So if a lens's focal length is defined by its distance to the camera's sensor, does that mean all lenses are identical and that the only difference is the length of the cylinder it is built into? Of course I know this can't be true and that there are more complications than that. But what are those complications? My guess would be that the curvature of each lens has to be adjusted depending on the lens's focal length. But if that's true, than that would mean that a 50mm lens adjusted so that it is 28mm away from the sensor does not make it a 28mm lens--which then diminishes the definition of focal length. Can anyone help me with this confusion? If you can extend this discussion to prime lenses and zoom lenses that would be the cherry on top.