Optical proximity correction methods and apparatus are disclosed. A simulated geometry representing one or more printed features from a reticle is generated using an optical proximity correction (OPC) model that takes into account a reticle design and one or more parameters from a process window of a stepper. An error function is formed that measures a deviation between the simulated geometry and a desired design of the one or more printed features. The error function takes into account parameters (p.sub.0 . . . p.sub.J) from across the process window in addition to, or in lieu of, a best focus and a best exposure for the stepper. The reticle design is adjusted in a way that reduces the deviation as measured by the error function, thereby producing an adjusted reticle design.