The Gauss beam describes the shape of the incoming light at pixel level
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Gauss-points
4 points on the main optical axis of lenses:
- first and second focal point F1,F2
- first and second principal point P1,P2
Knowledge about the position of these 4 points is enough to calculate the position of an image point from the position of an object point.
That holds for single lens elements, but also for complex lens systems.