----- Original Message ----- From: "Bob Talbot" > You will be telling us next that focal length of a lens does not change when you > switch "film" formats ;o) Hmmm... Bob, I suppose that you meant that you would change the focal length of your lens if you wanted to maintain the same angle of view when switching film format, the change being a function of the diagonal of the film format. Otherwise, the focal of a lens is and will always be the focal of that particular lens. ;o) > > Of course pinholes have focal lengths - how else could you focus them? > It's just that any one pinhole has lots of focal lengths, all positive. Hmmm... Did you say " "focus"-ing" a pinhole? I guess you refer to the complex calculation one uses to measure the optimal pinhole diameter, using the film-to-pinhole diatance (focal length) and some constant related to the wave length of the light... (See below) Hence, while any pinhole will always create an image, irrespective of the focal length, there corresponds only a single "optimal" pinhole diameter for any single focal length. Check Larry Fratkin's excellent "Pinhole Camera Design Calculator" on this subject http://www.mrpinhole.com/calcpinh.php Of course, any object in front of an optimal pinhole will always be in focus on the film plane (as long as the object-to-pinhole distance is greater than the pinhole-to-film distance). Regards, -:)) Guy P.S. Optimal pinhole diameter: For those interested, the basic formula is: "Pinhole diameter = 0.0073 * SQR(focal length) "where Diameter and focal length are in inches and SQR stands for square root. "For metric system the formula becomes: "Pinhole diameter = 0.03679 * SQR(focal length) "where Diameter and focal length are in millimeters " Source: Guillermo Penate http://members.rogers.com/penate/pinsize.htm Note that the image-creation process of the pinhole is very different from that of the lens. Whereas the lens will "focus" (bend) light to their specific focal point, pinholes create images because, from the myriad of light rays reflected by an object point, they only let a single ray through to the film plane.
