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The determination of the image forming wavefunction is considered for normal operation of the conventional transmission electron microscope. No assumption is made about the relative strengths of unscattered and scattered radiation. Spatial coherence and monochromaticity are assumed. For objects which are represented by a finite number of Fourier components, an algorithm for phase determination is given. Only the locations and not the intensities of the reflections in the diffraction pattern are needed. The main information used is contained in two or three images with different defocus. Combinations of defocus values, which should be avoided since they yield insufficient information, can be determined beforehand from the diffraction pattern.It is shown that with this data there are only one or (in some cases) two solutions of the phase problem. An example is given where this ambiguity cannot be avoided even by the use of an arbitrarily large number of defocus values. In this context an erroneous statement, which can be found in the older literature, about the determination of the wavefunction by the density distribution and its temporal derivative, is corrected.