Using image qualification theory of Linfoot, we studied the deforcusing character of imaging of weak phase object in HREM. The power spectrum of electron micrographs of an amorphous carbon thin foil specimen of 10 μm thick is measured from which the distribution of the relative structral content is determined. It is further verified by numerical analysis. The correlation factor, the image fidelity, the choice of the defocus amount in the electron microscope for the unkown crystal objects and automatic focusing is discussed.