The light intensity distribution in the focal region of a high-numerical aperture optical system (NA=0.85). The focused beam propagates in the vertical direction (see arrows). In the lower horizontal plane the intensity distribution is shown in the nominal focal plane. The two other horizontal cross-sections correspond to defocused image planes. The state of polarization of the beam initially was linear. The arrows represent the power flow in the focal region (Poynting vector). The cone illustrates the ray picture of image formation

Special thanks to Arthur van de Nes for writing the software that generates the above picture.