Aspects of Camera Lenses: Secondary Axis
Camera lenses are made out of several pieces of glass and follow the complex science of optics. The glass in the lenses are made into perfect circles with the centers slightly wider than the edges. An imaginary line passing directly through the center at a perpendicular angle is called the principal access because it is a straight line that cuts the lens in half.
Any other invisible line that passes through the center at an angle different from the principal axis is called the secondary axis. To be considered as the secondary axis, the line must pass through the center of the lens and not through the curvature.
How Lenses Are Made
Manufacturing lenses is a complex process. Several pieces of glass, all roughly the same size, are cut out of a large glass block. They are then grounded down and polished into the correct size and curvature. There is a lot of accuracy and skill needed when manufacturing the glass and assembling the lenses, because all of the principal axis' in the lens need to line up perfectly. Otherwise, the lens will be unable to focus. Recently, plastic has been used in some lenses as a substitute for glass.