1. A method to align a first image and a second image of a subject, the method comprising: for each of a series of blur kernels that vary according to alignment parameters, evaluating blur-kernel sparseness to locate a sparsest blur kernel, the sparsest blur kernel relating a select sharp image to a select blurred image, each of the select sharp image and select blurred image chosen from or computationally derived from the first or second images; and applying an aligning operation to the first or second image, the aligning operation being dependent on the alignment parameters corresponding to the sparsest blur kernel.