1. A method for determining a process model, the method comprising: using at least one computer for: receiving a first optical model which models an image that is formed by an optical system of a photolithography process at a first distance which is equal to the optical system's best focus distance; receiving a second optical model which models a second latent image that is formed by the optical system at a second distance which is different from the first distance; receiving a third optical model which models a third latent image that is formed by the optical system at a third distance which is different from the first distance and the second distance; receiving process data which is obtained by subjecting a test layout to the photolithography process; determining an uncalibrated process model by combining the first optical model, the second optical model, and the third optical model; and fitting the uncalibrated process model using the process data.