Fig. 1.

(a) Diagram showing how underfocused images keep two approximately Friedel-related diffraction patterns completely separate so that they do not overlap in the image. Since they do not overlap physically in the image, their amplitude and phase can be recovered by an appropriate image processing procedure. At underfocus B, the two diffracted beams are fully resolved from the low-resolution shadow of the particle, whereas underfocus A is the minimum underfocus needed to avoid any overlap of the high-resolution information. (b) The result of applying the two CTFP and CTFQ procedures is shown in comparison with the result of applying conventional CTF correction. The conventional CTF correction, termed CTFR, is equivalent to the application of $CTFP+CTFQ$. (c) Diagram of the Ewald sphere construction, and how the deviation from the central section, Δz^⋆, is calculated. The two diffracted beams are shown intersecting the Ewald sphere on the right and left sides of the diagram. The intersection to the left is also shown as its Friedel-related partner, dashed, on the right. The 2Δz^⋆ distance is the separation of the two Fourier components in reciprocal space.