The result of using extension tubes is that the distance from the back of the lens to the film surface is longer, which means you can take photographs at shorter distances to the subject. Extension tubes are thus commonly used for macro photography and sometimes for closeup portraits.
When using the camera's TTL metering system, no exposure compensation will be required (exposure compensation is required for handheld meters).
This updated version II is now also fully compatible with Canon Digital EF-S lenses, as well as standard photo EF and TS-E lenses.
Canon Extension Tube EF 25 II