What is the Axis and Principle Focus?

-The Axis is a Line that Passes through the Middle of the Lens -This line will always stay Straight, Convex or Concave -The Principle Focus is a Spot where either the Rays that are Parallel appear to Come From, Simply just Trace it Back to a Spot [Concave] or when the Rays hitting the Lens Converge, hitting a Specific Spot