Frederick Wiseman has been making his brand of long, implicitly narrative documentaries about institutions for almost 50 years, and I’ll be damned if his most recent, National Gallery, isn’t among his very finest. The movie, which opens today at New York’s Film Forum, examines the London art museum after which it is named from inside out—we see scenes of its patrons looking at art, its guides explaining the art, its administration discussing how to properly share the art with the public, its restorers showing how they preserve the museum’s priceless pieces. It is a calmly brilliant, regularly fascinating three-hour look at what constitutes art and how to best share it.