ABSTRACT
Lenses are distinguished mainly by their focal length,which is measured from the rear nodal point (or principal plane H). Focal length is an optical characteristic built into the lens. You can change it only by adding or removing optical components, such as a teleconverter. Focal length is unchanged when you add extension tubes or bellows, switch a lens from one camera to another, or use a lens for different film formats. On zoom lenses, you can change focal length within a certain range by moving some of the optical components inside the lens.
