Increasing the depth of field of a lens is one approach of relieving computational and mechanical refocussing mechanisms of lenses. In this paper we introduce a three element lens, consisting of an achromatic doublet, a singlet lens and a variable thickness element in the form of a refractive plate. The plate element provides an increase in depth of field comparable to 2.8 times that of a conventional lens. The lateral sag of the plate alters the optical path length for each bundle of rays with respect to the object distance so that all objects are imaged on to a fixed position detector. The surface is described by a combination of Zernike terms along the sagittal plane. Imaging is carried out with NIR narrow-band illumination where the fine details of the iris can be sharply captured in the image.
We propose a method of extending the depth of field (EDOF) of conventional lenses for a low cost iris recognition front-facing smartphone camera. Longitudinal chromatic aberration (LCA) can be induced in the lens by means of dual wavelength illumination. The EDOF region is then constructed from the sum of the adjacent depths of field from each wavelength illumination. The lens parameters can be found analytically with paraxial raytracing. The extended depth of field is dependant on the glass chosen and position of the near object point.