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.