Focusing by a high-power, low-Fresnel-number lens: the fly facet lens

Diffraction by fly facet lenses has been investigated by photographing the diffraction patterns at various distances from a facet lens whose power was estimated to be 2.03 x 10(4) D. We studied three different aperture diameters with Fresnel numbers of the order of unity. A large focal shift was prominent at the lower Fresnel number. The measured diffraction patterns correspond to those calculated with the Kirchhoff diffraction theory for an aberration-free lens with low Fresnel numbers.