A method of determining focal planes during a photolithographic exposure of a wafer surface is provided. The method may include receiving data corresponding to a surface topography of the wafer surface and determining, based on the received data corresponding to the surface topography, a plurality of regions having substantially different topographies. Reticle design data is received for exposure on the wafer surface, whereby, from the received reticle design data, reticle design data subsets that are each allocated to a corresponding one of the determined plurality of regions are generated. A best fit focal plane is then generated for each of the determined plurality of regions.