Real lenses do not behave according to the laws of Gaussian optics to which the teaching of lens theory is often confined; Hamilton and Maxwell showed already that this would contradict Fermat’s principle. This paper shows that the rudiments of a correct treatment can be easily incorporated in an undergraduate course. This will help students to develop a clearer notion of what they can expect from the various lenses they will encounter in the laboratory. In addition, it will give them some insight into the value of variational principles.