Parallax is an apparent shift in the position of an object when viewed from different angles. In surveying instruments like theodolites and levels, parallax occurs when the image formed by the objective lens is not in the same focal plane as the crosshairs in the eyepiece. To eliminate parallax, two steps are necessary: 1. Focus the eyepiece This brings the crosshairs into sharp focus for the observer's eye. 2. Focus the objective This ensures that the image of the distant object is formed at the same plane as the focused crosshairs.