This paper describes a feedback system in which a neutral density wedge automatically controls the light flux applied to a photoreceptor, so that an intracellular recorded response is held constant. We describe here a "differential sampler" that subtracts the photoreceptors dark response from the light response induced by a chopped light source. The resulting signal drives the feedback system. The response time of the feedback system can be adjusted with a "digital integrator" to match a range of variable conditions that occur in different experiments.