1. A computer implemented method for rendering a three dimensional scene to generate a two dimensional image, the method comprising the steps of: (a) computing an exit pupil of a lens system; (b) selecting a ray that passes through the exit pupil to an image point on an image plane; (c) tracing the ray from the three dimensional scene through the lens system to the image point; (d) calculating a radiance value for the ray; wherein the calculation of the radiance value for the ray in step (d) comprises setting the radiance value to zero if the ray is vignetted; (e) repeating steps (b)-(d) to obtain a plurality of radiance values; and (f) combining the plurality of radiance values to generate the two dimensional image on the image plane.