______ in a geodetic leveling instrument exists when the plane of the image and the plane of the cross-hairs do NOT coincide, and should be eliminated by _____ the objective, eyepiece, or both.

• A. Parallax, changing
• B. Non-collimation, refocusing
• C. Parallax, refocusing
• D. Non-collimation, changing

Answer: (C)

Parallax occurs when the image plane and the plane of the crosshairs do not align. In a geodetic leveling instrument, if the image you see through the telescope sits on a different focal plane than the reticle, moving your eye changes the apparent position of the target relative to the crosshair, causing reading errors. The way to fix this is by refocusing so that the image plane coincides with the crosshair plane. You can achieve this by adjusting the focus of the objective, the eyepiece, or both. Non-collimation would involve misalignment of the optical axis, which is a different issue and isn’t addressed by refocusing.