A106/A206/A209 Camera Back Focus Adjustment Procedure
? E. Altman, J Kovacs, Aug 14, 1997

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.