| 摘要 |
PURPOSE:To reduce the size of the whole system body and to obtain the excellent optical performance of a semitelephoto type which is about 25-30 deg. in photographic field angle when a converter lens is mounted by employing a rear converter system and specifying the lens constitution. CONSTITUTION:The lens surface of the 1st lens on an image plane side is made convex and the lens surface of the 2nd lens on an object side is brought to negative refracting power to set the shapes of both lenses properly, thereby compensating the spherical aberration. A converter lens is composed of three lenses having a positive, a negative, and a positive refractive index and an increase in Petzval sum is reduced as much as possible to prevent curvature of field from increasing. Meniscus lenses which have convex surfaces on the image plane side are used as the 1st and the 2nd lenses to reduce the generation of the comatic aberration of the whole image plane. The negative distortion when the converter lens is mounted is compensated by making the image-side lens surface of the 3rd lens convex.
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