| 摘要 |
PURPOSE: To obtain a bifocal optical device suitable for an image pickup device for day time and night time by providing a diaphragm integrally formed into one body as a part of movable holder and for keeping the brightness in a night time image pickup mode in the case of switching to short focal distance, and positioning the diaphragm in the vicinity of a front lens group. CONSTITUTION: This bifocal optical device in a long focal distance LF is constituted of an objective lens having a fixed lens constituted of the front lens group FGR centering on an optical axis XX' and a back lens group BGR. The front lens group having focal distance LF is provided with three lenses L1, L2 and L3, and the back lens group BGR similarly is provided with a lens 4 having magnificationγ. This optical device becomes an optimum condition by making an incident pupil positioned in the vicinity of a front side pupil on a long focal distance position. When the optical device is used as the image pickup device for the day time and the night time, the diaphragm capable of being controlled is arranged on a front position so that the brightness of the day time image pickup mode with respect to the night time image pickup mode can be adjusted.
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