A metal cylinder placed between the camera body and the lens. This device shortens the focusing distance to a few inches. For example, an ISO 100 film requires less light than an ISO 50 film. Each time you double the film speed, half as much light is needed for a correct exposure. Faster films need less light but they produce grainier pictures.
Slower films have a finer grain and they produce more contrast.
Pictures taken on slower films are sharper in appearance. A camera where focus is not adjustable. These cameras are set for a typical focusing distance of more than three feet. Usually, everything from three feet to infinity appears in focus. This distance is measured with the lens focused at infinity.