The leader is the short length of film at the very beginning of a roll of film that extends beyond the film cassette or backing paper and is used to thread and attach the film to the camera's take up spool, initiating the film advance mechanism and allowing the roll to be wound on correctly in preparation for the first exposure. It is a purely functional component of the film roll that is not intended to be used for making exposures, serving solely as a means of connecting the unexposed film to the camera's transport mechanism.
On 35mm film cassettes, the leader is the tapered or tongue shaped portion of film that protrudes from the light trap of the cassette when the film is new and unloaded. Its characteristic shape - typically cut or moulded to a narrower width at its tip, broadening out to the full 35mm film width over its length - is designed to facilitate easy threading through the film transport guides and secure attachment to the take up spool inside the camera. Once the leader is correctly threaded and attached, advancing the film winds the leader onto the take up spool and draws the first unexposed frame of the roll into position behind the lens ready for the first shot.
The leader also serves as a protective buffer for the unexposed film within the cassette, as the outermost portion of the roll is inevitably exposed to light during loading. Because the leader and the first few frames of the roll nearest to it may receive some degree of light fogging during the loading process, many photographers advance the film by one or two frames after loading before beginning to shoot, ensuring that the first recorded image falls on fully unexposed and unfogged film.
When a roll of film has been fully exposed and rewound back into its cassette, it is standard practice to rewind the film completely so that the leader disappears back inside the cassette, clearly distinguishing the exposed roll from an unexposed one. Some photographers and processing laboratories prefer to leave a short length of leader protruding from the cassette even after rewinding, as this facilitates easier retrieval and handling of the film during processing, and certain dedicated leader retrieval tools are available for extracting a rewound leader from a cassette when required.