Based on the random phase perturbation of light waves in laser optical paths, the formation mechanism of mode fringe patterns is analyzed in the real-time image subtraction using an electronic speckle pattern interferometry (ESPI). An amplitude-fluctuation measurement method is proposed. The systems of ESPI and digital shearing speckle pattern interferometry (DSSPI) are established and used for the out-of-plane vibration analysis. In addition, the vibration properties of the intact and the cracked cantilever aluminum plates are investigated experimentally. The experimental results show that the visibility of mode fringe patterns obtained in the real-time image subtraction mode is obviously superior to those by the other methods. The obtained first 10 orders of modal fringe patterns are well consistent with the calculation results by the finite element method. Compared with ESPI, DSSPI is more sensitive to the local stiffness variation and flaws of specimens.