Mike,
You are making a symmetry argument - but the situation is NOT symmetric.
First, throw ratio is defined with respect to
width - not height. Of course, one could define
it based on height.
However, there is a substantiative difference.
If you do not have either lens and show a 16:9 movie as letterboxed - then you have setup the
projector so that it projects the proper width image. Assuming a 16:9 screen, the "dark" pixels
are targeted above and below the active area of the screen. With the addition of the Panamorph
and vertical compression - you don't have to change the setup. It's OK horizontally, and the
Panamorph merely redirects the previously unused pixels that were firing high and low onto the
active screen.
With the addition of an ISCO, and its horizontal expansion - if you did nothing else, you would have
an 16:9 image that was both too wide [ due to the ISCO's horizontal expansion ], and too tall -
because you already have the "dark" pixels overscanning the screen vertically anyway.
You must then zoom down this oversized image with either a zoom lens or repositioning the projector
so that the image properly fills the 16:9 screen.
Unlike the ISCO, the Panamorph does not require a change in zoom or projector position - which is
why it is so coveted.
The situation is NOT symmetric - because when you do not have a lens and project a letterboxed image
you are overscanning the 4:3 image vertically but not horizontally. That's why ISCO requires a change
in throw, but Panamorph doesn't.
Greg