This function measures the full-width-half-max of a microscope point spread function (PSF) along 4 different dimensions: both x and y at the z-plane with the brightest intensity, and also along z/x and z/y. The user simply needs to provide a z-stack containing a PSF (e.g. by imaging a sub-micron fluorescent bead) and also the number of microns per pixel along x/y and z.