Crippled mount?

Well it can't and when it is "exposure errors may occur" so there seem to be technical reasons. I did notice someone mention that the metering coupling was non-linear in some designs and linear in later ones, so perhaps that's the problem. It's easier to use A lenses and later and perhaps the development costs for enabling the relatively few older lenses that will be used was not deemed to be cost effective.

As things stand compatibility extends back some 30 years at least, and that's pretty good by any standard. You can use lenses going back almost 50 years if you want to but it's not unrealistic or unreasonable to find that in that case there are some restrictions.

Sorry I explained it a bit wrong.
Don't know what a stop-down coupler so I don't talk about that.

What I mean is you've the "optical review right" where it stops down the lens.
Why can't it take a metering wide open (lens in rest position) and a metering when it preforms a "optical review" so that the camera could calculate the difference in exposure so that you can get a correctly exposed image.
This way the camera doesn't need to know which aperture is used.

I ain't talking nonsense here right?

@John, it might indeed have an epxosure error since it takes some time to make metering but it's atleast better then nothing. I don't really see though why it would metter if the metering is linear or not but maybe you misunderstood my idea.

That would be an improvement, but still the calculation (and stopping down) would need to be done after every change of aperture.

OK.... Now I'm sure I have it! - The crippled mount lacks the little lever, seen on the right hand side of the mount throat, that tells the camera what aperture the lens is set to, without the lens iris having to be physically in that position.

Thanks, folks.

To add to Stefan's idea, there are other solutions that could be done in firmware. How about the user simply dialing in the aperture manually on the camera so that it knows what calculation to perform*? It may not work perfectly, but I bet it would be better than what we have now.

Sadly, Pentax show no interest in improving support for legacy lenses. They're concentrated so hard on new technology. We're lucky we can use old lenses at all.

* Of course you'd have to set the aperture on the lens, too. But we could do both!

EDIT: Also you'd need to enter the len's maximum aperture. I wouldn't mind doing each time the lens was fitted.

That might also do Pete but it's more work for the one writing the firmware and yours also requires changes when moving the aperture ring the good side is though that there would be an aperture value in the EXIF data

I can't fathom why the metering is so skewed in M mode either, as it's spot on in Av.

What metering pattern are you using?
If you use spot then it is quite critical at what you aim it at.

It's not much bother to press the green button to stop down IMHO

That has nothing to do with the difference in metering between M and Av mode (with the lens wide open). I notice some with my K-7. Didn't notice with previous models.

No, but it's a bit of bother than wouldn't be necessary with better hardware or firmware.

I wouldn't mind so much if it worked, ie. resulted in correct exposure. It doesn't with my M and K lenses. It's a common problem that many users have mentioned on this and other forums.