JPH09159911A - Inner focus telephoto lens - Google Patents

Abstract

(57) [Abstract] [Problem] Correcting aberration fluctuations during focusing,
To obtain an inner focus type telephoto lens having good optical performance over the entire object distance. SOLUTION: In order from the object side, there are three lens groups of a positive refractive power first group, a negative refractive power second group, and a positive refractive power third group, and the first lens group 3 One positive lens and at least one negative lens, the second group has at least one positive lens and at least one negative lens, and the second group is moved on the optical axis to perform focusing. When the focal length of the entire system when the second group is focused on an object at infinity is f and the focal length of the i-th group is fi, 0.65 <f1 / f <0.95 0.4 <| f2 / f | <0.65 0.55 <f3 / f <0.9 must be satisfied.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inner focus type telephoto lens suitable for a photographic camera, a video camera or the like, and in particular, it has a focal length of 135 mm when converted for a 35 mm film in which aberration variation during focusing is well corrected. ,
The present invention relates to an inner focus type telephoto lens having an F number of about 2.0 and having excellent optical performance.

[0002]

2. Description of the Related Art Generally, focusing in a taking lens is performed by moving the entire taking lens or moving a part of the taking lens. If the taking lens is a telephoto lens having a long focal length, the taking lens becomes large and heavy, and it is mechanically difficult to move the taking lens as a whole to perform focusing.

For this reason, in many telephoto lenses, some of the lens groups are moved for focusing. Among these, various proposals have been made to use the inner focus type in which a part of the central lens group in the relatively small and lightweight lens system other than the front lens group of the taking lens is moved for focusing. ing.

For example, in JP-A-55-147606, an inner focus type telephoto lens having a focal length of 300 mm and an F number of 2.8 is disclosed in JP-A-59-65820.
In JP-A-59-65821 and JP-A-59-65821, the focal length is 1
An inner focus type telephoto lens of 35 mm and an F number of about 2.8 is proposed.

In all of the inner focus type telephoto lenses proposed by these, the first group having a positive refractive power, the second group having a negative refractive power, and the third group having a positive refractive power are arranged in this order from the object side. It has three lens groups, and the second group is moved on the optical axis for focusing.

[0006]

In the inner focus type, the focusing lens group is small and lightweight, so that the operability is easy and high-speed operation is possible, and the lens when focusing on an object at infinity and a near object is used. There are advantages that the position of the center of gravity of the entire system does not change and holding is easy.

On the other hand, when the inner focus type is adopted in a bright telephoto lens having an F number of about 2.0, the aberration variation at the time of focusing becomes large, and it is difficult to satisfactorily correct the aberration variation at this time, and the optical performance is improved. It is causing the decrease.

In particular, the spherical aberration fluctuates remarkably in the higher-order region of the fifth-order aberration or more, and the spherical aberration becomes largely under, and the image surface deteriorates and the coma aberration also increases in the near distance photographing. . In order to reduce these aberration fluctuations,
There is a method of forming the front lens group with three or more positive lenses and two negative lenses. That is, with such a lens configuration, the light flux passing through the front lens is smoothly converged to reduce the occurrence of high-order aberrations, and as a result, the aberration fluctuation due to the movement of the subsequent focus lens group is reduced to some extent. .

However, even if the high-order spherical aberration can be removed to some extent, it becomes difficult to remove the variation of the annular aberration and the variation of the coma aberration. Therefore, conventionally, the floating lens is used for the (A1) focus lens group. (A2) The focus lens group has a lens configuration that eliminates fluctuations in aberrations generated by itself. (A3) The lens group subsequent to the focus lens group has a lens configuration that eliminates aberration variation. Etc. are used.

As the method (A1), for example, Japanese Patent Laid-Open No. 176717/1984 proposes a telephoto lens adopting the so-called floating method in which the inner focus type is adopted and two lens groups are independently moved. ing. However, in this method, since the two lens groups are moved, the mechanical structure becomes complicated, the eccentricity accuracy becomes strict, and the driving torque increases as compared with the case of moving one lens group. When it is applied to a focus detection device or the like, there is a drawback that the drive motor becomes large.

The method (A2) is disadvantageous when it is applied to an automatic focus detection device in the same manner as the method (A1) because the number of lenses is increased and the weight is increased.

In the method (A3), the structure of the focus lens group is simplified, and the eccentricity accuracy is loosened.
Even when applied to an automatic focus detection device, it is advantageous from the viewpoint of motor torque and the like. However, in the telephoto lens having a large aperture ratio, there have been few cases in which the focus lens group is provided with such a lens configuration for eliminating the aberration variation and is positively applied to the aberration correction.

For example, in Japanese Patent Publication No. 56-13926, focusing is performed by moving a second lens group out of three lens groups of positive, negative, and positive refractive powers in order from the object side. And an inner focus type telephoto lens in which the third lens unit is balanced with the first lens unit and the second lens unit to correct aberrations.

However, when an attempt is made to increase the aperture ratio in the telephoto lens disclosed in the above publication, variation in aberration at the time of focusing increases. Further, for example, in JP-A-1-102413, focusing is performed by moving the second lens group out of three lens groups of positive, negative, and positive refractive powers in order from the object side. , A so-called Gauss type lens group is constructed in the third group, and an inner focus type telephoto lens is proposed in which fluctuations of higher-order spherical aberration and coma aberration are corrected well.

However, in the telephoto lens of the publication, the focal length is converted to a value of 13 mm for a 35 mm Leica plate.
When trying to obtain an angle of view of about 5 mm, the aberration in this angle of view region increases, and the aberration variation during focusing also increases. Further, there is a problem that the lens system becomes large, and in particular, the aperture and diaphragm diameter of the first lens unit become large.

On the other hand, in JP-A-4-255813, the first group is composed of three positive lenses and one negative meniscus lens, and the spherical aberration generated by the three positive lenses is balanced by the negative meniscus lens. However, since a glass material having a relatively high refractive index is used for the positive lens, there is a problem that the variation of chromatic aberration increases when trying to make the focal length telephoto.

It is an object of the present invention to provide an inner focus type telephoto lens having an F number of about 2.0, which is extremely bright and has a high aperture ratio of about 135 mm in terms of a large aperture ratio of 35 mm.

A further object of the present invention is to use the inner focus type and to improve aberration variation and axial chromatic aberration during focusing in a wide range of object distances from infinity objects to short-distance objects up to the secondary spectrum. It is to provide an inner focus type telephoto lens with a corrected and simple structure and a large aperture ratio.

[0019]

The inner focus type telephoto lens of the present invention comprises a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit having a positive refractive power in order from the object side. A third group of lenses, the first group having three positive lenses and at least one negative lens, the second group having at least one positive lens and at least one negative lens,
The second group is moved on the optical axis to perform focusing,
When the focal length of the entire system when the group is focused on an object at infinity is f and the focal length of the i-th group is fi, 0.65 <f1 / f <0.95 (1) 0.4 < | F2 / f | <0.65 ... (2) 0.55 <f3 / f <0.9 ... (3) The condition is satisfied.

[0020]

1 to 6 are sectional views of lenses of Numerical Examples 1 to 6 of the present invention, and FIGS. 7 to 12 are various aberration diagrams of Numerical Examples 1 to 6 of the present invention. In the figure, 1 is a first group having a positive refractive power, 2 is a second group having a negative refractive power, 3 is a third group having a positive refractive power, and SP is a diaphragm.

In this embodiment, the second group 2 is moved to the image plane side as shown by the arrow to focus from an object at infinity to a near object. In the present embodiment, the first group, in which the incident height of the light beam is high and the aberration correction is relatively easy, is composed of three positive lenses and at least one negative lens. Then, the refractive power of each lens group is set as in conditional expressions (1) to (3), whereby a telephoto lens having high optical performance with little aberration variation during focusing over the entire screen and object distance is achieved. ing.

Next, the technical meaning of each of the above conditional expressions will be described. The conditional expression (1) is to correct various aberrations of the entire screen in good balance with respect to the positive refractive power of the first group, mainly while shortening the total lens length. When the upper limit of conditional expression (1) is exceeded and the refracting power of the first lens unit becomes weaker, the diameter of the third lens unit and after in the first lens unit increases, and at the same time, the moving amount of the second lens unit during focusing increases. As a result, the total length of the lens becomes long in order to secure a space for movement, which is not desirable.

On the contrary, when the refractive power of the first lens unit becomes strong beyond the lower limit value, the total lens length is shortened, but the aberration correction, particularly the meridional image plane is insufficiently corrected, and the distortion aberration is excessively corrected, so that these aberrations are caused. Is difficult to correct. In addition, a large amount of outward coma is generated, and when an even higher degree of chromatic aberration correction is performed, an extraordinary dispersion glass such as fluorite (CaF2) or UD glass is used, a large amount of g-line outward coma is generated. It is difficult to balance these aberration corrections well.

Conditional expression (2) relates to the refracting power of the second lens group, and is mainly for favorably correcting the fluctuation of aberration when focusing is performed. If the upper limit of conditional expression (2) is exceeded and the refractive power of the second lens unit becomes weaker, the amount of movement during focusing increases, and the overall lens length increases to secure a movement space, which is not desirable. On the contrary, when the refracting power of the second lens group becomes strong beyond the lower limit value, the spherical aberration is overcorrected, and in particular, it is remarkably overcorrected in a short distance object. In addition, it becomes difficult to correct chromatic aberration in the second group by laminating in a well-balanced manner, and flare particularly on the g-line tends to occur.

Conditional expression (3) relates to the refracting power of the third lens group, and mainly relates to securing the back focus and the rear lens diameter. If the upper limit of conditional expression (3) is exceeded and the refractive power of the third lens unit becomes weak, the total lens length increases and the diameter of the first lens unit increases, which is not preferable. On the contrary, if the lower limit is exceeded and the refractive power of the third lens group becomes strong, the total lens length is shortened and it is difficult to secure the back focus, especially it is difficult to secure the space for the extender to be attached to the rear part of the lens to increase the focal length. Becomes

Next, features of each embodiment of the present invention will be described.

(A1) In Numerical Embodiments 1 to 6 shown in FIGS. 1 to 6, the first lens unit has a positive eleventh lens element on the object side.
When the focal length of one lens is f11, the condition 1 <f11 / f <3.5 (4) is satisfied.

Conditional expression (4) relates to the distribution of the converging action of the first positive lens on the object side in the first group. When the upper limit of conditional expression (4) is exceeded and the refracting power of the positive lens becomes weak, the converging action of the subsequent second and third positive lenses becomes large, and the occurrence of higher-order aberrations increases, Aberration correction becomes difficult.

On the other hand, when the refractive power of the first positive lens is increased beyond the lower limit, the lens thickness of the first positive lens becomes thicker and the weight increases. In addition, the effect of sharing the converging action is weakened, and the occurrence of higher-order aberrations also increases. Further, when a glass material having anomalous dispersion is used after the second positive lens as in the present embodiment, the effect of the anomalous dispersion glass material decreases as the converging action of the first positive lens group increases. Absent.

(A2) Generally, when the incident height H of the light beam on the lens surface is high, the effect of aberration correction is high, and it is also effective for the correction of variation in chromatic aberration.

(A2-1) Therefore, in the present invention, the first group is set to 3
In the numerical embodiments 1 to 3 of FIGS. 1 to 3 in the configuration having one positive lens and at least one negative lens, the first group is the positive eleventh lens whose lens surfaces are convex, A positive twelfth lens having a convex surface having a strong refractive power, a negative thirteenth lens having a concave surface having a strong refractive power toward the image side, a positive fourteenth lens having a convex surface having a strong refractive power facing the object side, and It is composed of a negative 15th meniscus lens having a convex surface directed toward the object side.

The term "strong refracting power on the object side" as used herein means that in comparison with the image plane side. Further, the strong refracting power on the image plane side means more than on the object side.

In Numerical Examples 1 to 3 shown in FIGS. 1 to 3, first, the incident light beam is gently converged by the two positive eleventh lens and the twelfth lens on the object side to suppress the occurrence of high-order aberrations. Further, the following negative thirteenth lens is used to correct various aberrations. Then, the positive 14th lens further converges the light beam, and the meniscus-shaped negative 15th lens behind the positive 14th lens suppresses aberration fluctuations during focusing, particularly inward coma aberration and aberration fluctuations that cause an excessive tendency of the meridional image plane. Corrected well.

(A2-2) Numerical Embodiment 4 of FIGS. 4 to 6
In Nos. 6 to 6, the first lens group is the positive 111st lens element with convex lens surfaces on both sides, and the meniscus-shaped positive 11th lens element with the convex surface facing the object side.
It is composed of two lenses, a positive meniscus 113th lens with a convex surface facing the object side, and a negative 114th lens with a concave surface having a strong refractive power facing the image side.

In Numerical Embodiments 4 to 6 of FIGS. 4 to 6, the convergence of the incident light beam is determined by the three positive 111th lenses, the 11th lens on the object side.
The second lens and the 113th lens are shared, so that the occurrence of high-order aberrations is suppressed and the meniscus-shaped negative first lens is used.
The spherical aberration is corrected by the concave surface of the 14th lens on the image plane side. The use of a glass material having a relatively high refractive index as the material of the positive 113th lens reduces the occurrence of high-order spherical aberration. Positive 111 lens, positive first
A low-dispersion glass material is used as the material of the 12 lenses, and fluctuation of chromatic aberration is suppressed to a low level.

In this embodiment, the first group is constructed as described above, so that various aberrations are corrected in a well-balanced manner in the first group having a high incident height of a light beam.

(A3) In Numerical Examples 1 to 6 of FIGS. 1 to 6, when the refractive index and the Abbe number of the material of at least one of the three positive lenses in the first group are Na and νa, respectively. 1.43 <Na <1.49 (5) 81 <νa <95.5 (6) The following conditions are satisfied.

Conditional expressions (5) and (6) relate to correction of chromatic aberration in the first lens group. In the present embodiment, at least one of the three positive lenses in the first group has a glass material in the range of conditional expressions (5) and (6), for example, fluorite (CaF2) that is anomalous dispersion glass or UD glass. Is used to correct not only the g-line but also the secondary spectrum.

(A4) The second lens unit is composed of a cemented lens composed of a positive lens and a negative lens. This suppresses the variation of chromatic aberration due to focusing and corrects the flare of spherical aberration on the g-line.

(A5) The third group has, in order from the object side, a cemented lens in which a negative 31st lens and a positive 32nd lens are cemented, and a positive 33rd lens, and the diagonal length of the effective screen is When IC is used, the condition 2 <f / IC <3.8 (7) is satisfied. Positive refractive power third
By constructing the group as described above, a so-called Gauss type lens configuration is provided as the entire lens system, and high-order spherical aberration fluctuations and coma aberration are satisfactorily corrected. Numerical Examples 1 to 5 in FIGS. 1 to 5 are a cemented lens in which a third lens unit is cemented with a negative lens having a strong concave surface facing the object side and a positive lens having a strong convex surface facing the image surface in this order from the object side. It is composed of a convex positive lens. Further, the third lens group can be composed of only a cemented cemented lens of a negative lens and a positive lens as in Numerical Embodiment 6.

It should be noted that it is more advantageous for the third lens unit to have a three-lens structure for correction of higher-order aberrations, and at the same time, the principal point interval of the third lens unit can be widened, so that focusing in the lens barrel is possible. This is advantageous for securing a space for an actuator or the like for driving.

Conditional expression (7) shows an effective range when the focal length of this embodiment is extended. If an attempt is made to apply the focal length to the telephoto side beyond the upper limit of conditional expression (7), the first lens group diameter, the diaphragm diameter, the final lens diameter, etc. are undesirably increased, and it is necessary to change the refractive power arrangement. Further, the influence of the secondary spectrum of the axial chromatic aberration becomes large, and the correction cannot be completed. On the contrary, if the lower limit value is exceeded and it is applied to the wide angle side, the high-order fluctuation of the spherical aberration of the first lens group cannot be sufficiently corrected.

Next, numerical examples of the present invention will be shown. In the numerical examples, Ri is the radius of curvature of the i-th lens surface in order from the object side, Di is the i-th lens thickness and air gap in order from the object side, and Ni and νi are the i-th lens in order from the object side. Are the refractive index and Abbe number of the glass. Table-1
Shows the relationship between the conditional expression and the numerical example.

[0044]

[Outside 1]

[0045]

[Outside 2]

[0046]

[Outside 3]

[0047]

[Outside 4]

[0048]

[Outside 5]

[0049]

[Outside 6]

[0050]

[Table 1]

[0051]

According to the present invention, each lens group of the telephoto lens is specified as described above, and the second group is moved on the optical axis for focusing, thereby shortening the overall lens length and accompanying the focusing. It is possible to achieve an inner-focus type telephoto lens suitable for a photographic camera, a video camera, or the like, which has a high optical performance in which aberration fluctuation is satisfactorily corrected.

[Brief description of the drawings]

FIG. 1 is a sectional view of a lens according to a numerical example 1 of the present invention.

FIG. 2 is a sectional view of a lens according to a numerical example 2 of the present invention.

FIG. 3 is a sectional view of a lens according to a numerical example 3 of the present invention.

FIG. 4 is a lens cross-sectional view of Numerical Example 4 of the present invention.

FIG. 5 is a lens cross-sectional view of Numerical Example 5 of the present invention.

FIG. 6 is a lens cross-sectional view of Numerical Example 6 of the present invention.

FIG. 7 is a diagram of various types of aberration in Numerical example 1 of the present invention.

FIG. 8 is a diagram showing various types of aberration in Numerical example 2 of the present invention.

FIG. 9 is a diagram showing various types of aberration in Numerical Example 3 of the present invention.

FIG. 10 is a diagram showing various types of aberration in Numerical Example 4 of the present invention.

FIG. 11 is a diagram of various types of aberration in Numerical example 5 of the present invention.

FIG. 12 is a diagram showing various types of aberration in Numerical Example 6 of the present invention.

[Explanation of symbols]

 1 1st group 2 2nd group 3 3rd group SP Aperture ΔS Sagittal image plane ΔM Meridional image plane d d line g g line

Claims (8)

[Claims] 1. A three-lens group including a first group having a positive refractive power, a second group having a negative refractive power, and a third group having a positive refractive power, which are arranged in this order from the object side. Three positive lenses and at least one negative lens, the second group has at least one positive lens and at least one negative lens, and the second group is moved on the optical axis for focusing, When the focal length of the entire system when the second lens unit is focused on an object at infinity is f and the focal length of the i-th lens unit is fi, 0.65 <f1 / f <0.95 0.4 <| f2 / f An inner focus type telephoto lens characterized by satisfying a condition of | <0.65 0.55 <f3 / f <0.9. 2. The first lens group has a positive eleventh lens on the object side, and when the focal length of the eleventh lens is f11, the condition 1 <f11 / f <3.5 is satisfied. The inner focus type telephoto lens according to claim 1. 3. The first lens group, in order from the object side, has a positive eleventh lens element having convex lens surfaces on both sides, a positive twelfth lens element having a convex surface having a strong refractive power on the object side, and a strong refractive power element on the image surface side. The negative thirteenth lens having a concave surface directed toward the object side, the positive fourteenth lens having a convex surface having a strong refractive power toward the object side, and the negative meniscus-shaped fifteenth lens having a convex surface directed toward the object side. The inner focus type telephoto lens according to claim 1. 4. When the refractive index and the Abbe number of the material of at least one of the eleventh lens, the twelfth lens, and the fourteenth lens are Na and νa, respectively, 1.43 <Na <1.4981 The inner focus type telephoto lens according to claim 3, wherein the condition <νa <95.5 is satisfied. 5. The negative third lens unit is arranged in order from the object side.
Having a cemented lens in which a lens and a positive 32nd lens are cemented together, and a positive 33rd lens, and satisfying the condition 2 <f / IC <3.8, where IC is the diagonal length of the effective screen. The inner focus type telephoto lens according to claim 4. 6. The first lens group, in order from the object side, has a positive lens 111 whose both lens surfaces are convex, a positive meniscus lens 112 having a convex surface facing the object side, and a meniscus lens having a convex surface facing the object side. The telephoto lens of the inner focus type according to claim 2, characterized in that it comprises a positive-shaped 113th lens and a negative-114th lens with a concave surface having a strong refractive power facing the image side. 7. The 111st lens, the 112th lens,
When the refractive index and the Abbe number of the material of at least one of the 113th lenses are Na and νa, respectively, the condition of 1.43 <Na <1.49 81 <νa <95.5 is satisfied. The inner focus type telephoto lens according to claim 6. 8. The negative third lens unit is arranged in order from the object side.
Having a cemented lens in which a lens and a positive 32nd lens are cemented together, and a positive 33rd lens, and satisfying the condition 2 <f / IC <3.8, where IC is the diagonal length of the effective screen. An inner focus type telephoto lens according to claim 7.