P3D formats & focal lengths

[Rod Sage <rsage@xxxxxxxxxxxxxxxxxxxx>]

Brian Reynolds wrote:
>Perspective is the relationship (location and size) of different
>subjects within a scene.  Perspective is only effected by the
>positioning of the taking lens with respect to the subjects in the
>scene.  The focal length of the taking lens does not effect
>perspective (assuming that you are far enough away from your subject
>that the difference in the physical size of the lenses is not
>significant as compared to the lens to subject distance).  If you take
>a picture from the same location with both 50mm and 100mm lenses and
>enlarge the 50mm picture so that its subjects' size match that of the
>100mm picture you will see that the relationships between the various
>subjects is the same in both pictures.  See Ansel Adams' "The Camera"
>(page 106).

After much brain twisting I realize this makes sense. At first I thought
that a 50mm lens at 20 ft. would produce the same image as a 100mm lens
at 40 ft. Then I remembered about the  foreshortening effect of longer
focal lengths. Distant objects appear to come closer.  I was confusing
angle of view with perspective. Then I found in the Time/Life Library of
Photography on page 114 of the Camera book "As long as you do not move
your camera, changes in focal length have little apparent effect on
perspective".
Sorry if I ruffled anyones feathers.
RDS