Contains three plastic tubes of different length (31mm,21mm & 13mm) which can be combined in seven ways to obtain various magnification. Perfect coupling with TTL metering and AE mechanisms. Full auto focusing with Canon EOS Cameras. 1. When using extension tubes the lens will not focus to infinity. The focus range will be greatly limited to a very close focusing distance. 2. There is light fall off when using any extension tube, sometimes the equivalent of 3 f-stops of light is lost when using multiple extension tubes together. This light lost can affect the camera's ability to auto focus. Manual focusing is recommended should the lens begin to hunt (not lock onto the subject). 3. Auto Extension Tubes are not auto focus compatible with Canon EF-S lenses