The effects of a thin lens on a real laser beam with an M2 factor larger than 1 is theoretically studied. The effect of weak lens aperture truncation is considered by introducing an increment to the M2 factor. A new thin lens equation is derived. This equation is a simple and effective tool applicable to real laser beams, and can be used to estimate the phenomena of focal shift and change in focused spot size caused by weak lens aperture truncation. Two examples of applications are described.