In the study of a diffraction field of a light wave passing through a symmetrical paraxial optical system, the Collins formula and its inverse are convenient for calculation. The algorithm study of the Collins formula demonstrates that both a single fast Fourier transform algorithm and a double fast Fourier transform algorithm can be used in diffraction calculation. But, whichever algorithm is adopted, only by meeting some specific conditions can the amplitude and phase distributions of a diffracted wave be calculated correctly. Based on the Nyquist sampling theorem, the indispensable conditions to calculate a diffraction field accurately are presented.
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