A extension tube is a non-optical device that increases the distance between the lens and the film. This reduces the minimum focusing distance and increases the magnification ratio. The EF 12 II extension tube extends the distance between the lens and the film by approximately 12 mm. Under certain conditions, autofocus works with both extension tubes, but not always. Since small changes in focusing can occur with close-up shots, we recommend manual focusing. The autofocus system of EOS cameras can be used for focusing confirmation.