(Color online): (a) Saddle-Point (SP) PSF, optimized for high background, low signal 3D precision over a 3 μm depth range. (b) Calculated PSF for various z positions (stated). Scale bar: 1 μm (in sample space). (c) Calculated CRLB of the SP PSF for x, y, z vs. astigmatic (Astigm.) and DH PSF as a function of z. 3500 detected signal photons per frame and β = 50 were considered. Astigmatic axes are at 450 relative to camera axes, hence the similar x and y behavior.