JPH077146B2 - Telephoto zoom lens - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a zoom lens, in particular, a so-called telephoto zoom lens having an angle of view at the wide-angle end of less than 45 °.

(Background of the Invention) Generally, as a focusing method of a zoom lens,
A so-called front lens feeding method is used in which a lens group arranged closer to the object side than a lens group that moves for zooming is moved. However, this system has large fluctuations in aberrations due to focusing at the telephoto end, especially fluctuations in spherical aberration and field curvature, and this tendency becomes more pronounced as the zoom ratio becomes larger and the aperture ratio becomes larger. For example, as a focusing method for a telephoto zoom lens, as in Japanese Patent Publication No. 59-4688, the front lens is divided into four groups of positive, negative, positive, and negative in order from the object side, and the third group is moved for focusing. The method of doing is proposed. However, in the specific example of this method, the zoom ratio is about 4 and the aperture ratio is about 1: 5.6. Therefore, when using this method with a zoom lens having a larger zoom ratio and a larger aperture ratio, Since the lens portion becomes large and it is necessary to make the variable power portion extremely small as compared with the front lens portion as a whole, the aberration fluctuation due to the variable power becomes large, and it is difficult to realize a high-performance zoom lens.

(Object of the Invention) An object of the present invention is to have a wide zoom range with a large zoom ratio,
An object of the present invention is to provide a telephoto zoom lens which has a small aberration variation due to focusing despite having a large aperture ratio.

(Outline of the Invention) In a telephoto zoom lens according to the present invention, a focusing lens unit arranged on the object side of a variable power lens unit has a positive refracting power.
The lens is divided into two lens groups, the object-side positive refractive power lens group is fixed to the image plane, and the image-side positive refractive power lens group is moved along the optical axis for focusing. It is a thing.

Specifically, as shown in the lens configuration diagram of the embodiment of FIG. 1 (state at the telephoto end), the first lens group as a focusing lens unit.
G ₁ is the first positive refractive power fixed to the image surface on the object side.
It is composed of a partial group L ₁ and a second partial group L ₂ movably provided along the optical axis on the image side thereof, and the combined focal length of the entire focusing lens unit is f, and the first partial group is When the focal length of L ₁ is f ₁ , the condition of 5.0 <f ₁ /f<12.0 (1) is satisfied.

If the upper limit of this condition (1) is exceeded, the second subgroup L ₂
Since the refracting power as the focusing lens part is concentrated on,
It becomes difficult to correct aberration variation due to focusing by combining with the first sub-group L ₁ . When the value goes below the lower limit of this condition, the refracting power of the second sub-group L ₂ becomes too small, so that the moving amount of the second sub-group for focusing becomes large and the focusing lens group becomes long. .
Further, since the burden of chromatic aberration on the first and second sub-groups increases, it is difficult to configure each sub-group with a simple lens structure, and the lens system becomes complicated and large.

Further, the first sub-group L ₁ has a negative meniscus lens component L ₁₁ with a convex surface facing the object side and a positive lens component L ₁₂ with a surface having a stronger curvature facing the object side, and the second sub-group L 1 ₂ is desirably configured to have a double-convex positive lens component L ₁₄ with its surface with a stronger curvature a positive lens component L ₁₃ and the object side cemented with its strong surface curvature the object side. The refractive index of the negative meniscus lens component L _{11 in} the first sub-group is Na, the radius of curvature of the image side lens surface is Ra, and the refractive index of the positive lens component L _{12 in} the first sub-group is Nb, When the radius of curvature of the object-side lens surface is Rb and the radius of curvature of the image-side lens surface of the image-side biconvex positive lens component L ₁₄ in the second sub-group L ₂ is Rc, 0.48 <Ra / f <0.72 (2) 0.48 <Rb / f <0.72 (3) 1.8 <Rc / f <2.6 (5) It is desirable to satisfy the condition.

Conditions (2) to (5) are for sharing the aberration between the first sub-group L ₁ and the second sub-group L ₂ so as to cancel the aberration variation due to focusing. At the telephoto end where the aberration variation due to focusing is the largest, by constructing each subgroup as described above, a large positive spherical aberration, a large non-spherical aberration is generated on the image side lens surface Ra of the negative meniscus lens component L ₁₁ in the first subgroup. A large amount of negative spherical aberration and astigmatism occur on the object-side lens surface Rb of the positive lens component L _{12 in} the first sub-group due to the occurrence of point aberration. The biconvex positive lens component L _{14 on} the image side in the group L ₂ is balanced so that its aberration value is almost equal to that of the aberration generated on the image side lens surface Rc.

When the second sub-group L ₂ is moved to focus on a near object,
Image side lens surface R of negative meniscus lens component L _{11 in} the subgroup
The spherical aberration generated by a changes in the negative direction and the astigmatism changes in the positive direction, and at the same time, the positive lens component in the first sub-group changes.
The spherical aberration generated on the object-side lens surface Rb of L ₁₂ changes in the positive direction and the astigmatism changes in the negative direction. As a result, the positive spherical aberration of the first subgroup is small and the positive astigmatism is small. Grows. On the other hand, due to focusing on a short-distance object, the second sub-group L ₂
The negative spherical aberration generated on the image-side lens surface Rc of the image-side positive lens component L ₁₄ is small, and the negative astigmatism is large.
For this reason, eventually, the variation in aberration due to focusing changes in the direction in which the first and second sub-groups cancel each other out, and the focusing lens group can be corrected to an extremely small value.

If the upper and lower limits of the conditions (2), (3), and (5) are exceeded, the aberrations generated on the lens surfaces Ra, Rb, and Rc will be out of balance, and aberration fluctuations due to focusing will be reduced. I can't do it. When the condition (4) is not satisfied, the negative meniscus lens component in the first subgroup is
The image side lens surface Ra of L ₁₁ and the positive lens component L _{12 in} the first sub-group.
The absolute value of the aberration generated at the object-side lens surface Rb of is reversed and can be canceled by the aberration generated at the image-side lens surface Rc of the image-side positive lens component L ₁₄ in the second sub-group L _2. It gets harder.

Further, the biconvex positive lens constituting the cemented positive lens component L ₁₃ in the second sub-group L ₂ is made of so-called anomalous dispersion glass or fluorite having an Abbe number of 80 or more, and the negative lens is lanthanum crown glass or lanthanum flint. It is desirable that the glass is a type of glass. By selecting such a glass material, it becomes possible to remove the secondary spectrum, which is a major cause of deteriorating the image contrast at the telephoto end.

(Examples) Examples according to the present invention will be described below. FIG. 1 is a lens configuration diagram of an embodiment according to the present invention. This zoom lens includes a first focusing lens group in order from the object side.
It includes a lens group G ₁ , a second lens group G ₂ having negative refractive power as a variable power lens group, a third lens group G _{3 having} negative refractive power as well as a fourth lens group G ₄ as a relay lens group. . Fourth
The lens group G ₄ is composed of a front group G _F on the object side and a rear group G _R separated by a relatively large air space, and the rear group G _R is the third group.
It is possible to move in conjunction with the movement of the lens group G ₃ . During zooming from the wide-angle end to the telephoto end, the second lens group G ₂ moves linearly toward the image side, and the third lens group G ₃ draws a locus with a convex surface facing the object side. The lens moves non-linearly, and at the same time, the rear group G _{R in} the fourth lens group G ₄ moves non-linearly so as to draw a locus with a convex surface facing the image side.

The first lens group G ₁ as a focusing lens group has the lens configuration as described above, and by moving the second partial group L ₂ along the optical axis toward the object side by 15.2 mm, It is possible to focus on an object at a short distance of 2.5 m from the apex of the lens surface.

As for the specific configuration of each group, the first lens group G ₁ includes, in order from the object side, a negative meniscus lens L ₁₁ having a convex surface facing the object side and a positive lens L having a surface having a stronger curvature facing the object side.
_A first sub-group L ₁ composed of ₁₂ and having a positive refractive power by synthesis, a second positive lens L 1 composed of a cemented positive lens L ₁₃ having a convex surface facing the object side and a biconvex positive lens L ₁₄ and having a positive refractive power synthetically. It consists of the subgroup L ₂ . The second lens group G ₂ includes, in order from the object side, a negative meniscus lens L ₂₁ having a convex surface directed toward the object side, a cemented negative lens L ₂₂ having a surface having a stronger curvature toward the image side,
Laminated negative lens L with the surface with stronger curvature facing the object side
Made _23, the third lens group G ₃ is composed of a negative lens L ₃ of the laminated with a concave surface facing the object side. The fourth lens group G ₄ as a relay group includes a front lens group G _F and a rear lens group G _R that can move along the optical axis, and the front lens group G _F has a stronger curvature toward the image side in order from the object side. Of the positive lens L ₄₁ , the biconvex positive lens L ₄₂ , the cemented positive lens L ₄₃ with the surface having a stronger curvature toward the object side, the positive lens L _{44 with the} surface having a stronger curvature toward the object side, and the image The rear lens group G _R is composed of a negative lens L _{45 with} a surface having a stronger curvature facing toward the side.
Laminated meniscus lens L ₄₆ with concave surface facing the object side,
It is composed of a negative meniscus lens L ₄₇ having a convex surface directed toward the object side, a biconvex positive lens L _48, and a positive lens L ₄₉ having a surface having a stronger curvature directed toward the object side. Between the fourth lens group G ₄ and the image plane I, a plane-parallel member P corresponding to a color separation prism or various filters is arranged.

Table 1 below shows the specifications of this embodiment. In the table, the leftmost number indicates the order from the object side, and the refractive index and Abbe number are d
The value is for the line (λ = 587.6 nm).

For the above embodiment, the wide-angle end (f = 24), the middle (f = 12)
FIGS. 2A, 2B, and 2C sequentially show various aberration diagrams in each state of 0) and the telephoto end (f = 275). In the aberration diagram, the reference ray is the e-line (λ = 546.1 nm), and in the spherical aberration diagram, the astigmatism diagram, and the chromatic aberration of magnification diagram, the g-line (λ = 43 nm) is used.
5.8 nm) and C line (λ = 656.3 nm) are also shown.
Further, in the spherical aberration diagram, the amount of violation of the sine condition is also indicated by a broken line.

Further, by moving only the second component in the first lens group G ₁ , the object at the wide-angle end (f = 24), the middle (f = 120), and the telephoto end (f = 275) can be obtained. distance
Aberration diagrams at a focus of 2.5 m are sequentially shown in FIGS. 3A, 3B, and 3C.

In this embodiment, a prism, a filter or the like corresponding to the parallel plane member P having a length of 69 mm, a refractive index of 1.60881 and an Abbe number of 59 is arranged between the fourth lens group G ₄ and the image plane. , It is designed including this.

According to these aberration charts, the zoom ratio is wide, 11.5, and the wide zoom ratio is F-number 1.8 to 2.8 (telephoto end), even though it is a telephoto zoom lens with a large aperture ratio. It is clear that various aberrations are corrected very well over the entire zoom range and also when focusing on an object at a short distance of 2.5 m in each focal length state. In addition to the conventional zoom lens for video cameras, it maintains excellent imaging performance far surpassing that of zoom lenses for 35mm SLR cameras. It can be seen that the product has sufficiently good performance as a product.

The following two tables show the infinity in-focus state and the short-distance in-focus state to the object distance of 2.5 m in order to show the state of aberration balance at the time of focusing in the first lens group G ₁ of the above-described example. 3 shows the third-order aberration coefficient of each lens surface. (All are at the telephoto end) In Table 2, I is spherical aberration, II is coma aberration, III is astigmatism, IV is Petzval sum, and V is each third-order aberration coefficient of distortion, and the leftmost number is , The order from the object side of each lens surface in the first lens group G ₁ . And
The subtotal of the aberration coefficients in each of the first and second sub-groups in the first lens group G ₁ and the total sum of the entire system are also shown.

From the above-mentioned aberration coefficient table, each lens surface as described above because the first lens group G ₁ as the focusing lens group according to the present invention is composed of the first and second partial groups having positive refractive power R
The state of aberration variation at a, Rb, and Rc can be read. That is,
It can be seen that the aberrations generated on each of these surfaces are generated so that they are skillfully canceled by focusing, and the fluctuations are suppressed to extremely small for the entire system. (However, the signs of the actual aberration and the above-mentioned aberration coefficient are inverted.) In this embodiment, the negative lens L ₁₁ in the first sub-group of the first lens group G ₁ is a lantern. The flint glass and the positive lens L _{12 are made of} phosphoric acid crown glass, which contributes greatly to the correction of the secondary spectrum in combination with the selection of the glass material of the cemented positive lens in the second sub-group. As shown in the aberration diagrams of FIGS. 3 and 4, the chromatic aberration is also corrected in a very balanced manner.

(Effects of the Invention) As described above, according to the present invention, despite the large zoom ratio and the large aperture ratio, the aberration variation due to focusing, particularly the variation of spherical aberration and image plane curvature is small, and the entire zoom range is achieved. It is possible to realize a telephoto zoom lens that maintains excellent imaging performance even when focusing on a short-distance object in.

Since the first sub-group L ₁ is fixed with respect to the image plane,
It is possible to construct a lens barrel with excellent impact resistance and dust resistance, and when attaching a special filter such as a cross filter to the object side, the front lens does not rotate, so it is desirable. There is also an advantage that focusing can be performed without changing the filter effect in the direction of.

[Brief description of drawings]

FIG. 1 is a lens configuration diagram at a telephoto end of an embodiment according to the present invention, FIGS. 2A, 2B and 2C are various aberration diagrams in the wide-angle, intermediate and telephoto zoom states of the embodiment, and FIG. 3A. , 3B
FIG. 3 and FIG. 3C are graphs showing various aberrations when focusing on a short-distance object in each of the wide-angle, intermediate, and telephoto zoom states of the example. [Explanation of Signs of Main Parts] G ₁ ... first lens group, G ₃ ... third lens group G ₂ ... second lens group, G ₄ ... fourth lens group L ₁ ... first lens group Middle positive refractive power first sub-group L ₂ --- Positive refractive power second sub-group in first lens group

Claims (2)

[Claims] 1. A zoom lens having a variable power lens group and a focusing lens group disposed on the object side of the variable power lens group, wherein the focusing lens group is a first sub-group having positive refracting power. And a second partial group, the first partial group on the object side of the two partial groups is fixed with respect to the image plane, and the second partial group on the image side is moved along the optical axis. When the combined focal length of the entire focusing lens group is f and the focal length of the first sub-lens group L ₁ is f _1, it is possible to obtain 5.0 <f ₁ /f<12.0 (1) Telephoto zoom lens characterized by satisfying the conditions. 2. The first sub-unit L ₁ has a negative meniscus lens component L ₁₁ having a convex surface facing the object side and a positive lens component L ₁₂ having a surface having a stronger curvature facing the object side, Subgroup L
Reference numeral ₂ has a cemented positive lens component L ₁₃ with a surface having a stronger curvature facing the object side and a biconvex positive lens component L ₁₄ with a surface having a stronger curvature facing the object side. The refractive index of the negative meniscus lens component L ₁₁ is Na, the radius of curvature of the image side lens surface is Ra, the refractive index of the positive lens component L _{12 in} the first subgroup is Nb, and the radius of curvature of the object side lens surface is Rb. When the radius of curvature of the image-side lens surface of the image-side biconvex positive lens component L ₁₄ in the second sub-group L ₂ is Rc, 0.48 <Ra / f <0.72 (2) 0.48 <Rb / f < 0.72 (3) 1.8 <Rc / f <2.6 (5) The claim 1 characterized in that the condition is satisfied.
The telephoto zoom lens described in the item.