1. A method for a lithographic process model calibration for separating optical and photoresist effects, said method comprising: identifying a set of test patterns for model calibration and simulating a printed image, wherein said test patterns are printed onto a substrate using said lithographic process; and determining best alignment of simulated and empirical best focus positions through modeling of the difference between simulated and empirical critical dimension measurements of said test patterns for a plurality of focus positions, including: determining said simulated and empirical best focus positions; and determining offset of said simulated and empirical best focus positions.