Light loss due to the magnification....imaging the OHP you see in school....when you move the OHP nearer to the screen, the image becomes smaller, but brighter. When you push the OHP away from the screen the image becomes larger (maginification in this sense), but the image becomes dim (due to spreading the same amount of light over a larger area, thus lower light density), therefore, longer exposure time is needed for the same exposure.
After viewing this post I asked: But can function normally shooting group photos?
Thats my question.
anyone knows how much light is loss per mm of extension tube? and magnification factor?
For example "36mm loss 1 stop?"