principal point
/ˌprɪnsɪpl ˈpɔɪnt/noun
1 Perspective. The point where the principal ray meets the picture plane.
2 Optics. Each of the two points at which the optic axis of a lens intersects the principal planes.
- In the case where the medium is the same on both sides of the lens, these points coincide with the nodal points, which have the property that the straight line between the first of these and any point of the object is parallel to that between the second and the corresponding point of the image..
Origin
Late 16th century (in an earlier sense). From principal + point. In sense 1 after Middle French principal poinct.
- knocker
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- knock someone into the middle of next week