An efficient method for optically actuating a micromechanical cantilever is presented for the first time to our knowledge. Measurable responses can be obtained for moderate light sources if electron tunneling occurs between the cantilever tip and a metallic contact below it. The small deflection of the cantilever that is due to light pressure is sufficient then to produce large tunneling current variations. On the basis of this effect several applications such as a miniaturized spectrum analyzer and one-step optical computing units for addition, integration, or differentiation of one-dimensional or two-dimensional optical signals are presented.