A film speed dial is a manual control found on film cameras that allows the photographer to set or adjust the ISO rating of the film loaded in the camera. By communicating the film speed to the camera's exposure metering system, the dial ensures that the meter calculates correct exposure settings for the sensitivity of the film being used.
The majority of modern film cameras set the film speed automatically by reading DX coding - a series of electrically conductive squares printed on the outside of the film cassette that the camera's internal contacts detect and interpret. However, most cameras that feature automatic DX coding also retain a manual film speed dial, allowing the photographer to override the automatic setting when required.
This ability to override the film speed setting is particularly useful in a number of situations. A photographer may choose to uprate a film - setting the dial to a higher ISO than the film's nominal rating - in order to shoot in lower light conditions, a technique known as pushing the film, which requires compensatory adjustment during development. Conversely, the dial can be used to downrate a film for creative effect or to suit particular shooting preferences. The override function is also a practical solution when using a camera whose exposure meter has drifted slightly out of calibration, as adjusting the film speed dial can compensate for any consistent over or underexposure without the need for the camera to be serviced.
On cameras without a dedicated dial, film speed is sometimes set via a menu system or an exposure compensation control, but the underlying principle and purpose remains the same.