It is well known that a direct Fourier Transform relation exists between the amplitude distribution at the front focal plane of a converging lens and that at its back focal plane. A general formula for the amplitude distribution at the back focal plane has been developed, basing on Huygen—Fresnel—Kirchhoff diffraction integral, for an arbitrary position of the object without restricting the characteristics of the lens. Proper account has been taken for deviation from the paraxial approximation and the inherent defects of the lens. The formula derived does not pertain to any system, in particular, but could be modified suitably as the need arises to any specific setup.