| 摘要 |
In an endoscope having variable magnification, which can be set between a wide-angle position of low magnification and a telescope position of high magnification, an objective is provided with a diverging front lens and a converging rear lens. A diaphragm is provided between the two near the focal plane of the converging lens. In order to set a high magnification, the distance between the objective and a transmitting optical system including an eyepiece is increased and the diverging lens of the objective is brought into contact with the object to be viewed. The diverging front lens makes a relatively small contribution to the aberrations in the position with high magnification. In the case of low magnification, the diverging front lens cooperates with the converging lens and forms a wide-angle objective. Stabilisation of the pupil is achieved by selecting the position of the diaphragm in such a way that the diaphragm is virtually ineffective in the position of high magnification. The maximum numerical aperture is thereby determined by the diameter of the converging lens. However, the diaphragm becomes effective in the position of low magnification. A field lens situated in the vicinity of the image plane in this case receives the edge beams of the light bundle at an angle which is virtually constant over the entire range of magnification. As a result, it is possible to accomplish transmission of images by conventional means, for example using alternating field lenses and intermediate lenses.
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