Based on the Extended Nijboer-Zernike theory, the effect of different amplitudes for exit pupils on the image intensity in the focal plane was analyzed. A novel approach was applied to testing the wavefront according to the actual condition of the amplitude in the exit pupil, which can help eliminating the error caused by the nonuniformity illuminated pupil and the Fast Fourier Transform in the original phase retrieval algorithms. A testing experiment was performed on an imaging optical system, and obtained results show that the tested wave fronts in the exit pupil of a camera lens are 0.196 5λ in PV and 0.022 4λ in RMS (the testing wavelength λ is 6328 nm). The aberrations in the wavefront are mainly astigmatism, coma and high order astigmatism. Furthermore, the approach can also be used to analyze the amplitude in the exit pupil of a camera lens and calculate the light intensity distribution on other focal planes. The experiment proves this approach available.