Hi Paul,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 have erased a few prodig digests, so this might already have been said:
If you have your view camera properly aligned in parallel to the wall of the building, and if the lens does not distort, the windows at ground level will be exactly the same size as those further up. The triangles you mention are similar on the object and on the image side of the lens, the image triangle is just a rotated and scaled down version of the object triangle. Consider that all windows are in the same object plane, and hence have the same distance from the film plane (not the same distance from the camera, of course).
If, however, you tilt the camera, you will get different sizes for top and ground level windows (and - if memory serves, and if I did my calculations were correct - horizontal and vertical scale factors depend on partly different parameters). You will have great difficulty to correct this, since there is no simple tool to scale an image with a gradient to the scale factor, at least I know of none in PS.
All the best
Otto
--
//_ Dr. Otto PETER Institute for Applied Physics
.0^0__ University of Innsbruck
/_ \ / Technikerstr. 25
\____/^ A-6020 Innsbruck, Austria
_/|_ Tel.: +43-512-507-6408, Fax: +43-512-507-2922
Email: [EMAIL PROTECTED]
===============================================================
GO TO http://www.prodig.org for ~ GUIDELINES ~ un/SUBSCRIBING ~ ITEMS for SALE
