Simply speaking, Fourier transform is a limiting case of Fourier series. The structure factor F(u) is proportional to the Fourier components Iim(u) of the HREM image. For a weak-phase-object, the Fourier transform of the HREM image Iim(u) can be given by ,
Δf -- The defocus value
Cs -- The spherical aberration
k’ -- A constant
F(u) -- Structure factor of electron diffraction (ED), which is the Fourier transform of the potential distribution φ(r) of the object
λ -- The electron wavelength
u -- The reciprocal-lattice vector within the resolution limit
D -- The standard deviation of the Gaussian distribution of defocus due to the chromatic aberration 
The values of Δf (defocus value), Cs (spherical aberration coefficient), and D (standard deviation of the Gaussian distribution of defocus due to the chromatic aberration) can be obtained by TEM image deconvolution.
 X. D. Zou. Crystal Structure determination by crystallographic image processing. in
“Electron Crystallography”, eds. D. L. Dorset, S. Hovmöller, X. D. Zou, Nato ASI Series C,
Kluwer Academic Publishers, Dordrecht, 1997, pp163-181.
 Fijes PL. 1977. Approximations for the calculation of high-resolution
electron-microscopy images of thin films. Acta Cryst A33:109–113.