1. A method for identifying focal plane variations comprising: measuring aerial images using an image sensor array at a plurality of focal planes in an exposure tool, the image sensor array including a plurality of image sensor elements; determining a value for at least one image quality metric for the aerial image measured by each image sensor element at each of the plurality of focal planes to produce image quality metric data; fitting a parametric function to the image quality metric data, the parametric function being a function of focus; determining a best focus for each image sensor element using the fitted parametric function; calculating a difference between the best focus for each image sensor element and a nominal best focus setting of the exposure tool; and creating a focal plane map of the combination of the image sensor array and the exposure tool that indicates the differences between the best focuses of the image sensor elements and the nominal best focus setting.