| 摘要 |
PROBLEM TO BE SOLVED: To form an observing optical system capable of suppressing the generation of flare, to improve parts working accuracy and to improve insertion by forming a real iris on the image side from the object side face of a specific visual field conversion prism and determining an aperture. SOLUTION: The optical system consists of a 1st group G1 having a concave lens L1 and negative power and a 2nd group L2 constituted of convex lens components L2 to L4, visual field conversion prisms P1, P2 arranged between the 1st and 2nd groups, G1, G2 satisfy an inequality 1.3×D<L3.2×D and at least one aperture iris S is arranged on the image side from the most object side incident face of the prisms P1, P2. In the inequality, L is the length of the longest part in the insertion direction of the prisms P1, P2 and D is the outer diameter of the maximum lens in the 2nd group G2. Since a beam passed through the part A of a prism face R2 or a beam reflected from the part C of the face R2 is totally reflected and the length L of a face R3 parallel with the inserting direction is long, a beam having a large aperture is not reflected by the face R3.
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