Originally Posted by Liaury
...Switching between 16:9<->2.4:1 should not involve lens shifting. Both aspect ratio should have the exact same center point (literally right in the center of the screen). The projector is just zooming large to fill the width of your 2.4:1 screen and zooming small to get the 16:9 image within the vertical limits of the screen...
That would only be true if the pj lens were at the exact vertical center of the screen. Few people have that mounting option. If the lens is higher or lower than the vertical midpoint of the screen, you must use the manual lens shift to center it (joystick). Once you do that, the "center point" of the projected image, as far as zooming is concerned, is no longer at the "center" of the projected image. So as you zoom, the projected image will move either up or down (depending on whether the lens is above or below the physical midpoint of the screen). That is the reason for needing V-AREA POSITION to "electronically" shift the active image area within the projected image when switching between different aspect ratios.Edit:
I'll try to explain it a little better. Imagine the projector at a fixed position and projecting on a wall with no manual lens shifting at this point in time. Draw an imaginary line from the center of the lens, parallel to the ceiling and side walls, and over to the wall. Now imagine a dot on the wall where that line hits. That dot is the "physical" center of the projector's optical zoom. When zooming in or out, the projected image will grow and shrink around that dot as its center.
Now imagine using the manual lens shift to reposition the image on the wall at a lower position. Neither the projector nor lens have physically moved; all that has really happened is that you've altered the angle of the light path through the lens. The lens' optical properties (as far as zooming is concerned) have not changed. The physical center point of zooming is still that imaginary dot on the wall, which is no longer at the center of the projected
image. When zooming, the projected image will still grow and shrink around that imaginary dot, not around what you perceive to be the center of the projected image.
Sorry...I'm not very articulate when trying to explain these things, although they make sense in my head. Perhaps someone else could explain it better. Also, there is a good diagram here
in the 4000 thread illustrating the concept.