JPH0360409B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a so-called standard zoom lens that spans the standard range from wide-angle to telephoto. Traditionally, a two-group zoom lens with a diverging lens group in front has been used as a wide-angle zoom lens.
In addition, although it is advantageous for aberration correction at the wide-angle end, if the configuration has a zoom ratio of about 3x or more, including up to telephoto, spherical aberration and coma aberration due to the diverging lens group will remain as residual aberrations and have a negative impact on the telephoto end. It is not suitable for high-power zoom lenses. On the other hand, in a three-group or four-group zoom lens in which a periphery lens group precedes the lens group, in order to keep the diameter of the front lens small, the refractive power of each group of the zoom lens must be strengthened and the amount of movement must be reduced. However, in this case, aberration fluctuations due to zooming become large, making it difficult to keep distortion, astigmatism, and spherical aberration small over the entire zooming range. If you try to create a compact zoom lens by increasing the refractive power of a zoom lens with a 3-group configuration of positive and negative lenses, or a zoom lens with a 4-group configuration of positive, negative, and positive lenses, the refractive power of the negative lens group will become extremely strong, so the above drawbacks will not occur. Especially noticeable. On the other hand, in a zoom lens with a four-group configuration of positive, negative, negative, and positive, the refractive power is distributed to two sets each of the positive lens group and the negative lens group. strong groups can be avoided, and the above disadvantages can be alleviated. In this way, in a standard zoom lens, there are four
For those that adopt the group structure, Japanese Patent Application Laid-open No. 49-76540,
These include JP-A-53-97451, JP-A-55-121418, and JP-A-56-48607. However, even though the burden on the refractive power of each group has been reduced, aberration fluctuations due to zooming still remain, astigmatism, especially distortion, fluctuations are still large, and the front lens diameter is still not large enough to be described as compact. The drawback is that it is difficult to downsize. This shortcoming is even more noticeable with high-power zoom lenses that extend from the wide-angle range to the telephoto end, even among standard zoom lenses. The present invention solves these drawbacks, reduces the diameter of the front lens, and achieves both a more compact lens overall shape and higher performance. Basically, the present invention adopts a four-group configuration of positive, negative, negative, and positive from the above-mentioned object, distributes the refractive power to two positive lens groups and two negative lens groups, and then adjusts the refractive power of each group. We created a strong and reasonable power distribution and adopted the zoom method described below. The conventional zoom system of a zoom lens consisting of four groups (positive, negative, negative, positive) from the object side is shown in Fig. 1, and the zoom system according to the present invention is shown in Fig. 2.As is clear from the figures, it is clear that the conventional optical axis A major feature is that the second group _G2 , which has negative refractive power and has the strongest refractive power, is now a fixed group, which was used to form a variable magnification system by moving above. Then, the first group _G1 with positive refractive power, which was moved only for focusing as a focusing system, and the fourth group _G4 with positive refractive power, which was fixed relative to the image plane as a relay system, were moved from wide-angle W to telephoto. By moving it toward the object side over T, the fourth group G ₄ also has a variable magnification effect, and the resulting movement of the image plane is controlled by the third group with negative refracting power.
It is a zoom method that uses G ₃ to compensate. 4th group
By moving both G ₄ and the first group G ₁ toward the object side, a variable power system is formed, and the variable power effect is transferred to the second group G ₂
and the fourth group _G4 , it is possible to easily achieve high-power zooming. Furthermore, as shown in the figure, since the first and fourth groups move toward the object side, the total length from the front lens surface to the image surface changes in this zoom system. This change in overall length increases from the wide-angle W side to the telephoto T side. In other words, since the total length becomes longer on the telephoto T side, it is possible to reduce excessive end ratio at the telephoto end even when zooming at a high magnification. This is advantageous in correcting aberrations without making it difficult to correct aberrations due to an unreasonable telephoto ratio at the telephoto end. However, in the zoom system according to the present invention, the diaphragm moves toward the object side together with the third group during zooming, so this also contributes to reducing the diameter of the front lens compared to the conventional system. On the other hand, since the second group _G2 , which has a strong refractive power, can be fixed and the first group _G1 and fourth group _G4 can be moved monotonically toward the object side, it can be configured with a simple mechanism, so sufficient accuracy can be ensured. . In the zoom lens according to the present invention, the focal length of the first group is f ₁ , the focal length of the second group is f ₂ , and the focal length of the third group is When the distance is f ₃ and the focal length of the fourth group is f ₄ , (1) 2.7<f ₁ /−f ₂ ≦3.0 (2) 3.2<f ₃ /f ₂ <3.9 (3) 1.3<f ₄ / It is necessary to satisfy the conditions −f ₂ <1.7 for compactness and aberration correction. If the lower limit of conditional expression (1) is exceeded, the refractive power of the first group becomes relatively strong, making it difficult to correct astigmatism caused by light rays incident at a large angle, and fluctuations in spherical aberration due to zooming. becomes larger. Conversely, if the upper limit is exceeded,
Although it is advantageous for correcting astigmatism and spherical aberration due to zooming, the overall length increases, the position where the principal ray passes through the first group moves away from the optical axis, and the amount of extension due to focusing also increases, leading to an increase in the diameter of the front lens. . In conditional expression (2), if the total length is smaller than the lower limit,
The front lens diameter can be made smaller, but the 2nd and 3rd groups on the telephoto side
As the interval between principal points of the group becomes narrower, the Petzval sum becomes negative. On the other hand, when the upper limit is exceeded, the distance between the third and fourth groups becomes narrower, and in conditional expression (3), the front lens diameter inevitably increases due to the increase in overall length and the separation of the chief ray from the optical axis. If the lower limit is exceeded, the overall length will be shortened, but the distance between the principal points of the 3rd and 4th groups will become smaller, causing mechanical interference between the two groups on the telephoto side.On the other hand, if the upper limit is exceeded, the overall length will increase. At the same time, the Petzval sum suddenly drops to negative. If you use a zoom lens that satisfies the above condition and set the zoom ratio to about 3x, the movement of the third group to correct the image plane movement due to zooming will be on the object side in the same direction as the movement of the first and fourth groups. Since the cam mechanism moves monotonically, the cam mechanism is considered to be simpler, which facilitates manufacturing and is advantageous in maintaining high accuracy. Although both the first group and the fourth group are moved toward the object side, various functions can be used to describe the relationship between the two groups mechanically or in terms of aberration correction.
If sufficient aberration correction can be achieved by devising the structure of each group, then this function can be made simpler, for example, by taking a constant, which will further simplify the mechanism. Further, by setting the link constant to 1, that is, by moving the first group and the fourth group as one unit, the structure can be simplified and the accuracy can be further improved. In the basic configuration of the present invention as described above, each group is specifically configured as follows. The first group G ₁ with positive refractive power consists of two components, a positive lens and a positive lens, which are bonded together in order from the object side, and the focal length of the final Menscus lens is f ₁₃ and the combined focal length of the first group is f ₁ . It is desirable to satisfy the following condition (4): 1.1<f ₁₃ /f ₁ <1.6. The laminated lens is a positive lens with a gentle curvature on the image side, and the final positive lens has a meniscus shape convex toward the object side, so that light rays that enter each lens at a position far from the optical axis are strongly refracted. This makes it difficult for higher-order spherical aberration to occur. When the lower limit of conditional expression (4) is exceeded, the refractive power of the image-side meniscus positive lens becomes strong, and fluctuations in spherical aberration and astigmatism due to zooming become large, making it difficult to correct aberrations. On the other hand, if the upper limit is exceeded, the refractive power of the bonded lens becomes strong, the diameter of the front lens increases, and higher-order aberrations occur, making it impossible to correct aberrations properly. The second group _G2 with negative refractive power is composed of three components: a negative meniscus lens with a negative refractive power convex to the object side, and a positive meniscus lens convex to the object side, and the radius of curvature is changed from the first group side. In order, r ₆ , r ₇ ,...
..., r ₁₂ , and the positive constituting the bonded lens,
When the focal length of the negative lens is f _a , the f _b Atsube numbers are ν _a and ν _b , and the combined focal length is f _ab (5) −2.0≦r ₇ +r ₆ /r ₇ −r ₆ ≦− 1.0 (6) −0.1＜f _ab (1/f _a 〓 _a +1/f _b 〓 _b )＜−0.01 (7) 3.0＜r ₁₂ +r ₁₁ /r ₁₂ −r ₁₁ ＜20.0 is satisfied. desirable. Conditional expression (5) defines the bending strength of the negative meniscus lens closest to the object in the second group. Ambient light passing through the first group enters the second group at a large angle and is largely refracted. Therefore, if conditional expression (5) is not satisfied, it becomes difficult to correct astigmatism due to the influence of higher-order aberrations. Furthermore, it becomes difficult to correct fluctuations in spherical aberration due to zooming, which poses a problem in making the lens compact. Conditional expression (6) is indispensable for effectively and reasonably correcting chromatic aberrations, especially chromatic aberrations of magnification, which occur from surfaces with strong curvature due to compactness. Conditional expression (7) defines the bending strength of the positive meniscus lens necessary to correct spherical aberration on the telephoto side. The third group _G3 with negative refractive power consists of a biconcave lens and a biconvex lens bonded together, and has a radius of curvature of
When r ₁₃ , r ₁₄ , and r ₁₅ are set in order from the second group side, it is desirable to satisfy the following condition (8): 1.5<r ₁₅ +r ₁₃ /r ₁₅ −r ₁₃ <4.0. By satisfying conditional expression (8) that defines the bending of the bonded negative lens as the third group, it is possible to ensure the symmetry of fluctuations in spherical aberration and coma aberration due to zooming. The fourth group G ₄ with positive refractive power is composed of three components in order from the object side: a biconvex lens, a bonded positive lens for achromatization, and a biconvex lens.The positive front group is followed by a biconcave lens, a positive lens, and a refractive lens. It consists of a composite of three components of a bonded lens with a positive force and a rear group with a negative force. The curvature of the bonding surface of the front group is r ₁₉ , and the refractive index of the positive and negative lenses before and after it is n ₁₁ and n _{12 ,} respectively.
The curvature of the last positive lens in the front group is r ₂₁ from the object side,
When r ₂₂ , it is desirable to satisfy the conditional expression (9) 0.002<n ₁₁ +n ₁₂ /r ₁₉ <0.02 (10) 0.4<r ₂₂ +r ₂₁ /r ₂₂ −r ₂₁ <1.0. Conditional expression (9) is a quantity representing the refractive power of the bonded surfaces, and by satisfying this condition, comatic aberration and chromatic aberration can be balanced and corrected at the same time. Outside the range of the conditional expression, comatic aberration and chromatic spherical aberration cannot be corrected at the same time. Conditional expression (10) is a regulation regarding the bending strength of the last positive lens in the front group, and is effective in correcting fluctuations in spherical aberration due to zooming, especially when increasing the aperture f-number at the telephoto end. The radius of curvature of the negative lens on the object side of the rear group is sequentially r ₂₃ and r ₂₄
When the thickness is d, the refractive index of the positive lens and negative lens of the image side bonded positive lens are n ₁₆ and n ₁₇ respectively, and the radius of curvature of the bonded surface is r ₂₈ , (11) 0.08<d/f ₄ (12) −0.9<r ₂₄ +r ₂₃ /r ₂₄ −r ₂₃ <−0.4 (13) It is desirable to satisfy the following conditional expression: 0.002<n ₁₆ −n ₁₇ /r ₂₈ <0.02. Spherical aberration is improved by satisfying conditional expression (11), and conditional expression (12) specifies the bending strength of the object side negative lens in the rear group, and if the above expression (12) is not satisfied, astigmatism occurs. Conditional expression (13), which makes it difficult to correct aberrations and coma in a well-balanced manner, is desirable for defining the surface power of the final bonded surface and correcting coma. In the present invention, it is possible to increase the zoom ratio from about 3 times to more, but in order to ensure high accuracy, in the embodiment, as described above, the third group forming the correction system is always set to the first group during zooming. group and fourth
We adopted a zooming range that was in the same direction as the movement of the group. Furthermore, the relationship between the first group and the fourth group is always 1:1, that is, the link constant is set to 1 to simplify movement. The specifications of three embodiments according to the present invention are shown below. In each specification table, the subscript number indicates the order from the object side, and the refractive index is for the d-line (λ = 587.6 nm).
is the value for

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[Table] Each of the above embodiments is a zoom lens for a 35mm single-lens reflex camera, and has a focal length of f = 36 to 102mm, which covers the wide-angle to semi-telephoto range, and a zoom ratio of about 3 times. For example, in the second embodiment, the F member is 3.4 to 4.6, and in the third embodiment, it is approximately 3.5 over the entire range. Furthermore, the front ball diameter,
The overall length is extremely compact, and the filter size installed at the front of the lens system is as small as 52mm in the first and second embodiments, and even though the F number is set to 3.5 over the entire area in the third embodiment. Regardless, it is set at 62mm. Lens configuration diagrams of the first to third embodiments are shown in FIGS. 3, 4, and 5, respectively. Also, 1st to 3rd
The aberration diagrams of the examples are shown in Fig. 6, Fig. 7, and Fig. 8, respectively.
Shown in the figure. All aberration diagrams are taken at infinite shooting distance, and show spherical aberration Sph, astigmatism Ast, and distortion aberration.
Dis is shown for each of the shortest, middle, and longest focal lengths. It is clear from the aberration diagrams that each example has a very small front lens diameter as a zoom lens for a 35 mm single-lens reflex camera, yet all aberrations are sufficiently well corrected. As described above, although the present invention has a wide range of variable power from wide-angle to telephoto, the diameter of the front lens is quite small and the overall shape of the lens is compact.
Moreover, a zoom lens that always has excellent imaging performance over the entire zoom range can be achieved.

[Brief explanation of drawings]

Fig. 1 shows the movement trajectory of a conventional zoom lens with four groups, Fig. 2 shows the movement trajectory of the zoom lens according to the present invention, and Figs. The lens configuration diagrams of the three embodiments, and FIGS. 6, 7, and 8 are diagrams of various aberrations at infinity for each embodiment. [Explanation of symbols of main parts], G ₁ ... 1st group, G ₂
... 2nd group, G ₃ ... 3rd group, G ₄ ... 4th group.

Claims (1)

[Claims] 1. In order from the object side, a first lens group with positive refractive power;
The second lens group has negative refractive power, and the third lens group also has negative refractive power.
The lens group has a fourth lens group with positive refractive power, and when changing magnification from wide-angle to telephoto, the second group is fixed to the image plane, and both the first group and the fourth group are fixed to the image plane. side, the third group moves independently of the other groups, and the first, second, third, fourth
When the focal lengths of each group are f ₁ , f ₂ , f ₃ , f ₄ , (1) 2.7<f ₁ /−f ₂ ≦3.0 (2) 3.2<f ₃ /f ₂ <3.9 (3) 1.3 A zoom lens that satisfies the following conditions: <f ₄ /−f ₂ <1.7.