Laser-induced surface deformation is a widely used technique for studying thin film coatings. In this technique the deformation is typically detected using the well known optical beam deflection technique. In this paper we report a different technique for studying surface deformation by using an optical diffraction effect. In this detection scheme, the laser-induced thermal bump behaves as a curved reflection mirror which can either focus or defocus a second probe laser beam through optical diffraction, depending on the specific geometry used. This surface thermal lensing effect is demonstrated to be a sensitive and easy-to-handle method for thin film characterization. The potentials of this technique and its advantages over the conventional method for thin film absorption measurement and defect characterization are discussed.