Unsymmetrical Photographic Objectives
14.1 THE PETZVAL PORTRAIT LENS
This ancient lens was the first photographic objective to be deliberately designed rather than being put together by an empirical selection of lenses out of a box. It consists of two fairly thin achromats spaced widely apart with a central stop.1 It has excellent correction for spherical aberration and coma, but because the Petzval sum is uncorrected, the angular field is limited by astigmatism to about 12° to 15° from the axis. Modified forms of the Petzval lens are still used, mainly for the projection of 16 and 8 mm movie films and other projection devices, although if a negative field flattener is added close to the image plane the lens becomes a true anastigmat, ...
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