I am pretty sure Marianne's experiment is flawed

bigpigbig wrote:

http://forums.dpreview.com/forums/post/50073569

Since this "experiment" that Marianne did is being used in several AF threads, I have to chime in here. Please don't take this personally, but when I read it, something didn't sit right with me.

Placing an "aperture" in front of the lens is NOT the same as reducing the aperture of the lens.

Lenses are geometrically designed to focus light passing through an aperture. If the aperture is moved, the lens doesn't focus in the same way.

A simple way to show this is create a camera obscura with a sizable aperture. Place an appropriate focal length lens over the aperture. Voila, the image is focused. Move the lens away from the aperture and the image is not in focus (not matter where the focal plane is located).

Another mind experiment would be to imaging focusing on a forest through a hole in a cave. The "mask" is further in front of the lens than the paper (and made of rock) in Marianne's experiment, but it is still in front of the lens. The focus achieved on the trees in the forest is not "stopped down" to the effective aperture of the cave walls.

Is it?