Join Date: Oct 2012
Location: Sydney, Australia
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If you are using a cylindrical lens that can be rotated in its mount, the deterioration in screen geometry from horizontal offset is minimized.
Say you are projecting from above/left of screen center. You will need to angle the offset so that the beam points down and to the right.
You will then interpose the anamorphic adapter into the beam so that the center ray of the beam passes through the center of the lens optics.
You will notice a slight dip in the image border in the bottom right corner (the opposite corner to where your projector or placed). The right hand half of the screen will be somewhat wider than the left hand half as well.
By rotating the cylindrical lens the dip in the bottom right corner of the screen can be lessened or gotten rid of altogether, for practical purposes. Often the slight misalignment of verticals that a static grid pattern would reveal after doing this goes completely unnoticed in an actual movie presentation scenario.
You can also slightly change the horizontal offset (so that a 16:9 image is not perfectly centered) and then use the anamorphic lens to re-center the 'scope image. This will lessen the imbalance between the left and right halves of the screen.
Cylindricals are much more adjustable than prism lenses. They can be rotated about the Z axis (center ray of beam), the X axis (tilt up and down) and the Y axis (yaw), and placed slightly out of perfect alignment without apparent image quality penalty, especially in cases of mild misalignment or centering.
This is why many cylincrical lenses have rotational capability (similr to polarizing filters that can be rotated also). You often find that with some adjustment in less-than-ideal conditions the cylindrical lens optics can solve an image problem that, with prism and other unadjustable lenses, only expensive structural work (e.g. knocking out a support wall just to line up the projector) was previously able to do.