Camera lens why so many lenses?
In theory, the lens can only be composed of a single lens, but due to optical reasons, the imaging of a single lens inevitably exists chromatic aberration, spherical aberration, coma aberration, astigmatism and so on some of the astigmatism problem, especially in the edge part, imaging is poor
The combination of multiple lenses can correct aberration to some extent. Second, if the lens zoom requires multiple lenses to change the spacing to achieve.
And zoom, the aberration will change, so the zoom lens generally need more lenses to complete the zoom and different focal length aberration correction. Again, many of the lens focus is achieved through the lens position change.