An electrically controllable focal length is proposed for the visible wavelength range. The proposed structure is composed of a graphene sheet placed on a silver grating. The effect of the chemical potential variation of the graphene sheet on the light wave deflection, and hence the focal length of the structure, is investigated. The results of the full-wave simulation analysis based on the finite difference time domain (FDTD) method show that the focal length of the structure varies by approximately 7.82λ through changing the chemical potential of graphene from 0.1 to 1 eV. In addition, a generalized equivalent circuit model is introduced for the analysis of the proposed tunable graphene structure. The achieved results based on the circuit model match well with those based on the FDTD method. The proposed structure has the potential to be used in imaging devices and optical sensors.