| 摘要 |
PURPOSE:To obtain a wide angle lens with long back focus at about 75 deg. field angle and about 1.4-2 aperture ratio by constituting the wide angle lens of a 1st group lens consisting of positive, negative and negative lenses, a 2nd group lens having weak refraction force, a 3rd group lens consisting of negative and positive lenses, and a 4th group lens consisting of a positive lens and satisfying these lenses with specific conditions. CONSTITUTION:The wide angle lens is constituted of four groups, the 1st group consisting of a positive lens turning its convex face to the object aide and two negative meniscus lenes turning their convex faces to the object side, the 2nd group having weak refraction force as a whole, the 3rd group consisting of a negative lens and a positive lens, and the 4th group consisting of at least one positive lens and turning its image sided face are projected to the image side so that these lens group are successively arranged from the object side. Thus, the generation of aberration due to strong asymmetical arrangement obtained by arranging the strong negative lens group on the object side and arranging the lens group having weak refraction force between positive lens groups can be reduced by arranging the positive lens in the storing negative lens group on the object side and various abberations can be effectively corrected by satisfying the inequalities I-V. Where (f) is the focal distance of the whole system, f1 is the focal distance of the 1st group, f2 is the focal distance of the 2nd group, f12 is the composite focal distance of the 1st and 2nd groups, f234 is the composition focal distance of the 2nd, 3rd and 4th groups, N3p is the refractive index of the positive lens in the 3rd group, and N4p is the refractive index of either one of positive lenses in the 4th group.
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