bookmark Gctf New Predicts, refines and corrects contrast transfer function (CTF). Gctf offers four customizable parameters: resolution range, astigmatism, box size and B-factors. It can serve to identify local defocus variation that can be useful to find abnormal micrographs. This tool simplifies the cryogenic electron microscopy (cryoEM) image processing. It can be employed to improve the final resolution of 3D cryoEM reconstructions.