> -----Original Message----- Jay Busse wrote - : 18 December 2002 06:03 > on 12/16/02 9:29 PM Rick Lee wrote: > > > Yeah... I would have thought that the "Perspective Correcting Crop" in > PS > > would have done this automatically to approximate the effect of shifting > the > > front standard of a view camera. I was rather surprised to see things > > getting squished out of shape when "correcting" the perspective. ... > May or may not be possible. If, for instance, the software detects there > to > be a 5 degree taper in the sides of a building, the amount of correction > to > be applied to the height may be different if that taper is caused by a > camera with wide angle tipped slightly back or a longer lens tipped back > somewhat more. But that's just a guess - I'm no optical guru.
Neither am I and it is quite a few years since I studied optics so I am happy to be corrected if wrong... Imagine using the rising front on a view camera to photograph a tall building with windows. Compare the size of the windows at the top with those at ground level on the ground glass or sheet of film. You will discover that the vertical size of the window frames at the top of the building is greater than that of the ground level window frames. It is simple geometry, imagine a triangle. Draw a line from the centre of the lens to the bottom of the window frame, up to the top of the window frame and then back to the centre of the lens. Do the same for a ground floor window and compare. Now remember when using a rising front the film plane is vertical, like the building, so you get the same triangles on the image side of the lens. If using a long lens the difference is negligible, but if using an extreme wide angle the difference is quite noticeable. I would suggest that the only way for software to simulate a rising front would be to specify the focal length of the lens in use and possibly also the distance to the subject (as in some 3D software). In comparison tilting a camera up at a building and correcting in Photoshop you can actually achieve a more accurate appearance! The distorting effect described above is still there but not as much and therefore closer to the perception of our eyes. Paul Lawrence --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.427 / Virus Database: 240 - Release Date: 06/12/2002 =============================================================== GO TO http://www.prodig.org for ~ GUIDELINES ~ un/SUBSCRIBING ~ ITEMS for SALE
