A very difficult sentence to understand. In my opinion, not very well written.
I wonder if the writer actually meant the objective of the representation.
If that were the case I would take subject to be the main focus of the photograph, a person or an animal for example. The objective would be the context of the picture: furnishings of a room or perhaps a landscape in the background perhaps. Clearly the long exposure times meant that nothing in the cameras lens could change if the picture were to be a success.
Others may be able to offer a different interpretation of the writer's meaning.
Student or Learner