A recent paper by Sherif et al. (S.S. Sherif, E.R. Dowski, W.T. Cathey, J. Mod. Opt. 51 1191 (2004).) reported the derivation of an aspherical phase plate, which when placed at the exit pupil of a conventional imaging system and combined with digital processing of the recorded images, increases the depth of field by an order of magnitude. An error in the derivation of this phase plate has been identified, which makes the reported extension of depth-of-field sub-optimum rather than invalid. In this comment, an optimum phase plate is obtained and the relevant results are repeated.