This paper proposes a simple method to design experiments for drawing the light paths at a lens to find its effective focal length and principal planes. In the designed experiments, long-exposure photography was used to record the light scattered using a moving sheet of paper, thus revealing the light path. According to the proposed experimental method, (1) the effective focal length of a lens and (2) the principal planes of a lens can be measured. Moreover, the system is easy to build, inexpensive, and can promote students' understanding of geometric optics by providing an intuitive physical phenomenon. The work is also of pedagogic interest as it reveals a new simple experimental method to find the principal planes of a thick lens and its effective focal length.