Re: Image diameter ....?

"CamArtsMag" <[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
>   Note that perspective is dependant exclusively on the
> camera position with regard to the subject. Once the
camera
> is placed the focal length does not affect the perspective
> only the size of the image on the film.
> >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
>
> I agree with the possible comment that can't the camera's
movements affect
> perspective? I am thinking of swing and tilt on the back.
>
> steve simmons

  Well, by perspective I mean the relative size of forground
and background objects.
  Shift seems to affect perspective beause most lenses
(other than fisheye type) are intended to be orthogonal. By
orthogonal I mean that the image is produced in a way that
the original relative dimensions are reproduced to an eye at
the correct viewing point. The "distortion" at the corners
of a wide angle picture will dissapear when the image is
correctly viewed. It is this "distortion" which is used to
make diminishing lines parallel by shifting the lens in
relation to the film. In fact, it is a distortion of the
original perspective, but one which looks right to the eye
when at other than correct viewing position.
  Tilts do something else. The image projected by a lens is
_three dimensional_ although we usually use only a two
dimensional plane. By tilting the lens or the film a
different plane of the image is chosen. Usually tilts are
used to increase apparent depth of field. What is actually
happening is that the plane in the object field is being
tilted. If this plane matches a tilted surface in the object
field all of it will be in focus. This is not really
increasing depth of field since the depth of the object
field is no greater than it would be with everything
parallel.
  Tilting the field can cause some geometrical distortion
but it is different from that produced by shifting since we
are usually still in the center of the field where shifting
off-sets the center toward one side of the image.
  This is much easier to explain where one can draw pictures
or demonstrate the effects.


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Richard Knoppow
Los Angeles, CA, USA
[EMAIL PROTECTED]