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| Optical axis | A straight line connecting the center points of the spherical surfaces on each side of a lens. In other words, the optical axis is a hypothetical center line connecting the center of curvature of each lens surface. In photographic lenses comprised of several lens elements, it is of utmost importance for the optical axis of each lens element to be perfectly aligned with the optical axes of all other lens elements. Particularly in zoom lenses, which are constructed of several lens groups that move in a complex manner, extremely precise construction is necessary to maintain proper optical axis alignment. |
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| Overall linear extension | The entire lens optical system moves straight backward and forward when focusing is carried out. Representative examples of lenses using this type of focusing include the EF 50mm f/1.8 II and TS-E 90mm f/2.8. |
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