Wide angle photographic lens

#The invention relates to a photographic wide angle lens of a small, light weight camera with very long back focus, which may also be used as a single-lens reflex camera with a revolving mirror.

#Examples of wide angle photographic lenses for single-lens reflex camera are disclosed in Japanese Laid-Open Patent Publication No. 5-45581. However, all of these lenses have a large volume, and there are few lenses having a field angle of about 100.degree. with an attachment size smaller than four times that of the focal length. With regard to lenses having field angles equal to or larger than 110.degree., the attachment size is in the range of 4.5 to 5 times the focal length.

#With the conventional wide angle photographic lenses described above, the attachment size becomes very large when the field angle is sufficiently wide. Conversely, the field angle is not necessarily very wide when the attachment size is small.

#FIG. 1 is a schematic side view of the structure of the lens of the first embodiment of the invention;

#FIGS. 2A-2C and 2D-2F are graphs that show spherical aberration, astigmatism and distortion, respectively, at infinite shooting state and at about 1/15 magnification shooting state in accordance with the first embodiment;

#FIGS. 3A-3C and 3D-3F are graphs that show spherical aberration, astigmatism and distortion, respectively, at infinite shooting state and at about 1/15 magnification shooting state in accordance with a second embodiment;

#In a wide angle lens, typically, a front lens group having a negative refractive power is arranged on the object side, a rear lens group having a positive refractive power is arranged on the image side of the lens system, and an iris diaphragm is normally positioned in the rear group. With the above structure, the back focus is longer than the focal length, the Petzval sum is negative, and negative distortion and internal coma occur. These are mostly due to the effect of the front group having the negative refractive power. Thus, the larger the refractive power of the front group, the more noticeable such characteristics become. Provided that the refractive power of the front group is not too large and the back focus and the field angle are relatively small, these characteristics may function as an advantage in designing the lens. However, if the field angle exceeds 100.degree., and a back focus of more than twice the focal length is required, the relative refractive power of the front group becomes large, causing astigmatism of the image plane in the positive direction and large negative distortion, making the above-stated tendencies undesirable in designing the lens.

#In view of the above mentioned problems, astigmatism of the image plane in the positive direction may be resolved by making the distance between the front and rear lens groups larger. When the distance between the front and the rear lens groups is increased while maintaining the ratio of the back focus to the focal length constant, the negative refractive power of the front lens group becomes smaller, but the change in the positive refractive power of the rear group is relatively small and the characteristics of the rear group having relative positive refractive power are emphasized. Hence, the characteristics and coma of the image plane of the total lens system are similar to the case in which only the rear group is present, provided that the distance between both groups is sufficiently large, and a wide angle photographic lens may be achieved by the simple application of the prior art. Further, although astigmatism in the image plane may be corrected by making the distance between the front and rear lens groups large, the effective diameter necessary for the front lens group in this case becomes large, making it impossible to achieve a small, light weight wide angle photographic lens. This results because the iris diaphragm is contained in the rear lens group and making the distance between the front and rear lens groups large does not result in much change in the relative position of the entrance pupil and the rear lens groups in the total lens system, causing the distance between the front lens group and the entrance pupil to change according to the distance between the front and rear lens groups. This is the reason that the attachment size of the wide angle photographic lens tends to become large.

#On the other hand, distortion is expressed in terms of the ratio of the heights of the ideal image and the actually formed image. A negative distortion may not directly be corrected by making the distance between the front and rear lens groups large and by making the effective diameter of the front lens group large, that is, by the above-mentioned means for correcting the astigmatism in the image plane. This is because, as the effective diameter of the front lens group becomes large, the position of the entrance pupil retreats, and the height at which the main light rays pass through the front group does not change much relatively. Thus, the value of the distortion that occurs does not change much. However, as a result of reducing the burden related to correction of astigmatism in the image plane and coma, correction of distortion becomes easier and the fact that the lens volume is large has an indirect advantage on the correction of distortion.