There are five parameters, i.e. the initial amplitudes of a pump beam and a signal beam, the ratio of the initial beam width of the pump to that of the signal, the initial separation between the two beams′ centers, and the wavelength ratio of the pump to the signal, which influence the induced focusing of the signal optical beam due to the cross phase modulation of the pump beam in self defocusing nonlinear media. The effect of the ratio of the initial beam width on the focusing of the signal beam is discussed under other different parameters. The numerical results show that there exists a pair of optimum values for the ratio of the initial beam width and the initial separation, which can result in the strongest focusing of the signal, and another pair of optimum values for the ratio and the separation, under which the optimum focusing can be obtained for the given length of the medium specimen. The detailed discussion about the effects of all five parameters on the induced focusing of the optical beam in self-defocusing nonlinear media can be used to design experiments leading to the best effect of the focusing.