There are many factors which make it difficult to relate instrumental CCD photometry to a fundamental standard system with an accuracy much better than about 1%. Here I will address only three of them: (1) infrared leaks in the filters; (2) the finite opening and closing times of mechanical shutters; and (3) changes in the air mass for long integrations. I will be approaching these subjects from the point of view of a visiting astronomer at someone else’s observatory, who gets three or four nights of observing time a year, and has only the afternoons preceding those nights to perform whatever tests can be carried out while the equipment sits on the telescope.