| 摘要 |
PURPOSE:To obtain a lens system where no restrictive conditions are imposed on the back focus, by satisfying prescribed conditions in an optical system provided with four, namely, positive, negative, positive, and negative lens groups arranged in order from the object side. CONSTITUTION:In case of varying power from the shortest focal length end (S end) to the longest focal length end (L end), the first positive lens group I and the forth negative lens group IV are moved from the image side to the object side, and the second negative lens group II or the third positive lens group IIIis moved at least during varying power, and as the result, the air gap between the first lens group I and the second lens group II is extended, and that between the third lens group III and the fourth lens group IV is reduced, and the fourth negative lens group IV is moved for focusing, and conditions of a formula are satisfied. In the formula, betaL4 and betaS4 are laterial variable powers of the fourth negative lens group IV in the L end and the S end respectively, and fS and fL are focal length of the whole of the system in the S end and the L end respectively. |
