Research about lesson study and other forms of collaborative, practice-based professional development has tended to focus on identifying structures that support teacher learning. This study builds on this work by examining preservice elementary teachers' efforts to conduct lesson studies in mathematics. Analysis showed that the structures of the lesson study project supported some groups in developing mathematical and equity lenses for looking at teaching; however, the use of these lenses simultaneously led to productive and problematic learning. This finding suggests that future research focus on challenges present in collaborative, practice-based work even when productive structures are in place.