| 摘要 |
PROBLEM TO BE SOLVED: To obtain the endoscope objective which has a large diameter, small size, and good optical performance while holding the number of lens elements small by composing the lens of a lightness stop and three lens groups, and meeting specific requirements. SOLUTION: The lens consists of a 1st negative single lens 10, the lightness stop S, a 2nd lens group 20, and a 3rd lens group 30 in order from the object side. The 2nd lens group 20 is a single lens and the 3rd lens group 30 is a cemented lens of a positive and a negative lens. Then, -7.0<SF<-1.5 and 0.05<|f/f1|<0.3 (f1 <0). Here, SF is a shape factor of the 1st single lens 10 and SF=(r2 +r1 )/(r2 -r1 ), where Pi is the radius of curvature of the object-side surface of the 1st single lens 10, r2 the radius of curvature of the image-side surface of the 1st single lens 10, (f) the focal length of the whole lens system, and f1 the focal length of the 1st single lens 10.
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