In the study of coherent optical imaging, the coherent transfer function is the fundamental basis for investigating the imaging quality of coherent light. At present, the coherent transfer function is defined under a particular approximation condition. The imaging formula of single lens system has been created based on coherent transfer function, which can only approximately calculate the amplitude distribution of the image field. Nevertheless, amplitude and phase of the image field are of equal importance. In this paper, the single lens imaging system has been studied. An accurate formula for the complex amplitude distribution of the ideal image and the image field near the image plane is established. The calculation method for the amplitude and phase distribution of the defocussing image field is given. Finally, the relative experimental verifications are performed.