| abstract |
An optical system capable of easily and accurately adjusting a lens position even when a lens is further arranged between two lenses provided with a position reference mark. An optical system according to the present invention includes a first mark lens L2 and a second mark lens L5 having position reference marks M1 and M2 provided on a surface of the lens, a first mark lens L2, and a second mark lens L2. An intermediate lens group LG2 including at least one lens disposed between the mark lens L5 and the mark lens L5. The intermediate lens group LG2 satisfies the following conditional expression. 0.1 <fm / f <2 Here, fm is the focal length of the intermediate lens group, and f is the focal length of the entire optical system. [Selection] Figure 3 |