nodal points

Definition: In geometrical optics: the two points on the axis which are positioned such that a ray entering one point leaves the other at the same angle as it meets the first point. * Generally, object distance is measured from the front nodal point, the image distance from the rear nodal point. * In practice, rotation of the optic about its rear nodal point changes the part of the subject imaged, but does not change the position of the image: it therefore defines the axis of lens rotation for panoramic photography.

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