“American Prospects,” the title of Joel Sternfeld’s retrospective, invites three readings of work that represents nine years of itinerant photographic practice. Prospects are first of all views, usually commanding views that extend from a particular outlook to the horizon. Sternfeld’s handsome color photographs are typically of this sort. He most often positions his large-format (8-by-10-inch) view camera at sites that survey certain segments of contemporary American landscape, those that tend to lie along the margins of our postindustrial, urban culture where signs of a different sort of prospect