| 摘要 |
PURPOSE:To prevent deterioration in focus detection accuracy by providing a focus detection cylinder to part of a lens barrel for power variation and moving it in one body when the power is varied, and varying the focus detection cylinder through a link member as a lens barrel for focusing is moved. CONSTITUTION:When a subject is at infinite distance, a spot image is formed at the boundary between photodetecting elements 2a and 2b regardless whether a photodetection system is moved or fixed during power variation, so there is no troubles in focusing detection. When the photodetection system is fixed during power variation, the spot image from the subject moves toward the photodetecting element 2b and an error in range measurement occurs in a front focus state. This tendency increases more and more as the subject distance is shorter and shorter. When, however, the photodetection system is moved in the same direction by the same movement extent with a focusing lens 3 which is moved during power variation, the spot image is formed at the boundary between the photodetecting elements 2a and 2b at any time, so that image is focused on a subject position PD at any time. Consequently, a zoom lens for automatic focus detection which provides high-accuracy detection at any subject distance, specially, in microphotography is obtained. |
