Patent ID: 8369030

Claim:
A fixed focal length image forming lens comprising: an aperture stop; a first lens group arranged on an object side relative to the aperture stop; and a second lens group having a positive power arranged on an image side relative to the aperture stop, the first lens group comprising a first F lens group having a negative power and a first R lens group having a positive power arranged in this sequence from the object side with a widest air space within the first lens group between the first F lens group and the first R lens group, the first F lens group comprising at least two negative lenses, the first R lens group comprising at least one positive lens, the second lens group comprising a second F lens group having a positive power and a second R lens group arranged in this sequence from the object side, the second F lens group comprising a first positive lens, a first negative lens, a second negative lens, and a second positive lens arranged in this sequence from the object side, wherein the second R lens group comprises one aspheric surface having a form of decreasing positive power from an optical axis of the second R lens group toward a periphery thereof, a focal length f 2 F of the second F lens group and a focal length f 2 R of the second R lens group satisfy the following relationship (1): 0.4 <f 2 F/f 2 R< 0.6 (1), and the aspheric surface of the second R lens group satisfies the following relationship (2): 0.3 <D 1/ D 2<0.5 (2), where D 2 represents a sag amount of the aspheric surface and a surface formed by a paraxial curvature radius at a position of an effective height of H2R of beams of light of the aspheric surface and D 1 represents a sag amount of the aspheric surface and a surface formed by a paraxial curvature radius at a position of 0.8×H2R.