Comment on "Reconstruction algorithm for high-numerical-aperture holograms with diffraction-limited resolution"

We comment on a recent Letter by Zhang et al. [Opt. Lett. 31, 1633 (2006)] in which the authors proposed a reconstruction algorithm for high-numerical-aperture (NA) holograms. Such an algorithm has been available for in-line holography for more than a decade. The authors' "achievement" of high NA for digital in-line holography, NA=0.17, is below what was reported already several years ago (NA=0.30) and is considerably lower than what is routinely achieved now. We present reconstructions of holograms acquired with NAs above 0.4 in which we show maximal achievable resolution. (c) 2006 Optical Society of America.