I am not interested in forum wars, so do forgive me that this will be my last post on this subjected.
a narrow mount will limit the angles from which light will hit the sensor. This will limit the f number physically. It is very similar to vignetting, but not quite the same - because with vignetting, you usually talk about a gradual drop off. Here it is a sharp barrier (that will look gradual on the image, unless stopped down).
Actually it's the ratio of mount diameter to flange distance that matters. With the FE mount, it's theoretically possible to achieve f0.65 about.
it is already an issue with some lenses, the 24-105 springs to mind, but whether or not it is because of the mount diameter may not be clearly inferred. It will obviously be more of an issue with faster lenses, but that has not prevented Sony from having fast lenses.
Nevertheless, anecdotally, it does seem like Sony is struggling more with vignetting, which is likely lens mount shading.
I don't know about that, looking at Nikon's recent 20mm f1.8 vs Sony's version, the Nikon suffers from way more vignetting and also is much softer at the edges (and also more CA, but that's another issue). However, Nikon has the largest mount, so what happening? It seems like the mount has no impact here.
obviously if you move the sensor around, you need a larger imaging circle, and a larger imaging circle, is harder without a larger mount diameter. Hence the connection. Whether the “8 stop” vs “5 stop” canon Sony different marketing is related to mount diameter, is not known. But it could be. It definitely seems like the larger relative mount and smaller sensor of mft has been helpful to ibis. So it may be related or may not be. At some point thought, the mount diameter would be a limitation for Sony ibis. Given that Sony is so close to the physical sensor size in mount diameter, it does not seem unlikely that it is related. And there is certainly no evidence, that I have seen, to suggest it is unrelated.
On balance, a large mount diameter is preferable, unless one consistently use smaller lenses. In addition to the advantages noted above, it allows for a sturdier connection between lens and body. Moreover, it avoids throating on the lens, which also requires more engineering. Finally it gives some flexibility in future design changes.
I see there's some confusion. Larger mount has 2 advantages when it comes to IBIS:
1) ability to work at longer focal lengths, so where Sony's IBIS would stop being effective at 150mm, Nikon's IBIS would still work up to 200mm, something like that.
2) video stabilization, where more movement is happening, a larger mount has the benefit.
However, a larger mount doesn't magically give more stops, Sony's IBIS is 5.5 stops even at 20mm, not because of the mount diameter (it's more than enough for a 20mm lens), but because the IBIS system is not fast enough (either the gyros respond too slow, or the mechanism cannot move the mass fast enough, or both). A larger mount won't solve any of those issues.
By the way, if you need more proof, just look at Sony's APS-C bodies, the A6600, it has 5 stops IBIS, however the mount is HUGE relative to the sensor size.
Now how does Canon or Panasonic get more stops on FF? Simply because the lens AND the body work in tandem. Sony and Nikon are not doing this yet. They're letting the lens IS handle 2 axes, so the body only handles 3 axes, but Canon/Panasonic combine them instead.
sure mount size has been overhyped as a disadvantage/advantage, but I think it likely that those who designed new mounts chose their parameters for a reason
Canon RF mount has exactly the same diameter as the EF mount, which was made when nobody even thought about IBIS, so that specific diameter had nothing to do with IBIS.
