The CTF-correction method.
The Contrast Transfer Function CTF compensation for low contrast
specimens (e.g. frozen-hydrated), for which phase contrast is the only
significant mechanism, then higher defocus levels must be used to
achieve any significant transfer, and several images at different
focus levels must be combined to complete the information lost from
the transfer gaps of any one image. The CTF correction can be applied
to each extracted particle separately or to the whole micrograph after
digitisation. The simplest level of compensation is to reverse phases
at the negative lobes of the CTF.