It is known to use infra-red densitometry to measure the variations in the optical density of photographic material. In photographic processing apparatus, it is important that the photographic material is transferred from one part of the apparatus to another at precisely the correct time to prevent damage to the material. Described herein is a method and apparatus for controlling the transfer or switching of photographic material from one processing tank to another of photographic processing apparatus during processing of the material. A threshold detector (30) is used to produce an output signal (40) indicative of a change in infra-red density of the photographic material. The output signal (40) is then used to control the transfer or switching of the material from one processing tank to another.