Here is some information about the extender that I viewed today. It discusses the changes, and explains that you need to mount the extender to the lens before placing it on the camera to get advantage of the improved communications. We've always been told to do this, and I do, but its interesting to get more info as to why.http://cpn.canon-europe.com/content/education/technical/latest_ef_lens.do
(go to page 4)
I've pasted it in below.
As a development of Canon's EF Extender Mark II models, the EF1.4x III and EF2x III take the performance of the previous generation of extenders and improve on it in key areas, while tailoring them to suit the requirements of shooting with digital SLR cameras. A new optical design helps to eliminate chromatic aberration, resulting in sharper images that show less colour bleed along high contrast edges.
Digital cameras have slightly different lens requirements to those of film cameras, so the Mark III Extenders employ different lens coatings to help to reduce the flare and ghosting that can be caused by stray light passing through the lens and either refracting off internal elements, or reflecting back off the sensor in the camera.
Both extenders also employ Fluorine coating to help to keep the front and rear elements clean, thereby avoiding dust and dirt on the lens that may degrade image quality with flare.
The handling of the extenders has been modified too â€“ the lens release button is now better placed and easier to use, while being out of the way and less likely to be knocked.
However, the biggest change is within the electronics of the extenders. Both models feature integrated processors to collate and transfer all the information from the lens back to the camera. This includes data about focal length, focus distance and Image Stabilization. The advantage of this is that the camera can adjust settings like AF Microadjustment (if you are using it) to suit the combination of lens and Extender. It also means the AF performance when using extenders is improved compared to the previous generation of extenders when used with earlier generations of Canon telephoto lenses.
To achieve the maximum benefits of this new processor, you need to ensure that you attach the lens to the Extender first, and then attach the whole lens and extender combination to the camera. This way the camera will see a combined "Lens + Extender". If you add the extender to the camera first the camera will recognise it, but will then not be able to deduce that a lens has been added to the front of it.
As with the previous Canon extender models, the lens mounts are dust and drip proof, but in the Mark III Extenders, the lens mount features more screws to provide a stronger and more durable attachment to the camera and lens.