An optical system is provided. The optical system includes a first optical module with a first light-entering hole, a second optical module with a second light-entering hole, and a third optical module with a third light-entering hole. The second light-entering hole is close to the first light-entering hole and the third light-entering hole. The focal length of the second optical module is different from the focal length of the first optical module and the focal length of the third optical module.