| 摘要 |
PURPOSE:To make a focus adjustment through a converter by composing the converter of a positive and a negative lens in order from an object side, obtaining negative power as the whole system, and satisfying specific conditions. CONSTITUTION:The converter consists of the positive and negative lenses in order from the object side, has the negative power on the whole, and satisfies the conditions shown by inequalities (1)-(3). In the inequalities, (m) is the power of the rear converter, r2 the radius of curvature of the 2nd surface of the positive lens, r3 the radius of curvature of the 1st surface of the negative lens, f1 the focal length of the positive of the positive lens, and (f) the focal length of the rear converter. Further, the whole system of the rear converter is shortened toward an image plane to vary the focus from infinite distance to short distance. Consequently, the converter which has low power and enables the focus adjustment is obtained and the number of lens elements is decreased to reduce the cost.
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