A method of determining the point-spread function (PSF) of an imaging system includes the steps of capturing image data, establishing an idealized source spot, establishing a functional form model, subtracting the captured image from the estimated image equation and determining a metric that measures the fit of the estimated image to the captured image. The functional form model may include both diffraction and aberration and stray light. The functional form model may be optimized to reduce the metric to an acceptable level.