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OVERVIEW
In a video microscope, telescope, or other optical apparatus
utilizing lenses, the exact length of the optical path is
critical to proper focusing. If the system includes both a
camera and eyepieces, the two branches of the optical path
must be closely matched in length. The light from the
subject is divided by a 'beam splitter' prism along these
two separate paths. Any difference between the lengths of
these paths will result in one being out of focus in
relation to the other. Bringing one into proper focus will
cause the other to blur.
Most microscopes have fine adjustments built into the
eyepieces to allow for variations in the human eye or slight
deviations from proper alignment. If the microscope is
adjusted to bring the monitor image into focus, the
eyepieces can then be fine-tuned to match. In the event that
the eyepieces do not have enough range to compensate for
these differences, it may be necessary to adjust what is
referred to either as 'back focus' or 'par focus'.
BACK FOCUS ADJUSTMENT
To adjust 'back focus', the eyepiece fine adjustments should
first be set to their center or zero positions. The
microscope should then be focused in the eyepieces rather
than on the monitor, without changing the fine adjustments.
Once the image in the eyepieces is clear, the camera side of
the optical path can be set to match.
Adjusting the camera side of the optical path involves
moving the camera's image sensor either toward or away from
the subject. Many of the video cameras which incorporate a
backfocus adjustment use a threaded collar (or 'back focus
ring') which fits partially into the camera body, held by
one or two small set screws. By loosening the set screw(s)
and rotating the camera body (containing the image sensor)
further forward or back on its ring, the monitor image may
be brought into proper focus. Care should be taken not to
twist or kink the cables attached to the camera, and to keep
the back focus ring and other adapters from turning along
with the camera body. Once the image in the monitor is set,
verify that the image in the eyepieces has not changed, and
then tighten the camera set screws. The optical paths are
now matched, or 'parfocal'. Slight corrections may now be
done at the eyepieces.
Cameras which lack a back focus adjustment must be
positioned at the correct distance either by adapter rings
and spacers or with other methods. Other back focus
arrangements have been developed by various manufacturers,
which are not covered here.
PROBLEMS
Occasionally, excessive torque applied to the camera while
threading it on and off of its mounts may force the back
focus ring to turn in spite of the set screws, particularly
with single set screw cameras. This is the most common cause
of back focus misalignment. If the threads become damaged
enough by this, the camera body may lean slightly on its
ring. Once this happens, the image on the monitor may become
out of focus on one side compared to the other, as the image
sensor no longer sits perpendicular (flat) in relation to
the microscope. Routinely checking and tightening the set
screws may help prevent problems of this type from
developing. Note that some cameras use multiple screws
around a floating image sensor assembly to set back focus,
and that all of the screws must be adjusted identically or
the sensor will tilt and exhibit the side-to-side focus
problem above.
Other than a mechanical problem in which the image sensor is
no longer held rigid and perpendicular inside the camera
body, or the camera body against its lenses, the camera
itself has no effect on the focus of the system.
Changing the adapter rings between the microscope and camera
will generally involve resetting back focus.
WARNINGS
Please exercise caution when installing or removing a camera
from its microscope, as this is when most damage to cameras,
cables, and threaded adapters occurs. Any connecting cables
should be detached from the camera beforehand, to avoid
twisting the wires loose while rotating the camera.
ADDITIONAL INFORMATION FOR THE A206, A206A and A209
CAMERAS
If the focus differs only slightly from the eyepieces to the
image on the monitor, check the focus adjustment on the
eyepieces. Each eyepiece has a fine focus adjust and may be
adjusted to another person's eyes. Focus the image in the
monitor first, then focus the eyepieces to each eye. Use low
magnification for this adjustment.
If there is a large variation in the focus, set the
eyepieces at zero and turn the microscope to the highest
magnification (do not use an oil immersion objective) and
focus the scope through the eyepieces. If the focus on the
monitor is off, loosen the two set screws that hold the back
focus adjust ring with the Allen wrench provided. These set
screws are located on the top and the side of the front
housing of the camera. Rotate the camera body until you
attain a focused image in the monitor. Make sure only the
camera is turning while the back focus ring, C-mount ring or
C-mount adapter remain stationary.
Turn the microscope back to low magnification and again
focus the scope through the eyepieces. If the focus on the
monitor is still off, rotate camera body again to attain a
focused image on the monitor. The camera should now be 'parfocal'
with the scope.
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