| 摘要 |
PURPOSE:To correct variation in aberrations with good balance by constituting a rear converter lens which has negative refracting power and vary the focal length of the whole system by using 1st-4th lenses, and satisfying specific conditions regarding some of air gaps, the mean refractive index of material of positive and negative lenses, and the focal length of the whole system. CONSTITUTION:The rear converter lens which has the negative refracting power and varies the focal length of the whole system consists of the 1st lens group with a cemented lens surface having its convex surface on an object side, the 2nd positive meniscus lens having its convex surface on the image plane side, the 3rd negative lens group having convex surfaces as both lens surfaces, and the 4th negative lens which has a larger-curvature refracting surface on the object side than on the image plane side. The inequalities hold for the air gap D3 between the 1st and 3rd lens groups, the means refractive indexes N'+ and N'- of the materials of the positive and negative lenses constituting the whole constitution, and the focal length F of the whole system. Consequently, a specific power variation rate is obtained and aberrational variations at the time of mounting, specially, variations in spherical aberration and astigmatism are corrected excellently.
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