| 摘要 |
PROBLEM TO BE SOLVED: To provide a telephoto lens with improved brightness and satisfactorily corrected aberrations in lens constitution having comparatively long back focus though it is compact. SOLUTION: The telephoto lens is equipped with: a diaphragm; a first lens whose both surfaces are made spherical, whose surface on an object side is made convex and which has positive refractive power; a second lens whose both surfaces are made aspherical, whose surface on the object side is made concave and which has positive refractive power; and a third lens whose both surfaces are made aspherical, whose surface on the object side is made concave and which has negative refractive power from the object side, and is constituted to satisfy following conditional expressions (1) to (5): (1) BF/2Y>1.5; (2) 0.5<f1/f<1; (3) 0.7<f2/f<3; (4) -1.5<f3/f<-0.5; and (5) tanθ<0.45; wherein f is a paraxial focal distance of an entire lens system;θis a maximum incident angle of a principal ray; f1 is a focal distance of the first lens; f2 is a focal distance of the second lens; f3 is a focal distance of the third lens; 2Y is a length of the diagonal of an effective image plane; and BF is a distance to an image side focus from the final surface of the third lens. COPYRIGHT: (C)2007,JPO&INPIT
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