| 摘要 |
PURPOSE:To reduce an inward comatic aberration generated when an off-axis light beam passes the periphery of a 2nd positive lens group by generating an outward comatic aberration by using a negative meniscus lens which is concave to the object field side in a 1st group. CONSTITUTION:This lens consists of the 1st group r1-r4 composed of a negative meniscus lens which is concave to the object field side, a 2nd groups r5-r8 of four or five-element constitution wherein the lens closest to the object field side is a positive meniscus lens concave to the object field side, and a 3rd group r9-r12 which is a low-power meniscus lens concave to the object field side. Consequently, even if this lens is a telecentric ftheta lens, the spot diameter is reduced by decreasing the F value and a spot can be scanned in proportion to the angle of incidence of luminous flux over a wide scanning angle range. |
