The exposure counter consists of a drum or cylinder that is coupled to one member of a gear train that operates to simultaneously index the drum and actuate a film-advancing apparatus so that the drum is sequentially indexed to change a character present at the camera's counter window while the film-advancing apparatus is recording. The gear train usually includes also a second member connected to a spring, and when the tension of this spring reaches maximum strength it then acts on the counter to reset it to "0". The gearing required to provide adequate speeds for modern cameras is rather complex, and usually involves several stages connected in series with different types of drives such as belt drives, rack and pinion drives, or helical gear drives. The purpose of the exposure counter is twofold: first, it provides a means of accurately measuring how much film has been exposed; second, it controls the timing of the next exposure based on its position. Without an exposure counter, there would be no way to determine how much film had been exposed until after the film was developed, which would prevent you from making adjustments during shooting if necessary. By using a simple mechanical device, photographers can capture images without relying on their memory or guessing how long the photo session will last. In addition to measuring film exposure, exposure counters can also be used to control other camera functions including self-timer mode, flash firing, or even moving panoramic shots by incorporating sensors and motors into the mechanism.