Parallax

Apparent alteration in the relative position of two objects that occurs when there is a change in the observer's viewpoint. Also, a term applied to the difference between the two viewpoints. Because of the parallax, the picture seen in the viewfinder of a camera is not the same as the picture seen by the lens that is a small distance below or to the side. The effect is not serious beyond about 2 metres, but closer than this the photographer needs to make an allowance. Some viewfinders have correction marks.