| 摘要 |
PURPOSE:To prevent the degradation in visibleness of a finder system due to a ghost image caused by mirror reflection, by introducing an aspherical face to the third convex lens and limiting a ratio of focal length of the second convex lens to the third convex lens, a ratio of the air converted length between a mirror face and a field frame to the focal length of the third convex lens, and the value of the aspherical face shape. CONSTITUTION:An optical system consists of a concave lens having a high curvature in the pupil side, a convex meniscus lens whose concave is directed to the pupil side, and a convex lens in order from the object side, and the concave of the second convex lens is formed to a mirror for the field frame, and the field frame is provided on a plane plate arranged on the third convex lens or between the second convex lens and the third convex lens, and an aspherical face is introduced to the third convex lens, and a formula I is satisfied. As aspherical face shape DELTAZi of the i-th face is expressed with a formula II where Ci, K, Ai, and Pi(Pi>=2.0) are the curvature at the apex, the constant of cone, the coefficient of the aspherical face, and the power number of the aspherical face respectively in the rectangular coordinate system with the apex of the face as the origin and the optical axis as the X axis. The aspherical face is the object-side face in case of i=1 but is the pupil-side face in case of i=2, and at least one face is formed to the aspherical face, and DELTAZi is zero for the spherical face. |
