developer solution. The toner concentration is preferably sensed as electrical resistance. A control circuit responsive to this sensed resistance preferably includes an analog-digital converter, a microprocessor, a stepper motor driver/controller, and a stepper motor rotating a cam.