| 摘要 |
PURPOSE:To obtain a compact long focus lens of large-aperture ratio which has the superior performance of image formation, by making a lens system consisting of three lens groups, i.e., a lens group having positive refractive power, a meniscus lens group and a lens group having positive refractive power in order from an object, and satisfying prescribed conditions. CONSTITUTION:In a lens system consisting of the 1st lens group G1 having positive refractive index, the 2nd lens group G2 of a meniscus lens facing its convex surface to the image side and the 3rd lens group G3 having the positive refractive power, 0.45phi<phiG1<0.6phi is satisfied when the refractive power of the whole system is phi and the refractive force of G1 is phiG1, and 0.26<(DELTA12-DELTA3)/ DELTA12<0.4 is satisfied when focusing is attained by relatively moving the united G1 and G2 to G3 and the quantity of movement of the united G1, G2 and that of the G3 are difined as DELTA12 and DELTA3, respectively. Consequently a focus lens having long large-aperture ratio which has the superior performance of image formation can be obtained. |
