Computer-implemented arrangements for adjusting the focus in original electron microscope image data are described. In an implementation, a fractional Fourier transform operation and a phase restoration operation, both responsive to a fractional power, are collectively applied to original electron microscope image data to produce computationally-focused image data. A parameter adjuster is used to provide a range of variation of the power, and can be adjusted by a user or under the direction of a control system. The fractional Fourier transform operation and the phase restoration operation can be realized by at least one numerical algorithm and can comprise an approximation.