This paper describes a very simple, but accurate method of finding the position of the infinity focus of a telescope objective relative to the rear face of its lens cell. The method is easily undertaken in the classroom and provides an opportunity for learners to conduct an interesting and novel optical experiment which introduces thick lens theory. An experimental procedure with an example set of results is described, along with an error analysis. The error analysis demonstrates that the technique can be used to determine the position of the infinity focus to a relatively high degree of precision.