This paper describes an attempt at evaluating a potentially critical "detail" of teaching practice; that is, using a particular diagram to illustrate the imaging role of a converging lens. This "basic" diagram has been designed to contribute to help students overcome the well-known "travelling image" syndrome. We conducted a comparative study with 125 students in all, at three academic levels: end of secondary school, degree students, and teacher training. The groups compared had previously been taught elementary optical imaging in a classical, uncontrolled manner, and were presented with two classical questions, commonly used to demonstrate students' difficulties. In each group, one-half of the students had a classic introductory diagram and the other half had the "basic" one. A positive reaction of students to the evaluated diagram was observed at a relatively high academic level (trainee physics teachers and degree students), in contrast with an apparent lack of effect at the end of secondary school. The paper ends with a discussion of the evaluation of a detail of practice in isolation, with respect to the distance between students' actual and targeted levels of comprehension. (Contains 7 tables, 4 figures and 1 note.)