| 摘要 |
PURPOSE:To enable an optical system to be adjusted finely and varied and to improve the accuracy of power setting by determining the focal lengths f1, f2 and f3 of 1st, 2nd, and 3rd group lenses of a variable power telecentric image formation optical system so that a specific expression is satisfied. CONSTITUTION:The 2nd group lens is arranged movably along the optical axis of the telecentric image formation optical system constituted so that the rear-side focal position of the 1st group lens and the front-side focal position of the 3rd group lens meet each other. Then the inequality is satisfied for the focal lengths f1, f2, and f3 of the 1st, 2nd and 3rd group lenses. Consequently, when the movement of the 2nd group lens is small, a shift in the composite focal position of the 1st and 2nd group lenses is small and the 3rd lens can be fixed. Further, when the image formation power is largely changed, the 3rd group lens can be moved to the composite focal position of the 1st and 2nd group lenses. Consequently, the power can be set with high accuracy.
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