Rear-focus conversion device lens for telephoto lens

A rear-focus conversion lens device which is interposed between an objective lens and a camera body, provides with said objective lens a synthetic optical system having a synthetic focal length that exceeds a focal length of said objective lens, and has a focusing function, comprising:

• a converter barrel detachably mounted between said objective lens and said camera body;
• a lens system having a negative refracting power and movable within said converter barrel along an optical axis;
• and
• moving means for moving said lens system along the optical axis,
• wherein said rear-focus lens device is focused on an object at infinity to an object at a close distance by moving said lens system along the optical axis, and the following relations are satisfied:
• 1.2<.beta.<2.5 . . . (1)
• 0.05<.vertline..DELTA.B.sub.f /f.sub.R .vertline.<0.35 . . . (2)
• 0.4<.vertline.B.sub.f /do..beta..vertline.<0.8 . . . (3)
• 0.41<.SIGMA.d/MB<1.5 . . . (4)
• where .beta. is an enlarging magnification of a focal length in an infinity in-focus state, .DELTA.B.sub.f is an amount of change in a synthetic back-focal distance when said synthetic optical system is focused from an object at infinity to an object at a close distance, f.sub.R is a focal length of said rear-focus lens device, do is a distance between a vertex of a lens surface of said rear-focus conversion lens device which is closest to the object and an image point formed by said objective lens, MB is a distance between a mounting surface of said camera body on which said objective lens is mounted and a film surface, and .SIGMA.d is a distance between a vertex of a frontmost lens surface of said lens system and a vertex of a rearmost lens surface thereof.