Tunable field depth: hyperbolic optical masks
For controlling the depth of field, in an optical system working at full pupil apertures, we unveil the use of a pair of hyperbolic phase masks. For suitably framing our proposal, we link the Strehl ratio versus defocus with the area under the modulation transfer function (MTF). We show that by using hyperbolic phase masks, one can simultaneously reduce the impact of focus errors as well as increase the area under the MTF. We show that hyperbolic amplitude masks, with moderate absorption, can reduce the artifact noise caused by the use of phase masks. Finally, by exploiting the Lohmann–Alvarez technique, we describe the use of pairs of hyperbolic masks for governing field depth at fixed pupil apertures.