My understanding of optics is poor. What I think I remember, from physics class, is this: light reflects from the subject and hits the front surface of the lens at all points. The lens will focus all of this light into an image at the focus plane. I was told that if you cut a lens in half (top to bottom not front to back) it would still work the same, but the image would be dimmer.
So my question is: why does putting a graduated neutral density or center filter over part of the lens dim only part of the image? It seems it should dim the whole image evenly, but I know it doesn't. So where am I wrong, or what information am I missing?
I am thinking the answer has to do with how far the filter is from the front of the lens, but it is just a guess.