1. A computer-implemented method for adjusting focus in original electron microscope image data, the method comprising: obtaining original electron microscope image data; providing a fractional power associated with a fractional Fourier transform operation; calculating approximated phase restoration information responsive to the fractional power; calculating an approximated fractional Fourier transform operation responsive to the fractional power; operating on the original electron microscope image data with the approximated phase restoration information and the approximated fractional Fourier transform operation to produce computationally-focused image data; wherein a focus plane of the computationally-focused image data is responsive to the fractional power.