P3D short, normal, long lenses

[bercov@xxxxxxxxxxx (John Bercovitz)]

Brian Reynolds writes:

> First off the terms should be short, normal and long. 
[...]
> Short, normal and long refer to the focal length of the lens as
> compared to the diagonal of the image on the film.

Good.  Someone needed to say this.

> 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.)   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.

John B


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