PT - JOURNAL ARTICLE AU - Alexis Rohou AU - Nikolaus Grigorieff TI - CTFFIND4: Fast and accurate defocus estimation from electron micrographs AID - 10.1101/020917 DP - 2015 Jan 01 TA - bioRxiv PG - 020917 4099 - http://biorxiv.org/content/early/2015/08/13/020917.short 4100 - http://biorxiv.org/content/early/2015/08/13/020917.full AB - CTFFIND is a widely-used program for the estimation of objective lens defocus parameters from transmission electron micrographs. Defocus parameters are estimated by fitting a model of the microscope’s contrast transfer function (CTF) to an image’s amplitude spectrum. Here we describe modifications to the algorithm which make it significantly faster and more suitable for use with images collected using modern technologies such as dose fractionation and phase plates. We show that this new version preserves the accuracy of the original algorithm while allowing for higher throughput. We also describe a measure of the quality of the fit as a function of spatial frequency and suggest this can be used to define the highest resolution at which CTF oscillations were successfully modeled.