A Z calibration file is a plain text file with three whitespace-separated columns. The first column gives an emitter's Z coordinate in nanometres, and the second and third columns give the standard deviation of a Gaussian describing the PSF for an emitter at the given Z coordinate. They describe the X and Y dimension of the camera, respectively, and are given in micrometers.

The range of the Z calibration file (lowest and highest Z coordinate) gives the working depth of the approximation.