Abstract: A method of controlling photographic processing apparatus when processing a given photographic material using the characteristic curve for that material, the characteristic curve being determined from a control strip of the photographic material, the control strip being produced exposing the control strip to a step wedge, and processing the exposed strip in the processing apparatus to be controlled, the characteristic curve being determined by measuring density values from the processed control strip in relation to the exposure applied to the strip in the step wedge, and plotting these density in relation to the exposure,characterized in that the characteristic curve is defined by:D=D.sub.s /[1+({E.sub.i /E}.sup..beta.)/.alpha.].sup..alpha.where E is the exposure,D is the density at exposure E,E.sub.i is the exposure at the point of inflexion of the curve,D.sub.s is the density at saturation, and.alpha. and .beta. are constants for the material.