Petzval curvature plays an important role in the design of a
triplet. Its value should be defined at the beginning of the design process,
and along with the field of view, it will characterize the type of design.
For these reasons, a look at the consequences of choosing various Petzval
values is interesting. Different image quality criteria versus the Petzval
radius for different glass combinations are studied. It is shown that the
natural Petzval radius of a Cooke triplet lies at about — 2.7 times the focal