| 摘要 |
PURPOSE:To obtain a zoom lens which is short in a back-focal distance, compact, and also satisfactory in various aberrations, by satisfying prescribed conditions, in a lens system constituted of three groups of a positive lens, a negative lens and a positive lens in order from an object side. CONSTITUTION:In a zoom lens constituted of three groups ( I -III) of a positive lens, a negative lens and a positive lens in order from an object side, the para- axial magnification is always maintained correctly while magnification and correction of movement of an image face following it are executed by monotonously decreasing an interval between the first group and the second group as a composite focal distance of the whole system is varied to the telephoto end from the wide angle end, and simultaneously, monotonously increasing an interval between the second group and the third group. Also, the following conditions are satisfied. 0.6<-f1/f2<2.0, 0.2<f1/f3<0.7 (provided that, fi is a focal distance of the i-th group). By this condition, a sufficient variable magnification ratio is obtained and also aberration is corrected easily. In this way, it is possible to obtain a zoom lens which is short in back-focal distance, compact, and also satisfactory in various aberration. |
