F(.theta.) Lens system of four-group construction

An f.theta. lens system having an entrance pupil at a remote position on the incident light side of the lens system and having negative distortion such that the image height H of a parallel light beam incident at an angle .theta. with respect to the optical axis with said entrance pupil as the center is H=f.theta., where f is the focal length of the entire system, said lens system including, in succession from the entrance pupil side:

• a first group having at least one negative lens and having a composite negative refractive power;
• a second and a third group each having a meniscus lens component having its concave surface facing said entrance pupil side;
• and
• a fourth group having at least one positive lens and having a composite positive refractive power;
• and wherein the lens system satisfies the following conditions:
• 0.08f<R.sub.III <0.35f
• .vertline.R.sub.III .vertline.<.vertline.R.sub.II .vertline.
• f<f.sub.II
• f<f.sub.III
• 0.8<R.sub.4 /R.sub.3 <1.2
• 0.7<R.sub.6 /R.sub.5 <1.5
• where R.sub.3 and R.sub.4 represent the radii of curvature of the opposite lens surfaces of said second group from said entrance pupil side, R.sub.5 and R.sub.6 represent the radii of curvature of the opposite lens surfaces of said third group from said entrance pupil side, R.sub.II =(R.sub.3+ R.sub.4)/2, R.sub.III =(R.sub.5+ R.sub.6)/2, and f.sub.II and f.sub.III represent the focal lengths of said second group and said third group, respectively.