1. A computer-implemented method for adjusting the focus in original electron microscope image data, the method comprising: obtaining original electron microscope image data comprising a plurality of spatially-indexed amplitude values, and further comprising a center located relative to the plurality of spatially-indexed amplitude values; providing a fractional power parameter associated with a fractional Fourier transform operation; calculating approximated centered two-dimensional discrete phase restoration information responsive to the fractional power value; calculating an approximated centered two-dimensional fractional Fourier transform operation responsive to the fractional power value; operating on the original electron microscope image data with the approximated centered two-dimensional phase restoration information and the approximated centered two-dimensional fractional Fourier transform operation to produce computationally-focused image data; wherein the focus plane of the computationally-focused image data is responsive to the fractional power parameter.