Figure 3732 gives a very simplified illustration of how the incident electrons travel through the TEM column. r is the real space coordinates in the sample or image, u is the reciprocal space coordinates and θ is the scattering angle.
Figure 3732. Simplified illustration of how an image is formed in a TEM column.
Ideally, an electron microscope can be simplified as a contrast transfer function (CTF). For a perfect coherent illumination in reciprocal space the CTF can be represented by T(k, g) as 
A(k) -- The aperture function describing the effect of the objective aperture,
k -- The reciprocal space vector,
|g| -- The spatial frequency,
χ -- The phase shift from defocus and spherical aberration.
The effect of the post specimen imaging process is given by applying the CTF to the frequency components of the wave function,
F -- The Fourier transform,
F-1-- The inverse Fourier transform.
Considering the non-constant wavelength of an incident electron beam and the astigmatism, the phase shift function can be described by,
The last and the second terms describe the effects of the non-constant wavelength and astigmatism, respectively.
Δf -- The defocus,
λ -- The electron
Cs -- The coefficient of third order spherical aberration,
C5 -- The coefficient of fifth order spherical aberration.
Adjusting defocus and defocus spread, in CTEMs with LaB6 or W electron guns and FE-EMs, affects the intensity of diffractogram and real images.
 Cowley, J. M., Diffraction Physics, Horth-Holland, Amsterdam, 1990.