Is "subject" same as the object or different? Is it "the photographer" or "the object itself"? If it is the object, it's repetitive, and if it is the photographer, maybe the photographer also had to immobilize himself by tie him up by something.
ex) Early photography continued the trend toward the imprisonment of the subject and the object of representation. During photography’s first decades, exposure times were quite long. For instance, the daguerreotype process required exposures of four to seven minutes in the sun and from twelve to sixty minutes indoors. Early photographs represented the world as stable, eternal, and unshakable. And when photography ventured to represent living things, they had to be immobilized. Thus, portrait studios universally employed various holding devices to assure the steadiness of the sitter throughout the lengthy time of exposure. The devices firmly held the person in place. In other words, a person who wanted to see his own image became a voluntary prisoner of the machine.
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.