| 摘要 |
PROBLEM TO BE SOLVED: To obtain a wide angle lens system suitable for an electronic still camera whose f-number is about 2.8, which shows excellent performance to a half viewing angle being≥30°at a short focal distance end and also whose back focus is much longer than a focal distance. SOLUTION: In a retrofocus type optical system consisting of a 1st lens group on an object side having negative power and a 2nd lens group on an image side having positive power with a boundary at a position where space is maximum, the wide angle lens system satisfies conditional expressions (1) to (4), that is, (1) 2.0<f<SB>B</SB>/f<2.5, (2) 4.5<¾f<SB>1</SB>/f¾<6.2 (3) 1.8<f<SB>2</SB>/f<2.0 (4) 1.0<d/f<1.5. Provided that f<SB>B</SB>shows the back focus, (f) shows the focal distance of an entire system, f<SB>1</SB>shows the focal distance of the 1st lens group, f<SB>2</SB>shows the focal distance of the 2nd lens group, and (d) shows axial space between the 1st and the 2nd lens groups. COPYRIGHT: (C)2004,JPO&NCIPI
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