Relative systematic bias of the commonly used method that estimates a_ij from the slopes of correlations, using a single illumination value at 75 ke. The top figure is for the full model, and the bottom one is for b = 0. In the top case the nearest neighbors a_ij are biased by +8% and −4%, respectively. For the bottom case, there is a global overestimation of the effect, increasing with distance.