This study investigates the impact of an observation framework on preservice teachers' abilities to engage in productive video-based reflections on mathematics teaching. The Lesson Analysis Framework draws from research on expert-novice teacher differences. Its central element is the analysis of the impact of teachers' instructional decisions on students' learning of mathematics. Through an experimental design, the effects of this framework on preservice teachers' abilities to reflect on teaching are compared to the effects of an alternative framework focused on the evaluation of separate elements of instruction. Findings suggest that the Lesson Analysis Framework facilitates preservice teachers' learning to elaborate on what they observe and to propose alternative teaching strategies. In addition, the framework provides a structure for evidence-based evaluation of observed instruction. (Contains 1 table and 1 figure.)