When the DP or First AC calls for a lens change, filter, or any other piece of equipment, it is the Second who will bring it from the camera cart (Figure 16.20) or the truck. The Second AC is also sometimes referred to as the clapper or the clapper/loader. This is because one of her main duties is to operate the slate or clapper, as it is sometimes called. The slate serves several functions. First, it allows the editor or video transfer person to coordinate the visual image of the stick coming down with the audio of the clap, thus achieving sound sync. The slate also identifies the scene number, the take number, and the roll number. It will also identify a scene as day-interior, night-exterior, and so on. It should also indicate whether a shot is sync sound or MOS. It should list the frame rate and any special filters being used. The slate will also indicate any effects such as different frame rates. We’ll talk about slating in more detail later. Another important duty of the Second AC is setting focus marks for the actors, and assisting the First in measuring focus. In addition, the Second keeps the camera notes (often taped to the back of the slate) and camera logs.
The Second AC may also be in charge of setting up monitors for the DP and director, cabling them, and making sure they are operating properly. If there is a DIT on the job, they might handle this task, although it’s best to have a digital utility or camera utility do this as obviously the camera assistants and DIT have many other urgent duties to attend to. The Second will also be in charge of making sure the carts are as close as possible. It is especially important to keep the lenses close as not having the correct lens up can hold up everything. Also note taking, cleaning, inventory, etc.
In summary: The First does everything that needs to be done at the camera and configures the camera as necessary and is responsible for its being able to function however it is mounted or used. The Second handles everything that needs to be done away from the camera.
from Cinematography: Theory and Practice, Blain Brown