Etienne said:
Your entire argument for Canon retaining the EF mount in a mirrorless body revolves around your personal desire to avoid buying new lenses.
Of course the EF was designed to optimize AF, but to spell it out for you, the lack of a mirror box also changes the engineering parameters for the lenses since they can be brought much closer to the sensor.
It's never been an issue that the EF mount is incompatible, it's just not optimal. The EF mount was designed to accommodate a mirror box. Clearly wide angle lenses, and probably normal lenses, can be made smaller and probably cheaper in a mirrorless optimized format. Sony has demonstrated so.
Your wishful thinking will, thankfully, be totally ignored by a company seeking to compete, or dominate, the emerging mirrorless camera market. Sorry, but you have absolutely no argument to support your dream. :
A bit of history here...
The EF mount (EF stands for Electro Focus) came out in 1987. The primary design feature of the mount was that all communication to and from the lens be done through serial data streams and that there be no mechanical couplings such as levers... It replaced the FD mount, which had levers and a couple of dedicated pins. FD mount was not conducive to automatic focusing. Canon had realized that it was a dead end, and eventually Nikon also realized that mechanical linkages were passe….
Fast forward to today, where the EF mount is still in primary use.... The mount still works well, but as computing power and data communication speeds have increased, the slow communications between lens and body have started to limit system performance. Canon has patented a new version of this mount which will allow for the negotiation between camera and lens of higher data rates. How this works is you turn on the camera, it sends a query (using the slow EF link speed) to the lens asking what the max supported lens link speed is. If there is no answer (legacy EF glass), then it continues at the EF link speed. If it responds by saying it supports a faster speed, then both switch to that faster speed. We expect to see this very soon, be it on new EF bodies, or if it is a new mount... Canon has stated that there will be an "elegant" solution with a new mirrorless mount, and the odds are exceptionally high that this will be part of it....
As far as mount size goes, you can make the flange shorter, but as a result the lens will have to bend light from regular and telephoto lenses more, and the result will be more chromatic aberration and there will be more vignetting as a result of the light hitting the sensor at a greater angle. For wide angle lenses, you are better off as there will be less vignetting and less chromatic aberrations.... a classic example of trade-offs... There is no A is better than B, they both have their strengths and their weaknesses. You can not say that the current EF mount is not optimal without acknowledging that whatever any possible new mirrorless mount is, that it will also not be optimal. You can not cherry pick your conditions.
There are very good reasons to keep the existing EF mount, just as there are also very good reasons to create a new mount. A lot of people argue that size is the over-riding criteria, yet when you include fast or long lenses into the system, the system savings are negligible. If you want a small system that is still of good quality, Canon should look at releasing some F5.6 or F6.3 L glass in whatever mount they pick.... That's where the real size savings will come from.