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 a plurality of optical image plane positions, including: determining said simulated and empirical best focus positions, including optical image plane positions; collecting empirical data at various focus points in the region of said best focus or at various exposure dose values, including collecting SEM measurements representing empirical data of said test patterns, wherein said SEM measurements are made on said test patterns exposed to nominal process conditions; determining a symmetry point of difference using said plurality of focus positions at said image plane position; determining optimal location of said optical image plane position by determining a minimum point of said difference using said plurality of image plane positions, wherein said optimal image plane position and said simulated best focus position form a saddle point of said difference using said plurality of focus and optical image plane positions; determining offset of said simulated and empirical best focus positions; adding a focus-blur correction to said offset to create a focus-blur-corrected offset; and calibrating a photoresist model based on minimizing said difference between empirical data for test patterns exposed at nominal focus and dose conditions and simulations of said test patterns, wherein a focus value used in said simulation corresponds to said offset or to said focus-blur-corrected offset, added to said exposed nominal focus value.