Talk:Nodal Point

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Revision as of 07:08, 15 May 2005

I think we should be a bit less equivocal in the perennial debate about "front nodal point" versus "entrance pupil". I had always favoured the front nodal point as the centre of perspective and centre of rotation, but have recently changed my mind.

As David Jacobson's Lens Tutorial says, "Nodal points are the two points such that a light ray entering the front of the lens and headed straight toward the front nodal point will emerge going straight away from the rear nodal point at exactly the same angle to the lens's axis as the entering ray had. Certainly one ray of the pencil of rays coming from an object will behave in this way, but this is not necessarily the ray which is heading for the centre of perspective.

On the other hand, all rays which form the image must pass through the aperture stop, which will approximate a point when the lens is fully stopped down. Since the entrance pupil is the front image of the aperture stop, it follows that the centre of perspective is the entrance pupil.

The authoratative view of the late great Rudolf Kingslake that "the entrance pupil of the lens is the center of perspective" is quoted by John Bercovitz in an email archive. Dr Kingslake confirmed this in a personal letter to John Bercovitz, and suggested a practical experiment which confirmed the theory. You can read the relevant passage in "Optics in Photography" using Amazon's "Look Inside!" feature at Amazon

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