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For beam diffraction by a half plane it is shown that, provided the arguments of the Fresnel integrals are sufficiently large and the beam axis is directed at the half plane edge, the diffracted far field is exactly proportional to the isotropic source diffracted field modified by the ratio of beam to isotropic source fields at the edge. Thus the beam diffraction pattern can be easily calculated from Fresnel integrals with real arguments. The accuracy of this approach is illustrated by comparison with the exact solution in a numerical example. Approximating beam diffraction patterns of other structures this way is suggested.