> Does anybody know why the image in an apartment door peep-hole is not
> inverted? Is there a tiny little penta-prism in there?
The real short answer is that the image is formed by a negative lens system and
is a virtual image. Such lenses form right side up and unreversed virtual
images of subjects. These virtual images can be seen and photographed but can
not themselves make an impression on photographic film (since they are
"virtual" ... not "real" as images made by positive lenses are).
Andy
