1. A method of making a photomask, the method comprising: constructing a transmission cross coefficient (TCC) matrix representing an illumination source for supplying light to transmit through the photomask and a pupil for focusing the transmitted light onto a target substrate to produce a set of main features; generating kernels through decomposition of the TCC matrix; selecting ones of the kernels having odd symmetry; generating a field map kernel as a sum of self-convolutions of the odd symmetry kernels; generating a first field map by convolving an area of the photomask corresponding to the set of main features with the field map kernel; and making the photomask corresponding to the first field map.