I just caught up with a Photokina interview with Stephan Schulz, in which the following passage appears:
Question: Similar to Sony’s E mount?
Answer (Stephan Schulz): Not exactly. That’s a big difference with the L-mount versus the Sony E mount. They started the mount with the NEX, which was APS. But, we assume that Sony wasn’t focused on full frame at that time due to the mount diameter. The Leica L-mount is much bigger which gives us more flexibility.
And we can look at the figures of various mounts from Wikipedia, and find some interesting data on throat diameters:
Clearly, Canon’s EF-M and Sony’s E-mount are designed to the same specification – APS-C. When Canon decided to go full frame with mirrorless, rather than retaining EF-M mount – clearly a possibility since Sony had done it before – they decided to start over with a completely new mount. And equally clearly, they did not do so for funsies, as there would be inertia against purchasing a camera with a small selection of native lenses as well as additional costs and compromises associated with using an adapter. The EF-M lens range at that point wasn’t huge – only some eight lenses, but there were voices of disgruntlement from Canon EF-M camera owners nonetheless – and perhaps some prospective Canon EOS R buyers. (Remember the video crop!)
So why would Canon “start over”? Clearly so as to reap the benefits of easier lens design, which Sony had neglected to do. In Sony’s design, the corners of the sensor are hidden behind the rim of the mount. This means special consideration needs to be given to illuminating the corners. However, this clearly sounds worse than it is. Dark corners when using adapted lenses are not a common complaint from Sony full frame users, so clearly all typical light cones do reach the corners of the sensor. But it would be a fair point to say that it’s by chance rather than by design, facilitated presumably by a need for lenses to be compact whenever possible, so back focal distances are a design consideration even without considering E-mount quirks.
The skeleton of that initial controversy was unearthed yet again when Sony got asked if they would make an f/1.0 lens, and flatly (and I think a little unfortunately) replied that it would not make business sense to do so. Unfortunate not least because Nikon have committed to releasing a Z-mount 58/0.95 in 2019, so the market for such lenses will very quickly become apparent.
Sony’s comment also comes on the heels of Nikon leaking via its French base that it could make f/0.65 lenses with the Z-mount. As a marketing triumph, I could just see one of the other two big players (Nikon or Canon) selling such a lens, initially at a loss and hiking up the price if significant demand turns out to exist.