Classification of photographic lens types has been undertaken in several ways of which the historical approach and the approach by Kingslake are the most important ones. A new classification is proposed which is based on the method of the Petzvalsum (P) correction. It is shown that the correction of P is the pivotal problem in photographic lens design and that all lenstypes can be considered as a consequence thereof. Most aberrations originate in the process of correcting P and their reduction can best be undertaken as an effort to improve the method of P correction. Fifth order field curvature is often characteristic of the method of P correction and is for most lenses the performance-limiting aberration. This classification is useful in understanding a lensform, as a guide to improve the aberration correction, and as a method to find new lensforms.