To extend the axial field of view in 3D SMLM experiments, data can be acquired in multiple z-stage positions [1]. However, the astigmatism method can estimate only the relative axial positions of molecules with respect to the focal plane. To find the absolute coordinates, the correct offset must be added to the axial positions of each molecule. The new coordinate can be determined as

where is the initial z-stage offset, is the estimated molecular position relative to the focal plane ( for 2D data), is the step between two positions of the Z-stage, is the image frame in which a given molecule appeared, is the number of images taken in each z-stage position, and is the number of z-stage positions. All  variables are in nm.