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A further check is necessary to insure that the masks will
not vignette for all blend line positions. With the plane mirrors returned to
proper match position the location of the overlap at top and bottom of the
screen should be noted or identified with tapes. Then when infinity and 3 foot
unvignetted targets are projected, the two overlap positions should fall well
within the tapes. Alternatively, perfect gray sky infinity and 3 foot vignetted
strips may be prepared and used. Any
asymmetry in appearances will be due to panel unbalance or to projector
vignetting.
A further, very serious effect is matchline "wedging" due to
improper alignment of the flat mirrors. To achieve proper alignment, a special
target film has been prepared but it is usually used in such a manner that it
is impossible to align to the requisite accuracy. In this target film, a
vertical line appears in panels in the overlap area. The mirrors are to be
aligned so that the two lines are absolutely parallel. This cannot be done
if the lines are superimposed on the screen. Instead, proper technique
calls for first separating the lines and then slowly bringing them back to
coincidence, noting whether the top or bottom touches first, adjusting the
mirror and repeating as often as necessary until both top and bottom touch at
the same time. This is the only approved technique. The target lines are
then brought into coincidence. At this time, after carefully rechecking focus
and framing in the center of the matchline, the magnification should be checked
and adjusted. If magnification is incorrect, adjustment should be made by
changing the path-length (mirror position) or the lens itself. If any
adjustment is made, the procedure is of course repeated. As a handy guide, a
magnification error of one inch on the screen calls for a path length change of
about one focal length.
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