| abstract |
The present invention provides a single focus optical system that can reduce the size and increase the performance of the optical system, and that does not affect other specifications and productivity, and that can solve the problem of adhering dust that has become a problem in recent mass production processes. A single-focus optical system forms an optical image of an object on a light receiving surface of an imaging element 16 having a diagonal length of 2Y ′, and in order from the object side, a first positive lens 111 having a convex object side surface and an aspheric surface. The first group 11 including the second negative lens 112 having a concave image side surface, the second group 12 including the third lens 121 having an aspheric surface, and the fourth positive lens 131 having an inflection point on the image side. An image side surface having a spherical surface is constituted by a third group 13 including a fifth negative lens 132 having a concave surface. The first and third groups 11 and 13 are fixed with respect to the image plane, and are focused by the second group 12. A single focus optical system having an optical stop 14 on the object side of the first lens 111, and satisfies 20 <| Δv1 | <70 and 0.541 <Y ′ / TL <0.9. [Selection] Figure 1 |