The Point Spread Function
The image of a point source of light imaged by an optical system is called the Point Spread Function (PSF). The PSF is a description of the 3-dimensional intensity distribution of the point source imaged by the system. The profile of the diffraction pattern is a result of the response of an optical system to the point source of light. The PSF is considered to be the fundamental unit of an image.
When an objective collects light to transfer onto the image plane of an optical instrument, the resultant image does not consist of a collection of infinitely small points, but rather every point on the plane is represented by an Airy diffraction pattern. The superposition of this interfering light field produces a three-dimensional Fraunhofer diffraction pattern.
The Airy disk consists of concentric rings of light surrounding a central, bright 2D disk or 3D ellipsoid. The radius of the central disk is governed by the NA and thus the resolving power of the objective in the lateral plane whilst the axial height of the ellipsoid governs the resolving power in the axial dimension.