Although the effect of light diffusion in paper on halftone reproduction, as captured in the optical point spread function or its Fourier Transform (MTF), has been known for about half a century, that effect on the microstructure of an image on photographic printing paper in particular, has not been widely studied. The purpose of this work is to construct a simple MTF model of photographic printing paper that specifically includes the effect of the optical MTF of the paper itself and the photographic layer MTF. The MTF for the paper substrate influences the overall MTF at two different stages in the process of exposing and measuring the print MTF. The first influence is during the exposing stage, where the paper substrate MTF acts in a linear manner. In the second, or measurement stage, the process is nonlinear. Because of this nonlinear stage, a Contrast Transfer Function (CTF) descriptor of model component parameters is developed. Model predictions show that it is the paper substrate MTF that dominates the normalized CTF of the resulting photographic print, and not the MTF of the photographic emulsion forming the image. Results from the model are compared with published data, and show good agreement.