[q] full-feature lens adapters of small frame mirrorless to larger frame mirroless

It seems that manufactures are not providing full feature (meaning autofocus, exposure, distance reading) adapters to adapt lenses for their own 'small-frame' offerings onto their larger-frame bodies.

For example (in my current understanding) no way to attach a M43 lens onto Panasonic-L.
Or Nikon 1 lens onto Nikon Z full frame.

For some situations, loss of pixels (and therefore ISO sensitivity), and other full-frame features -- is an acceptable trade off, in exchange for lighter carry rig.

Is this lack of lens transfer-ability -- a business decision, or there is a technical reason (eg on-sensor auto-focus will not work, or something like this) ?

The small sensor lenses will not, in most cases, cover the larger frame.
So the user would be left with a semi-circular image in the middle, with vignetting and/or loss of corner resolution.

I though that 'Auto cropping' mode is already a standard technology.
For example Nikon FX (full frame body) such as D800 will automatically switch to 'DX crop mode', when a DX (APSC-sized) lens is mounted on the body.
See this reference
There should be no vignetting, or semi-circular image in the middle of the shot taken with that mode on.

The reasons that M43 lenses cannot be mounted (with full features) on Panasonic-L bodies, or Nikon N1 lenses on Nikon Z-series full frame mirrorless -- perhaps are driven by more nuanced technical or marketing considerations, but I do not quite understand what those might be

MattKing said:

The small sensor lenses will not, in most cases, cover the larger frame.
So the user would be left with a semi-circular image in the middle, with vignetting and/or loss of corner resolution.

Click to expand...

The people I know who have full frame Nikon DSLRs aren't interested in being able to use cropped sensor lenses on them. They want to use film camera lenses, or modern lenses designed for the digital bodies.
With the sales of digital cameras in free-fall, I wouldn't expect any manufacturer would be putting efforts in increasing the ability to use already owned lenses on new bodies.

I can use Canon EF-S lenses made for their APS-C DSLRs on my EOS R by putting them on any of the EF/RF adapters that Canon offers. The adaptation is seamless and supports all functions of the lenses; the camera automatically switches to crop mode when an EF-S lens is mounted.

However, there are no EF-M to RF adapters, because the flange focal distance of the EOS M cameras is 18.0 mm while the flange focal distance of the EOS R cameras is 20.0 mm. It is mechanically impractical to build an adapter that sits recessed inside the RF mount, which is what would be required.

Looking at the Nikon example you cited, the Nikon 1 mount has an FFD only 1 mm longer than the FFD for the Z mount, so it would be a technical challenge to begin with. But regardless, I suspect that the market demand for a lens that projects an image circle designed for an 8.8 x 13.2 mm sensor on to a 24 x 36 mm sensor is nil.

@Oren Grad , thx for pointing out flange distance.
Yes, this might be at least one of the reasons.

Specific reason I was thinking about it -- is bird photography in a lighter package, without buying another body or 1-inch hybrid (like Sony RX10 IV , or Panasonic FZ2500).
Full frame lenses that go to 600 -- are just way way too heavy (even the ones by Tamron or Sigma).