EDIT: I need to clarify the question.
Kodak sells A gray scale card. It is used to calibrate B&W filming. Film stock exposure is measured with a densitometer. If the film has been properly exposed and developed, each region of the card will register a specific density when measured with the densitometer.
This is different than a gray card. A gray card is used to adjust the exposure of a camera that has presumably already been calibrated, usually by the manufacturer.
Given the camera location and plane position in the blend file, it should be possible to set the lighting so that middle gray is properly exposed and then render. In the render, the luminance value of each patch should match that of the original image. This is similar to using a gray scale to calibrate a camera, but not exactly the same, but should be possible.
Is there some combination of color management settings that will accomplish this or do I have to use RGB curves to compensate?
(original question: I'm a black and white photography nerd. One of the things I've done with film cameras is to calibrate them to a grey scale card. I've tried this with no luck in Blender. With 50% gray properly focused, the values do not match the gray scale. How do I do this?
I'll attach a blend file with the lighting setups I've tried.
I've tried with three different lighting setups - Remove the world lighting (set it black) and use a sun lamp pointing perpendicular to the image - Remove the world lighting and use an area lamp parallel to the image directly behind the camera - Remove the lights and use the world lighting.
In each case, I adjust the strength until the 50% value on the gray card shows a value of ~.5000 when a point in it is clicked in a render. I understand that there might be enough fall off from the area lamp that the card is not evenly lit, but for the sun lamp and the world light this should not be true. I've tried this with various combination of color management parameters to no avail. The closest I've managed is filmic/raw, but it doesn't match a gray scale curve.)