P3D Projection and mounting

["David W. Kesner" <drdave@xxxxxxxxxx>]

Several people have alluded to the difference in on film deviation 
and on screen deviation and how this is compounded by very large 
screen sizes. It has been suggested that by changing the horizontal 
convergence of the projector (and thereby moving the placement of 
the window from the screen to somewhere in front of it) you will be 
able to compensate for this.

As I understand it, zero deviation occurs at the screen; negative 
deviation increases as the scene moves in one direction from the 
window; positive deviation increases as you move in the other. This 
means changing this alignment decreases deviation in one area 
but is also increases the deviation in another.

Say you have an image with convergent points in the center of the 
scene and a 1.5mm deviation on close objects and a 1.5 mm 
deviation on far objects. If you adjust the projector to reduce the 
close objects to 1.0 mm doesn't that also mean you increase the far 
objects to 2.0mm?

I know I should study the technical papers on the subject, but I find it 
much easier to ask the questions here and get an immediate and 
pointed answer.

That's all for now,

David W. Kesner
Boise, Idaho, USA
drdave@xxxxxxxxxx