| 摘要 |
PURPOSE:To miniaturize the whole of a lens system and relax the assembling precision and reduce the variation of aberrations due to variable power by setting the focal length of the second lens group so that specific conditions are satisfied. CONSTITUTION:A converter lens C which is set on the image surface side of a main lens system M to change the focal length of the whole of the lens system is provided with the first lens group 1 where a negative lens or a positive lens or a positive lens and a negative lens are stuck to each other in order from the object side and the second lens group 2 consisting of a single negative lens. These lenses are constituted to satisfy conditions of inequalities (1)-(3) where f2 is the focal length of the second lens group 1 and DELTAn12 is the difference of refractive index of glass between the negative lens and the positive lens in the first lens group and r2 is the radius of curvature of the sticking lens surface in the first lens group and (f) is the focal length of the system of the converter lens G. Though this converter lens has a simple lens constitution consisting of three lenses of two groups, the assembling precision is relatively loose and a prescribed variable power ratio is easily obtained and variation of aberrations due to variable power is well compensated.
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