If the image formed by the objective lens is not in the same plane with cross hairs, then it is known as
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Think of holding your finger in front of your face and looking at a distant object. If you close one eye and then the other, your finger appears to move relative to the background due to parallax.
Parallax occurs when the image formed by the objective lens and the crosshairs in a surveying instrument are not in the same plane. This causes an apparent shift in the position of the object being viewed when the observer's eye moves laterally.