Lens System Capable of Converting Focal Length by Addition of a Lens

1. Field of the Invention

This invention relates to a lens system in which a positive auxiliary lens system is detachably mounted between a photographic lens and an image surface to change a focal length of the photographic lens, thus making it possible to take a photograph at a wide angle of view.

2. Description of the Invention

Conventional systems for changing the focal length of the photographic lens include a system using a zoom lens and a system using an auxiliary lens. Separately from the system using the zoom lens which can easily form the lens into a large type, a known magnification system, in the system using the auxiliary lens, for switching a photographic angle of view into a wide angle or a subwide angle and a standard or tele includes:

(1) A tele-type front converter is provided in front (on an object side) of a main lens in a wide angle system,

(2) A wide angle type front converter is provided in front of a main lens in a tele-system; and

(3) An auxiliary lens having a negative refractive power is mounted between an image surface at rear of a main lens in a wide angle system to increase the resultant focal length.

The aforementioned system (1) has a disadvantage in that the front converter is large. This is particularly conspicuous if the main lens is in wide angle and therefore, aberration is difficult to be well corrected. Where the wide angle is included in a magnification area as in the present invention, this system is extremely difficult to be employed.

The aforementioned system (2) is advantageous, as compared with the system (1), in preventing the front converter from being formed into a large type but has a disadvantage in that since the main lens is in the tele-system, the lens system is long, and if the front coverter is mounted thereon, the lens system is lengthened.

The aforementioned system (3) uses a lens of a wide angle of view as a main lens, and Petzval's sum thereof is naturally designed to make it small. If a negative auxiliary lens is mounted to form a long focal point lens, the negative Petzval's sum increases to make it difficult to obtain a good image surface and in addition, positive distortion aberration also increases.

In addition to the aforementioned systems, the fourth system is considered in which an auxiliary lens having a positive refractive power is mounted at rear of a standard or narrow angle main lens to shorten the resultant focal length. However, this system is hardly known for the reasons below:

a. Since the auxiliary lens having a positive refractive power is added at rear of the main lens, the positive Petzal's sum increases, and the flatness of an image required by the wide angle lens is deteriorated.

b. In the wide angle lens obtained by adding the auxiliary lens having a positive refractive power at rear of the main lens, the positive refractive power is applied to the rear portion of the lens system and therefore, the negative distortion aberration increases. These drawbacks likely occur in the fourth system, rendering design difficult.

For example, in the lens system disclosed in Japanese Patent Application Laid-Open No. 111,825/81 which employs the aforementioned system, the distance from the main lens to the image surface is constant in case where only the main lens is used and in case where the auxiliary lens system is mounted. However, a main point of the auxiliary lens system has to be arranged backwardly due to said restriction, and this deteriorates the magnification efficiency and the full length of the auxiliary lens system is longer than the full length of the main lens, making the system large. As the result, the maximum angle of view is just 55.degree. and the magnification ratio is less than 1.4. Despite this, deterioration of the image-surface characteristic due to the increase in Petzval's sum and an increase in negative distortion aberration are conspicuous.

In accordance with the present invention, when an auxiliary lens system is mounted, a main lens is slightly moved towards an image and the full length of the auxiliary lens system is reduced to decrease an amount of occurrence of aberration, whereby the aberration can be well corrected while making an angle of view into a wide angle of 60.degree. to 70.degree..

In a focal length coverter lens system in which a positive auxiliary lens system is mounted at rear of a main lens to provide a wide angle of view, as in the present invention, it is desired that the main lens suitable for a wide angle of view is used. To this end, it is necessary to have a structure which includes at least two positive lenses and at least one negative lens. As one example, there is a triplet as shown in FIG. 1(A). However, it is of course possible that a lens system suitable for a wider angle of view can be used.

It is necessary for the auxiliary lens system to

(1) be small in size, in addition to

(2) prevent an increase in positive Petzval's sum, and

(3) prevent an increase in negative distortion aberration, as described above,

in order not only to incorporate it into a camera but to enhance the performance of image formation at a wide angle. When a wide angle is obtained by a combination of a main lens and an auxiliary lens added to a portion at rear of the main lens, an angle incident on the auxiliary lens naturally increases. Thus, the auxiliary lens system likely becomes large, and an amount of aberration occuring in a peripheral portion thereof also increases as the system increases in size. Therefore, if the full length of the auxiliary lens system is reduced, the height of light, from an optical axis, which passes through the auxiliary lens system, also reduces not only to miniaturize the lens system but to minimize the amount of occurrence of aberration. Because of this, in the present invention, the full length L.sub.A of the auxiliary lens system is made less than at least the full length L.sub.M of the main lens system.

Also, it is desired to reduce the number of lenses in order to miniaturize the auxiliary lens system. In the present invention, the positive auxiliary lens system is composed of at least two positive lenses and at least one negative lens. However, in the auxiliary lens system which is composed of just three lenses where an angle of view at a wide angle is 63.4.degree. and a magnification ratio is 1.43 as can be seen from a seventh embodiment which will be described hereinafter, and which is composed of five lenses where an angle of view at a wide angle is 63.4.degree., a magnification ratio is 2, and brightness is F:2.8 as in a six embodiment, good aberration is obtained. Furthermore, in the auxiliary lens system which is composed of four lenses where a magnification ratio is 1.56 as in first to fourth embodiments, aberration can be well corrected up to an angle of view of 70.degree..

To make the magnification ratio great and obtain a good aberration condition, it is advantageous to employ a lens arrangement, which is great in magnification effect, even if the refractive power of the auxiliary lens system is small, in preventing an increase in Petzval's sum. It is therefore desired that a first lens on the side of the main lens in the auxiliary lens system is made to comprise a positive lens and a main point on the side of an object in the auxiliary lens system is made to close towards the main lens. Such an optical arrangement brings forth an advantage in that a negative distortion aberration generated when the auxiliary lens system is added decreases.

Moreover, preferably, a first surface of the first lens on the side of the main lens is made to form a convex towards the object. This can prevent an increase in negative distortion aberration and can miniaturize the auxiliary lens system by the converging action thereof. The aforesaid first lens comprises not only a single lens but a cemented lens or a lens including a little air spacing.

Needless to say, it is advantageous, to prevent an increase in Petzval's sum in the auxiliary lens system that the refractive index n.sub.p of a glass material of positive lenses in the auxiliary lens system is made to make it high and that of negative lenses is made to make it low. Preferably, the refractive indexes and Abbe's number taken are in the range as follows:

FIGS. 1 and 2, FIGS. 3 and 4, FIGS. 5 and 6, FIGS. 7 and 8, FIGS. 9 and 10, FIGS. 11 and 12, and FIGS. 13 and 14 are sectional views of lenses and aberration views of a first embodiment, a second embodiment, a third embodiment, a fourth embodiment, a fifth embodiment, a sixth embodiment, and a seventh embodiment, respectively, of the lens system in accordance with the present invention, in which drawings, (A) showing the case of only the main lens and (B) showing the case where an auxiliary lens is added.

Embodiments of the present invention will now be described.

In each embodiment, (A) shows data for only the main lens and a sectional view and an aberration view thereof, and (B) shows data where the auxiliary lens is added and a sectional view and an aberration view thereof.

In the embodiments 1 to 4, when only the main lens is provided, the focal length is 48.5, the aperture ratio is 1:5.6, and the angle of view is 48.degree.. When the auxiliary lens is added rearwardly of the main lens, the focal length in the resultant system is 31, the aperature ratio is 1:3.6, the angle of view is 70.degree., and the magnification ratio is 1.56. The embodiment 5 shows a long focal lens in which in only the main lens, the focal length is 70, the aperature ratio is 1:6.7, and the angle of view if 34.4.degree., and when the auxiliary lens is added rearwardly of the main lens, the length in the resultant lens is 35, the aperature ratio is 1:3.36, the angle of view is 63.4.degree. which is a wide angle, and the magnification ratio is 2. The embodiment 6 shows a long focal lens in which in only the main lens, the focal length is 70, the aperture ratio is 1:5.6, and the angle of view is 34.4.degree., and in the resultant system in which the auxiliary lens is added, it comprises a bright wide angle lens in which the focal length is 35, the aperture ratio is 1:2.8, the angle of view is 63.4.degree., and the magnification ratio is 2. In the embodiment 7, in only the main lens, the focal length is 50, the aperture ratio is 1:5, and the angle of view is 46.8.degree., and in the resultant system in which the auxiliary lens system is added, the focal length is 35, the aperture ratio is 1:3.5, the angle of view is 63.4.degree. which is a wide angle, and the magnification ratio is 1.43.

Reference characters in data:

ri: radius of curvature of refractive surface

di: spacing on axis of refractive surface

ni: refractive index of glass material of each lens with respect to line d

.nu.i: Abbe's number of glass material of each lens

f: resultant focal length of the whole

bf: backfocus

F: aperture ratio

M: magnification ratio

As will be apparent from the sectional viw of the lens and the aberration view, the system is small, the magnification ratio is great, and the curve of image surface and distortion and various aberrations are well corrected .