Petzval is a starting design for a large aperture narrow-field photographic lens. The Petzval design has been around for more than one hundred years, and is still widely used. The Petzval lens is a very old design form ( > 150 years!) that is still a mainstay in lens libraries. The original Petzval Portrait lens used a cemented doublet and air-spaced doublet, but the term Petzval lens is now generally applied to lenses containing two separate groups (usually doublets) in which both groups contribute positive power. The Petzval lens is a good design form for high-aperture narrow field applications. Curiously, although invented by Petzval to improve the field coverage of high-aperture systems, it makes no attempt to correct the Petzval curvature. In modern designs, the Petzval lens often incorporates a negative element near the image plane to flatten the field. The lens included here is a typical design that can be used as a starting design for specific modification. It is scaled to 50mm focal length, so you can compare its performance to demotrip.len and dblgauss.len.