I want to compare and quantify the light transmission of various lenses (with some custom modifiers). So I have a target setup with constant illumination, and a camera body for them ready to shoot RAW with a fixed ISO, aperture, shutter speed, and distance.
Suppose I shoot some comparison images controlled in this fashion. Now, I want to pick some points on the target and see, relatively, how bright they are through each lens. How should I do this?
I could open the images in Adobe Camera RAW and get a HSL lightness value using the eyedropper. Is the Adobe "luminance" number linearly proportional to transmission? E.g., if lens A has luminance of 90 and lens B on the same point has luminance of 81 then is B transmitting only 90% as much light as A?
What if I want to express this in terms of relative Exposure Values or Stops?
And are there any other variable that I would have to control to fairly compare the transmission of different lens configurations?