| 主权项 |
1. An objective lens comprising
a first lens group in which a lens having negative power is arranged nearest to the object side, the first lens group having negative power and being immovable in changing magnification from a normal observation state to a close-up observation state, where the normal observation state is a state in which magnification of the objective lens has the minimum value and the close-up observation state is a state in which magnification of the objective lens has the maximum value, a second lens group which is arranged nearer to the image side than the first lens group is, the second lens group having negative power and moving along the optical axis in the change of magnification from the normal observation state to the close-up observation state, a third lens group which is arranged nearer to the image side than the second lens group is, the third lens group having positive power and being immovable in the change of magnification from the normal observation state to the close-up observation state, and an aperture stop which is arranged between the second lens group and an image plane, the aperture stop being immovable in the change of magnification from the normal observation state to the close-up observation state,the objective lens including no lens group other than the first, second, and third lens groups, and the objective lens satisfying the following condition:
1.24<|F1f/Fn|<1.8where F1f denotes a focal length of the lens that is placed nearest to the object side in the first lens group and Fn denotes a focal length of the whole system in the normal observation state. |
