| 摘要 |
The improved eyepiece variable focal length optics comprises, in order from objective optics with which it is used in combination, a negative first lens group, a positive second lens group and a positive third lens group, the first and second lens groups moving in opposite directions along the optical axis during variable focal length but the third lens group being stationary, and the eyepiece variable focal length optics satisfies the following conditions (1) and (2): <paragraph lvl="0"><in-line-formula>2.0<f3/f2<4.0 (1) </in-line-formula><paragraph lvl="0"><in-line-formula>7.0<L12/f(h) (2) </in-line-formula>where f2 is the focal length of the second lens group, f3 is the focal length of the third lens group, f(h) is the shortest focal length of the overall eyepiece variable focal length optics, and L12 is the axial air gap between the first and second lens groups at the shortest focal length. The eyepiece variable focal length optics achieves a variable power ratio of about three and yet is compact and allows for adequately large apparent visual field and eye relief.
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