Previously, for each pupil diameter we had computed the mean and standard deviation (over eyes) of the magnitudes at each radial spatial frequency. Candidate functions were fit to the mean radial MTF for each pupil diameter by minimizing the sum over all radial frequencies of squared errors between the function and the mean at each frequency, divided by the corresponding variance at each frequency. At each pupil diameter, fitting was restricted to magnitudes greater than 0.01. This means that there may be different numbers of magnitudes at different pupil diameters. One or more parameters were estimated for each function for each pupil diameter. Finally, we computed an RMS error for the complete set of pupil diameters by summing the minimum squared errors, dividing by the total number of data points, and taking the square root. A number of formulas for the human average MTF have been proposed. Jennings and Charman (1997) provide a table of examples, which we extend here. These functions may be divided into two groups, which we describe as the exponential group and the Lorentzian group. Some of the formulas also make use of the diffraction-limited MTF, which we discuss first.