An extension tube is a non-optical device that increases the distance between the lens and the film. This allows the distance between the lens to focus much closer than normally possible and thus, gave the increased magnifications.
The EF 12 tube provides a 12mm extension while the EF 25 version adds just over 25mm of extension.
These close-up accessories maintain full electronic coupling between the camera body and lens, often providing greater magnification than is possible with close-up lenses. Can be used with most EF lenses, and are ideal for nature photography with single focal length telephotos. Manual focusing is recommended.