This article combines different conceptualizations of teachers' diagnostic competence in listening to students' mathematical thinking processes on the levels of general perspectives, noticed aspects and activated didactic categories. An empirical study of 159 prospective mathematics teachers' diagnostic judgments investigated how these levels are related and how they can be enhanced by focus question and an intervention introducing relevant categories. For this purpose 2 x 159 written diagnostic judgments, before and after short intervention, were compared with respect to general perspectives, activated categories and aspects noticed in the video. As a means of comparing, a coding scheme was developed with respect to a sound conceptualization of diagnostic competence and to interrater reliability. The comparison shows that video alone cannot enhance prospective teachers' capacities of noticing, hence the activation and reflection of didactic categories is crucial. Possible theoretical consequences for a multi-faceted conceptualization of diagnostic competence and practical consequences for teacher education courses are discussed.