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A compound lens (a lens made of multiple glass elements, like basically any camera lens) has two nodal points. The front nodal point is the point in the optical system in which all the rays of light which enter the lens converge. The rear nodal point is the point in the optical system from which all the rays of light leaving the lens seem to radiate. That is, it’s the point on the optical axis from which the emergent ray leaves. The focal length of the lens is measured from the rear nodal point. The rear nodal point is also important for photographers who want to take VR panorama images, since the lens must be rotated around the rear nodal point to avoid parallax error. Simple lenses made of one piece of glass, such as magnifying glasses, have one nodal point - or the front and rear points are the same, depending on your point of view.