Diagrammatic representations are ubiquitous in science education, with students and teachers alike being firmly committed to their use. However, students interpret the visual data imparted by diagrams in different ways. In this study involving diagrams of cells, it was found that first-year university students apply various criteria to evaluate the quality of diagrams ranging from the instrumental to the aesthetic. A recurring theme is the misconception that diagrams are intended to portray real objects in a "photographic" manner as opposed to being models thereof. Teachers need to pay more attention to how their students conceptually process diagrammatic information