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The magnification of electron microscopes is defined as the ratio of a specimen size and an image size recorded on a CCD camera or photofilm.
As discussed in BellShaped Field in EMs, by introducing dimensionless coordinates y = r/a and x = z/a = cot φ (where we have the variable φ varying from π (z = −∞) to π/2 (z = 0) and then to 0 (z = +∞).), by following the theory of trajectories of electron in electron lenses,
and by assuming a ray passing through a point P_{0}(y_{0}, φ_{0}) in front of the lens, a image point P_{1}(y_{1}, φ_{1}) can be theoretically obtained,
 [4274a]
where,
M  Magnification
ω  Lens strength, given by,
 [4274b]
Figure 4274. Rotationally symmetric magnetic fields and electron lenses.
Newton’s lens equation of light optics suggests Z_{0}Z_{1} = f_{0}f_{1}. Further calculation based on bellshaped field, we can obtain,
 [4274c]
 [4274d]
We can see that the focal lengths f_{0} and f_{1} are not the same as the distances z(F_{0}) and z(F_{1}) of the foci from the lens center at z = 0, indicating that electron lenses cannot be treated as thin lenses.
The magnification M in Equation 4274a can be rewritten in terms of f and Z,
M = f_{0}/Z_{0} = Z_{1}/f_{1}  [4274e]
