Compact Photographic Camera Lens System Having an Aspherical Surface of Short Overall Length

(a) Field of the Invention

The present invention relates to a compact photographic camera lens system having an aspherical surface of short overall length, and more particularly, to a compact photographic camera objective using Leica size film, which is so designed that an aspherical surface is introduced into part of the lens system to minimize overall length as well as reducing the number of lens component of the lens system and where the aberrations of the lens system are properly corrected up to a field angle of around 63.degree..

(b) Description of the Prior Art

In general, a field angle of around 63.degree. is chosen for compact photographic camera objectives using Leica size film. In order to design lens systems of this kind as very compact systems having a telephoto ratio of 1.0 and lower, it is necessary to locate the rear principal point of the entire lens system before the first lens surface. For the purpose of satisfying this requirement, it is considered to adapt a lens type generally used as telephoto lenses comprising a front lens group designed as a converging lens system and a rear lens group designed as a diverging lens system. With the telephoto type lens system, however, it is extremely difficult to properly correct offaxial aberrations.

A lens system construction similar to that used in the present invention is disclosed in the Japanese Laid Open Patent Application No. Sho 55/1980-73014 in which a lens system is formed of only spherical lens systems. On the other hand, a lens system in which aspherical surfaces are formed on both sides of a fourth lens component is disclosed in the Japanese Laid Open Patent Application No. Sho 56/1981-59217 of the same inventor which has been made with the object of improving performance of the lens system in the former application in the marginal portions of the image plane. Specifically, a compact photographic camera lens system of short overall length is provided by introducing an aspherical surface at an extremely effective position for correcting aberrations in marginal portions of an image plane so that the aberrations are properly corrected. To achieve this purpose, the prior application indicates that the sufficient requirement is to introduce two aspherical planes into part of a lens system. However, the aspherical surface is disadvantageous in manufacturing due to difficulty of processing and inspection and accordingly presents an unavoidable expense. Therefore, it is apparently advantageous in manufacturing to reduce the number of aspherical surfaces to one. In this case, a problem is that, even if the number of aspherical surfaces to be introduced is reduced to one, how the same lens performance as that of a lens having two aspherical surfaces can be obtained.

It is an object of the present invention to provide a compact photographic camera lens system having a telephoto ratio smaller than 1.0 which is so adapted as to have a very short overall length by using a lens system of the telephoto type including four components of four lens elements and by introducing an aspherical surface into only one side of a fourth lens component and which is arranged such that the aspherical surface has an especially large aspherical constant of the sixth order and a small aspherical constant of the lower orders than the fourth order so as to be effective only to the higher order correction regarding offaxial rays, thereby properly correcting offaxial aberrations without adversely affecting paraxial aberrations so that a field angle up to around 63.degree. can be obtained and the overall length of the lens system can be minimized.

According to the present invention, the number of aspherical surfaces is limited to at most one so as to facilitate manufacturing and yet wherein noticeable effect on the correction of aberrations can be obtained even with one aspherical surface which is designed such that terms higher than the sixth order of the aspherical surface is made much larger than terms lower than the fourth order and the airspace between a front and rear lens groups is increased. As shown in aberration characteristics curves in the accompanying drawings, correction of aberrations particularly at a position of around 0.8 to 0.9 times the maximum image height is fully improved compared with a spherical surface system and it is possible to obtain the same effect as that of a system having two aspherical surfaces.

FIG. 1 is a sectional view illustrating a lens construction according to Embodiments 1 and 2 of the present invention;

FIG. 2 is a diagram illustrating aberration characteristics curves of a lens system of Embodiment 1 shown in FIG. 1;

FIG. 3 is a diagram illustrating aberration characteristics curves of a lens system of Embodiment 2 shown in FIG. 1;

FIG. 4 is a sectional view illustrating a lens construction according to Embodiment 3 of the present invention; and

FIG. 5 is a diagram illustrating aberration characteristics curves of Embodiment 3 shown in FIG. 4.

A lens system of the present invention is an objective for use in photographic cameras which comprises a first positive meniscus lens component having a convex surface on the object side, a second biconcave lens component, a third biconvex lens component consisting of a single (FIG. 1) or compound (FIG. 2) lens and a fourth negative meniscus lens component, said fourth lens component having an aspherical surface on the image side and being so designed as to satisfy the following conditions:

The condition (i) defines the resultant focal length from the first lens component to the third lens component. When the lower limit of the condition (i) is exceeded, the overall length of the lens system can be advantageously reduced but correction of offaxial aberrations intended in the present invention is insufficient even if an aspherical surface is introduced. When the upper limit of the condition (i) is exceeded, the overall length can not be reduced. The condition (ii) defines an airspace d.sub.6 between the front lens group of the so-called convergent lens system from the first lens component to the third lens component and the rear lens group of the so-called divergent lens system of the fourth lens component. When the airspace d.sub.6 increases to some extent the overall length of the lens system is advantageously reduced and aberrations are advantageously corrected. For example, when the airspace d.sub.6 is increased with the same overall length, refracting powers of the front and rear lens groups are reduced with the result that aberrations, especially spherical aberrations and astigmatism, are advantageously corrected. There is a tendency to reduce sagittal and meridional curvature of fields so that astigmatism in the marginal portions of an image plane can be reduced. However, the increase of the airspace d.sub.6 attended with the increase of asymmetry in the front and rear groups of a lens system. Furthermore, outward coma and distortion of the pin cushion type are apt to be produced. However, these can problems be solved by making the surface of the fourth lens component on its image side an aspherical surface deviated on the object side from the standard spherical surface (r.sub.8).

As described above, spherical aberration, astigmatism, coma and distortion are properly corrected and the imaging performance in the marginal portions of the image plane which is intended in the present invention is also corrected properly.

The introduced aspherical surface is intended for correction of higher order aberrations with respect to an image plane (coma higher than the fifth order, distortion higher than the fifth order) and is required to have no adverse effect on spherical aberrations which can be fully corrected in a spherical system. Such requirements are most effectively achieved by introducing an aspherical surface which has a low offaxial ray, a higher principal ray, smaller aspherical constants which are lower than the fourth order on the image side plane of the fourth lens components than that on the object side plane thereof and large aspherical constants higher than the sixth order.

Specifically, assuming that an equation of the aspherical surface is expressed as follows: ##EQU1## and the sum of all terms other than the above spherical component is expressed as follows:

it is preferred that the following conditions are satisfied:

and

(where at least the limits of 0.15f.ltoreq.y.sub.8 .ltoreq.0.2f).

The fact that spherical aberrations and aberrations of lower orders regarding paraxial rays are properly corrected is disclosed in the Japanese Laid Open Patent Application No. Sho 55/1980-73014. Accordingly, it is preferred that the foregoing conditions are satisfied in order than pincushion type distortion is not remarkably produced by producing higher order distortion on the negative side without adversely effecting aberrations corrected in the Japanese Patent Application, higher order coma flare is removed and a sufficient quantity of marginal rays is ensured by enhancing the aperture efficiency over marginal portions of field angle. The deviation from these conditions will disadvantageously increase distortion and lower order aberrations regarding paraxial rays.

In addition, the following conditions (iv) through (viii) are imposed for further improvements in correction of aberrations.

The condition (iv) defines an air space between the second and third lens components. When the upper limit of the condition (iv) is exceeded, the overall length of a lens system increases and it is difficult to reduce astigmatism over the entire image plane. When the lower limit of the condition (iv) is exceeded, it is impossible to obtain a sufficient effective aperture due to the mechanical interference between the second and third lens components.

The condition (v) defines thickness of the third lens component. When the upper limit of the condition (v) is exceeded, astigmatism is advantageously reduced over the entire image plane but the overall length is apt to increase. When the lower limit of the condition (v) is exceeded, it is difficult to correct astigmatism.

The condition (vi) defines an air space between the first and second lens components. When the upper limit of the condition (vi) is exceeded, it is difficult to correct spherical aberrations and chromatic aberrations. In addition, the overall length of a lens system is apt to increase. When the lower limit of the condition (vi) is exceeded, it is impossible to obtain a sufficient effective aperture due to the mechanical interference between the first and second lens components to cause a shortage of a quantity of marginal rays.

The condition (vii) defines a focal length of the fourth lens component. When the upper limit of the condition (vii) is exceeded, aberrations are advantageously corrected but the overall length of a lens system is apt to increase. When the lower limit of the condition (vii) is exceeded, it is easy to reduce the overall length but correction of aberrations becomes inconvenient. In particular, offaxial aberrations (coma and distortion) are not fully corrected even if an aspherical surface is introduced.

The condition (viii) defines the Abbe's number of a convex lens of the third lens component and is for correcting chromatic coma of lower side rays in offaxial aberrations. When .nu..sub.3 (.nu..sub.3b in Embodiment 3) is more than 58, the g line of the lower side rays is overcorrected, namely lateral aberration is apt to disadvantageously become a negatively large value.

Referring now to the drawings, Embodiments of the present invention will be described below. In the drawings, r.sub.1 through r.sub.9 represents radii of curvature on the surfaces of the respective lens elements, d.sub.1 through d.sub.7 represent thickness of the respective lens elements and airspaces therebetween, n.sub.1 through n.sub.4 represent refractive indices of the respective lens elements and .nu..sub.1, .nu..sub.2, . . . , .nu..sub.4 represent Abbe's numbers of the respective lens elements.