Projection lens lengths

[P3D Bob Aldridge <bob.aldridge@xxxxxx>]

Amongst his relatively long posting re viewing with different focal
lengths, Paul Talbot wrote:

->How far from the screen is the ortho seat with the
->4" lens?  With the 5" lens?  Does the "ortho seat" result in depth
->reconstruction similar to what would be observed with an ortho viewer
->such as the Gold Button? 

Surely the focal length of projection lenses doesn't affect the position
of the ortho seat at all? It's the size of the projected image that
affects the position of the ortho seat. And the degree of overlap of the
images. e.g. with a 48 inch high projected image from a Realist, and the
frames exactly overlapping, the ortho seat is theoretically at 7 feet -
the distance of the window as set up by the camera/mount system. 

With larger projected images, you leave the infinity distances separated
by 65mm. The frames are no longer co-incident. This results in a window
that is off the screen - closer to the viewer. The ortho seat is now 7
feet from this window position.

The changes introduced by the different projection lens focal lengths
are in the length of throw - not the projected image (all things being
equal, of course. i.e. ignoring distortion effects from the individual
lenses). The actual images would be the same from each focal length if
the throw was adjusted to give equivalent image sizes.

Well that's how I see it. Any comments?


Bob Aldridge
Stereoscopic Society Projectionist
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 * POW 2.0  0007 * Disinformation is not as good as datinformation.


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