Single Focus Lens System Compact Size Long Back Focal Length

The lens system is divided into multiple lens groups (first lens group with negative refractive power, second lens group with positive refractive power) that can move independently. This segmentation allows the back focal length to be extended through group separation while keeping individual group sizes compact, resolving the contradiction between long back focal length and compact overall size.

The patent employs a focusing mechanism where the first and second lens groups move along the optical axis in opposite directions. This dynamic adjustment enables the system to maintain a long back focal length during focusing operations while keeping the lens barrel compact, as the moving groups optimize the optical path rather than requiring a physically longer structure.

Different lens groups are assigned specific optical functions: the first lens group with negative refractive power handles wide-angle light gathering, while the second lens group with positive refractive power provides focusing and image formation. This local specialization allows each group to be optimized for its specific function, achieving high overall optical performance without requiring every element to be complex.

The patent optimizes specific parameters including the refractive powers of the lens groups, their separation distances, and the focal length ratios (|F1/F2| between 0.3 and 0.7). By carefully controlling these parameters, the system achieves large aperture and wide viewing angle with improved optical performance while avoiding unnecessary complexity through systematic parameter optimization rather than arbitrary design.