At 02:21 PM 7/5/04, I wrote:
>I'm not surprised that you're seeing a shrinkage in the field when you
>shift in any direction off-axis (center). I would expect that from the
>geometry of the portion of the image cone you're using centered versus the
>portion you're using shifted. The angle at the apex of the light cone for
>the portion of the cone being used by the film gate shrinks slightly as it
>is moved from center toward the edge of the cone.
Addendum:
A rectilinear lens maps flat planes in space parallel to the film plane . .
. to the film plane. If you've ever used a WA to photograph a group of
people, especially anything shorter than 35mm (in 35mm format), and had
someone's head too close to an edge, or especially a corner, it looks
unnaturally distorted. This is because a rectilinear lens preserves angles
parallel to the film plane, not areas. Look at a brick wall parallel to
the film plane and the bricks' rectangular angles are preserved, but not
their relative areas across the film plane. [A true fisheye does the
opposite; preserves areas at the expense of angles by mapping hemispheres
in space to the flat film plane . . . And Shazaaam! . . . Its images looks
like a polar projection maps of the Earth.]
-- John Lind
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