Photo-resists are photographical materials whose transmittance is a complex function. It modifies the phase of the incident wave mainly by variations in the thickness of the photo-sensitive layer. It will be shown in this paper that their response follows a well-determined relation; the thickness of the dephasing layer is a definite function of the energy received. In that relation the values assumed by three parameters defined in this paper permit the characterization of the contrast, deviation from linearity, sensitivity and threshold energy for each pair of photo-resist-developer. Photo-resists are polymers in solution. For the same kind of polymer, changing the way of preparation and the developer used gives a wide range of contrasts and changes the linearity of the response, etc. The justification for these changes can be found in the physicochemical characteristics of polymers. Photo-resists can be used for the synthesis of optical correcting plates and filters, kinoforms, phase holograms, etc. Knowing the response curve should allow for the use of a specific pair of photo-resist-developer that could suit the needs for a certain intensity distribution; a photo-resist with better reproducibility, that could have a greater precision and a wide dynamic of response, or with a better approach to linearity, could then be prepared based on the general principles that we are expounding.