| 摘要 |
PROBLEM TO BE SOLVED: To provide a finite zoom lens having a zoom ratio being about 1.8 as a projection lens for magnifying and projecting a DMD and having extremely high performance all over the zoom area and all over the projection screen while having a very long back focus (about three or more times of the focal distance). SOLUTION: This zoom lens is composed of a 1st group, a 2nd group and a 3rd group in this order from the screen side. In the case of varying power from a wide side to a telephoto side, the 1st lens group is non-linearly moved on an optical axis to the DMD side once and then moved to a home position. The 2nd lens group is non-linearly moved to the DMD side and the 3rd lens group is linearly moved to the screen side. The focal distances F1, F2 and F3 of the 1st, the 2nd and the 3rd lens groups and a synthetic focal distance (f) satisfy a specified condition. Then, anomalous dispersion glass is used for two or more concave lenses of the 2nd lens group and the convex lens of the rear group of the 3rd lens group, and further low dispersion glass is used for the positive lens of the front group of the 3rd lens group.
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