P3D Re: short, normal, long lenses

[Brian Reynolds <reynolds@xxxxxxxxx>]

John B wrote:
> 
> Brian Reynolds writes:
> 
> > By the way, focal length has nothing to do with perspective.
> > Perspective is controlled by the subject to lens distance
> 
> That's exactly correct but focal length can help with the 
> correct image-viewing distance (so that you will view from 
> the perspective point).  (This is a different subject from 
> what Brian was talking about.  He was talking about the 
> perspective of the shot and I am talking about the perspective 
> of viewing the image.)

Actually I believe both taking and viewing perspective are controlled
by the lens to subject placement.  Viewing has the additional
complication of enlargement factor (for the miniature formats. :)

> For a first order answer, let's look 
> at the pinhole camera.  The perspective is set by the placement 
> of the pinhole relative to the objects in the scene.  The correct
> viewing distanct to maintain the perspective when viewing the
> image is the distance from the pinhole to the negative.  This
> distance will give you, as the viewer, the same object angles 
> that the camera's pinhole saw at the scene.  To the degree that
> the distance from the pinhole to the negative simulates the 
> focal length of a lens, the focal length is useful in 
> establishing the image's perspective point.
> 

I believe this example assumes no enlargement.  (I also believe I may
be beating a metabolically chanllenged equine here. :)

-- 
Brian Reynolds                  | "Humans explore the Universe with five
reynolds@xxxxxxxxx              |   senses and call the adventure science."
http://www.panix.com/~reynolds/ | - Edwin P. Hubble


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