A computer-implemented method for correcting misfocus in original image data using at least one approximation of a fractional Fourier transform operation. Original image data may comprise a light image, an electron beam image such as produced by an electron microscope, a particle beam image, or other types of images admissible to the technique. The approximation of a fractional Fourier transform operation may comprise a portion of a Taylor series expansion, a Hermite function expansion, a perturbation approximation, a singular integral approximation, or an infinitesimal generator. The method provides for the reconstruction of phase information associated with the original image data, and for the reconstructed phase information to also utilize an approximation of a fractional Fourier transform operation. The focus correction can be selected and controlled by user interaction or an automatic feedback control system responsive focus-related metrics such as to image sharpness signified by high spatial frequency content.