[davidmanze]

Give some examples if you like ...... I didn't follow the reasoning  !

[Brightcolours]

So how do you propose to measure T-stop? Without a sensor?

It doesn't have to be a camera sensor although I don't see any major problem with using one. Any suitable light detector could be used.

And many brands have the aperture closed when not connected.

If that happens, then for sure some way around would have to be found. I recall EF lenses can be forced to a given aperture by using DoF preview and disconnecting the lens in that state for example. I don't know how other lenses might react to similar.

Major problems: cameras add unknown amount of extra "amplification" to counter light loss. Canon does that a bit, depending on lens mounted, Sony and Panasonic a LOT, see DXO links above. Then there is the sensor light loss which depends on the angles a certain lens lets light fall onto it. This depends on focal length/exit pupil? and aperture size. Good luck navigating that shark infested bloodbath.

EF has the aperture open when disconnected. Other lenses can't be fooled like with EF and have it closed when powered down (= disconnected).

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Give some examples if you like ...... I didn't follow the reasoning  !

I gave examples a few posts above?

[Klaus]

Some manufacturers are cheating more than others when it comes to the aperture and there may be minor differences in the kind and amount of coating.
The focal length is also not the same as the one that is printed on the lens. Neither is the ISO rating on the camera.
But then ... who cares? Is that really something that you think about when shooting? I, for one, don't. Life's too short for that.

[Rover]

Some people like to obsess over numbers and combinations thereof...