Rough Optical Focus
Now that mechanical aim of the guns has been done, it is time to do an
initial, rough optical focus.
Display a crosshatch pattern. Adjust optical focus using the two knobs. The
front most knob on the lens controls edge optical focus. The rear most knob
adjusts center optical focus. Dial in center focus first. Then adjust the outer
(front knob) focus to while watching the corner lines of the crosshatch flare
inward and outward. Try to minimize flaring using the outer focus knob. Go back
and forth between the two optical focus knobs to get both center in focus and
outer edge minimally flared. Use of binoculars aids this process tremendously.
Rough Scheimpflug (Lens Flapping)
Once rough optical focus is completed, you can perform lens flapping. The
following is primarily for projectors which have continuously variable lens
flapping. For projectors with lens flapping rings, simply set the lens rings per
the installation manual specified settings for your projection distance.
Display a focus pattern. Look ONLY at the center of the top edge. Adjust
center optical focus to make that edge sharpest. Note the position of the focus
knob. Pay attention to the center of the bottom edge. Adjust lens center focus
to make the bottom edge sharpest. Is the lens center focus control in the same
position as when the top was sharpest? If so, vertical flapping is correct. If
not, adjust vertical flapping using a wrench. Note the wrench position when the
bottom edge is focused vs top edge. Set the vertical flapping to put the wrench
in exactly halfway between the two positions. Horizontal flapping is performed
in an analogous fashion paying attention to only the left and right edges.
Now go back and refine center and edge lens focus. Once that is done you
should have the lenses in fair optical focus and flapping. Advanced owners may
consider the 3 x 5 card technique fine focusing technique presented later in
this document for even better optical focus and Scheimpflug.
Phosphor Grain Optical Focus Technique for when the Electron Beam Has not
been Focused Yet
Most people have difficulty deciding whether optical, beam or both kinds of
focus problems are present, particularly if they have not already achieved good
beam focus. Here is a trick for setting good optical focus without being
confused by beam focus. You must have a good pair of binoculars which focus at a
short distance to use this technique.
The phosphor surface of a CRT has an inherent grain pattern. Because this
grain is visible and is exactly at the plane of light generation, one can use
the grain to set optical focus independent of beam focus. A small piece of 3M
easy release blue masking tape aids in keeping your eyes correctly focused on
screen. Display a bright window pattern and intentionally defocus the electron
beam to make the scan lines disappear. Adjust center optical focus while viewing
the screen through binoculars. When optical focus is correct, the inherent grain
pattern of the phosphor surface suddenly snaps into view. This is nearly
impossible to see with naked eye, but binoculars make it readily evident.
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