On Wed, May 20, 2009 at 05:08:52PM -0600, William Robb scripsit: > ----- Original Message ----- > From: "Graydon" > > Or maybe I'm just confused, and I'm not sure I can manage ASCII art > > diagrams for the first couple floors being right here, nicely parallel > > to the sensor, and the top two floors, fifteen floors above those first > > two floors, not being (in perspective) parallel, although actually in > > parallel unless it's a very funky building. > > Ok, lets presume that it's a perfect world, because it makes the explanation > easier.
It certainly does! > The walls of the building are perpendicular to the absolutely level ground > you are standing on. > If the sensor/film is also perpendicular to the ground, it will be parallel > to the walls of the building. > In this scenario, there will be no keystoning. > Keystoning (actually, I think this is when the building appears to be tilted > forwards due to over correction, but I'll happily misuse the term because > it's one that doesn't see the light of day often enough) arises when the > building is taller than the lens in use can capture, and so the camera must > be tilted upwards to capture the netire building. As soon as that happens, > the building will have the falling over backwards look. Ah, ok! I thought keystoning meant the apparent divergence of actually parallel lines. > The ways to correct it are: back off far enough that you get the entire > building in the picture without having to tilt the camera, and then crop out > the forground, or move the lens up or film/sensor downwards (with the camera > set up square and perpendicular to the ground) or a combination of the two > to get the entire building in the picture (generally this is a view camera > thing, rarely do shift lenses have enough movement), or take the picture > with the camera tilted upwards and correct the image during processing, > either with the transform perspective tool in Photoshop (or whatever passes > for the same thing in the software you are using) or if you are shooting > film, tilt the easel to correct the image (only works if the mage is only > slightly skewed due to depth of focus considerations). Appreciate the explanation. -- Graydon -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
