| 摘要 |
A variable objective is disclosed with finite transmission length L from the object plane 0 to the image plane 0', comprising three axially displaceable lens groups, wherein, successively in the imaging direction, a first lens group has positive refractive power, a second lens group has negative refractive power and the third lens group has positive refractive power. In a variable objective of the type mentioned above, the first lens group and third lens group are displaceable jointly while maintaining the same relative distance L2, wherein the distance L1+D1actual measured between the first lens group and the object plane 0 changes by the amount D1actual=0 . . . D1max, while the distance D2actual between the second lens group and the object plane 0 is subjected to a positively-guided nonlinear change. The movement of the second lens group is preferably carried out approximately according to the following relationship:where C1, C2 and C3 are command variables characterizing the nonlinearity. A mechanical compensation of the focus position is achieved in this way.
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