One of the experimental challenges in Fluorescence Correlation Spectroscopy (FCS) is efficiently computing correlations between photon arrival times coming from one or more detectors. This function uses an algorithm proposed by Laurence TA, Opt Lett. 2006 Mar 15;31(6):829-31 to calculate a log2-spaced correlation function between two arrays containing photon arrival times. The function can be modified to process arbitrarily spaced bins. The internal function photons_in_bins performs the efficient calculation and can technically accept bins of any size (not just logs-spaced).