Scherzer defocus

In Scherzer defocus, one aims to choose the right defocus value Δf one flattens χ(u) and creates a wide band where low spatial frequencies u are transferred into image intensity with a similar phase. In 1949, Scherzer found that the optimum defocus depends on microscope properties like the spherical aberration Cs and the accelerating voltage (through λ) in the following way:

[img[1][EmbeddedImages]]

where the factor 1.2 defines the extended Scherzer defocus. For example, when Cs=0.6 and an accelerating voltage is 300keV result in ΔfScherzer = -41,25 nm. The point resolution of a microscope is defined as the spatial frequency ures where the CTF crosses the X-axis for the first time. At Scherzer defocus this value is maximized.

[img[2][EmbeddedImages]]