David Brooks wrote: > I know the image on the film is upside down,same > as our eye see's it only the brain flips it. My ???? > is is it upside down and backward or just upside down > on the film.
Hi Dave, Each point in the object plane is ~inverted~ through the center of the lens to transform into the corresponding point in the image plane. This inversion operation must be symmetric with respect to rotation about the optic axis, since this is the symmetry property of your lens. Thus, an object point ~above~ the center of the lens corresponds to an image point ~below~ the center of the lens. Likewise, an object point to the ~left~ of the center of the lens corresponds to an image point to the ~right~ of the center of the lens, and so forth. The confusion arises because of the fact that you can view your processed film from either the emulsion side or the back (non-emulsion) side. The act of flipping your film over from front side to back is a ~reversion~ (i.e., left becomes right, but top does not become bottom). This operation doesn't have the same symmety property as your lens. This is probably more easily demonstrated with an example. Imagine, for example, that you're making an exposure of someone standing in front of your camera, and that you're standing ~behind~ your camera. Everything you see toward the top of this person will be imaged onto the bottom of your film, and everything you see to the left side of this person will be imaged onto the right side of your film. Keep in mind, though, that "left" and "right" in this case are with respect to your position ~behind~ the camera; thus, you're "looking" at the back side of the film. If you go around the front of the camera so that you are now "looking" at the emulsion side of your film, "left" and "right" are reversed. Bottom line, then, is as follows. If you're viewing processed film from the back side, the picture is ~inverted~ (top to bottom and left to right, or upside down and backwards as you describe). If you're viewing processed film from the emulsion side, the picture is ~reverted~ (top to bottom, but not left to right -- upside down, but not backwards, as you describe). Hope this helps. Bill Peifer Rochester, NY - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
