Thanks for the answers but I needed a little more clarification and I found it at this web site. http://webphysics.ph.msstate.edu/javamirror/ipmj/java/dlens/. This applet explains your mysterious remark about this type of lens not forming an image on film Andy. Thanks Bob and Andy. Greg > -----Original Message----- > From: ADavidhazy [mailto:ANDPPH@ritvax.isc.rit.edu] > > > 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 > There is no little penta prism in there. The construction is similar to a "spy glass" (Galilean telescope) in that it doesn't developed an image, you just look through it backwards (optically speaking). Look at it like a wide angle adaptor similar to those built for the normal lens of a cheap camera - only for your eye. Regards, Bob..
