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A simple model of the photolithographic process is used to analyze the effects of defocus on resist profiles and process control. An expression is obtained for linewidth variation as a function of exposure and defocus, whereby a process window can be established. The parameters which must be specified in a practical and self‐consistent definition of the depth of focus are identified: the resist process, printdown, nominal linewidths, linewidth variability, and profile slope. The model also provides a framework for interpreting empirical results obtained on a new generation of wafer steppers. Measurements of the effects of lens astigmatism and barometric pressure on the depth of focus are presented.