What max. aperture does the X-mount support

[ThorstenMUC]

From the discussion around the huge new Nikon Z-mount with 55mm diameter to support f/0.95 lenses, while the old Nikon F-mount with 46,5mm was struggling with lenses faster, than f/1.4 I was wondering, whrere our X-mount's limits would be.

As I understand this limitation it's not a hard border - depending on the lens construction you could route the light "around the mount-borders" - but the lightrays will then fall less and less perpendicularly on the sensor, resulting in vignetting and other IQ tradeoffs. That's why Nikon had almost no f/1.2 lenses in the past.

Since the X-mount with 44mm diameter and 17.7mm  flange distance almost has the dimensions of the Sony E-mount (46.1mm and 18mm flange dist.) I would expect, with just an APS-C sensor it would support pretty high apertures... but I can't do the maths behind that assumption myself.

[bergat]

If you see the  Fuji X-mount, you can easily understand that it could support also FX, if fuji would have liked. I think this problem is for the  sony E-mount

Edited August 29, 2018 by bergat

[Doug Pardee]

It's a non-trivial question. The size of the lens's exit pupil is focal length divided by f-number. So, a 50 mm f/0.95 lens has an exit pupil of about 52-1/2 mm. Then you need to concern yourself with the (diagonal) size of the sensor and the distance the exit pupil is from the sensor. Then do the math, and you can determine how large the mount opening has to be at any given distance from the sensor.

But you probably don't know the distance the exit pupil is from the sensor.

More realistically, you can determine how far the lens's exit pupil has to be for a given sensor and mount size. Fuji's X-Trans III sensor is about 28.2 mm in diameter, so the maximum exit pupil size increases by about 0.89 mm for each mm of exit pupil distance. So that 50mm f/0.95 lens needs to have its exit pupil at least 27-1/2 mm from the sensor.

The problem comes in when you increase the sensor size, as Sony did. Sony's full-frame sensor is 43mm diagonal. Maximum exit pupil size then increases by only 0.17mm per mm of exit pupil distance. That 50mm f/0.95 lens will need to have its exit pupil at least 55.7 mm from the sensor.

[No guarantees my math is 100% correct. Feel free to check me.]