[Brightcolours]

https://www.dxomark.com/Lenses/Sigma/Sig...D800E__814
https://www.dxomark.com/Lenses/Sigma/Sig...9-II__1120
https://www.dxomark.com/leica-apo-summic...ns-review/
https://www.dxomark.com/sony-fe-85mm-f1-...nt-choice/
https://www.dxomark.com/sony-fe-50mm-f-2...ns-review/

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Their published T-stop values are just nonsense, and do not take into account the extra amplification a camera applies because of the F-stop used.

Surely T-stop is by definition only the optical component. Anything the sensor does to adjust for it doesn't matter.

It is NOT doable with most lenses using cameras, and even if the lens would allow what you have to do, you still have to have very specific knowledge about what the camera is up to to kinda measure T-stops.

The simple solution is not to have any electrical coupling between the camera and lens. The camera can't adjust the output if you don't know where that light is coming from.

How would you go about measuring T-stops? In the middle of the frame you get a lot more light than in borders, let alone corners.

I don't know if there is an industry definition, but some kind of peak or center-average might make sense.

And how to distract the light lost by the sensor microlenses?

That should not be counted as it is not part of the objective. You can have a separate sensor efficiency measure if you really want.

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

And many brands have the aperture closed when not connected.