This paper explores whether it is effective to use real-time on-screen guidance to help users take better photos with mobile devices. Using a three-camera array, we developed a photo-taking interface that provides real-time feedback on how to position the subject-of-interest according to a photography composition rule: rule-of-thirds. We conduct a user study to compare the aesthetic quality of photos taken with our real-time guidance interface against a static gridline interface common to existing digital cameras. Expert photographers and Mechanical Turk workers rate the aesthetic quality of these pairs of photos. Results indicate the photos taken with our real-time guidance interface have significantly higher aesthetics scores. This study shows the potential in using camera array, computational photography, and real-time guidance interface to help non-expert users take better photos.