| abstract |
[PROBLEMS] To solve the problem of adhering dust that has not been affected by other specifications and productivity, and that has recently become a problem in mass production processes, while achieving downsizing and high performance. A focusing optical system is provided. A single focus optical system A1 of the present invention forms an optical image of an object on a light receiving surface of an imaging element 16 having a diagonal length 2Y ′, and a first positive lens 111 having a convex object side surface in order from the object side. Inflection point with first group 11 consisting of second negative lens 112 including an aspheric surface and concave image side, second group 12 consisting of third lens 121 having an aspheric surface and fourth positive lens 131 having convex surface on the image side The image side surface having an aspheric surface having a concave surface and the fifth negative lens 132 is constituted by a third group 13, and the first and third groups 11 and 13 are fixed with respect to the image plane. A single-focus optical system A1 which is in focus and has an optical aperture 14 on the object side of the first lens 111, and 20 <| Δv1 | <70, 0.1 <T23 / TL <0.4 and 0.541 < Satisfy Y ′ / TL <0.9. [Selection] Figure 1 |