Technical Terms

Principle Axis:
The principle axis of a lens is the line joining the centres of of curvature of its surfaces.
Principle focus, F:
The principle focus of a lens is the point on the priciple axis to which all rays originally parallel and close to the axis converge, or from which they diverge, after passing through the lens.
Focal length, f:
The focal length of a lens is the distance between the optical centre an the principle focus.