The paper presents the mathematical technique for precise calculation of the three dimensional point spread function (3D PSF) of an optical system. The proposed technique is based on the Huygens-Fresnel principle: a spherical wave at an exit pupil is considered as a numerous set of elementary secondary light sources. They emit spherical coherent electro-magnetic waves. All these waves form a definite distribution of summarized complex amplitudes in a three dimensional space near a focal point. This distribution is used for calculation of the distribution of effective intensity which takes into account inclinations of optical beams. The possible approximations of the 3D PSF are discussed. The results of calculations of 3D PSF using the precise and approximated expressions are compared.
The paper presents the mathematical technique for calculation of the diffraction depth of focus of an optical
system of a widefield microscope. The proposed technique applies the Rayleigh criterion based on evaluation of the
wave aberration appeared due to defocus in a high aperture optical system. The maximal value of a linear
approximation of the defocus wave aberration is used to define the depth of focus. It is proven that in optical systems
with numerical aperture higher than 0.5 have the diffraction depth of focus 25 - 40 % smaller than the widely known
formula defines. This fact is important for implementation of autofocus and digital focus extension algorithms. The
non-sophisticated formula for calculation of the depth of focus is proposed. The results of experimental measurements
of the depth of focus are presented and discussed.