During the a6400 press event, Sony had an entire presentation on how the Sony E-mount was originally designed for full-frame mirrorless cameras and how they can (theoretically of course) make an f/0.63 E-mount lens. This response was targeted directly at Leica - in a Photokina interview with Leica executives on the new L-Mount Alliance, Stephan Schulz from Leica Camera AG claimed that the Sony E-mount was not designed for full-frame mirrorless cameras. There are several other articles online making the same claim. Here is the video from the presentation (watch from the 8:10 mark):
Of course Sony will never make an f/0.63 lens. This is just a marketing move to top Nikon's claim that they can make an f/0.65 lens and of course Sony has to be better with their calculation of f/0.63...
