We present a Point Spread Function (PSF) model for visible light in fog. Our approach is based on approximations of the Mie-Lorentz scattering model. We developed fittings to the phase function, scattering cross section and attenuation coefficient. These fittings were obtained using a modified gamma distribution describing the fog size distribution. Consequently we derived the single-scatter PSF. Our PSF has low computational complexity and high similarity to the Mie-Lorenz model. The PSF is based on the droplet-size distribution and the distance to the point source. To verify our model, we compare it to results from a special fog chamber. The fog chamber is 21 meters long, 2.35 meters wide and 2.4 meters in height. The drop size distribution generated in the chamber closely resembles the distribution of natural fog outdoors. Accurate monitoring of light attenuation exists in the chamber. We show high similarity between the fog lab test results and our model.