Large parts of the visible area of the camera are shielded from the light source through the film plate and the camera mount rail. To compensate for that, I opened small areas of those parts to let light through, to see if the adjustment of the raspberry camera would better adjust the brightness. In my test it did and I'm very pleased with the result. 

I put the files in the gdrive folder: camera mount rail V2, film plate V3, light box V2