Aspects of Camera Lenses: Principal Axis
One feature common to all lenses, including camera lenses, is the principal axis. On a simple lens, say a curved piece of glass, the principal axis is an imaginary line drawn directly through the center of the lens. If the lens was standing up on a table, the principal axis would be parallel to the table.
Camera lenses are made of multiple simple lenses, or lens elements, but they also have a principal axis. It too is drawn directly through the center of the lens. Taking the lens as a cylinder, the principle axis goes right through the middle, parallel to the sides of the lens.
Importance of the Principle Axis
The focal point of a lens is where the light entering a lens will converge. This is located on the principle axis. Ideally, a principle axis should go through the middle of the lens and strike the film or digital sensor direct in its center.