# Talk:Lagrange invariant

I'm a bit confused what the terms mean. If I am at a point (z,x) on the edge of the image surface, for example, I assume the chief-ray height is x. Is the marginal-ray height there also x (since they come into focus on the image)? Is the chief-ray angle then the angle between the chief ray and the optical axis? Is the marginal-ray angle the angle with the optical axis or with the chief ray? Also, I'm inclined to see ${\displaystyle nu{\overline {y}}}$ as the chief-ray height times the NA since ${\displaystyle nu\approx n\sin u}$. Is that the right intuition? —Ben FrantzDale (talk) 14:54, 13 December 2011 (UTC)