JP3140489B2 - Zoom lens - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a zoom lens having a large aperture ratio and a high zoom ratio, which has a short overall length and uses a rear focus in a four-group configuration.

[0002]

2. Description of the Related Art In recent years, miniaturization and cost reduction of video cameras have been rapidly progressing. 2. Description of the Related Art In an image pickup unit of a video camera, an image pickup device (device) has been reduced in size from 2/3 inch and 1/2 inch to 1/3 inch and 1/4 inch. At the same time, as for a zoom lens for a video camera, a small and low-cost lens suitable for a 1/3 inch or 1/4 inch lens is desired.

Conventionally, a zoom lens having a high zoom ratio of 6 times or more for a video camera has four groups of positive, negative, negative, and positive from the object side. The most frequently corrected the image position. However, recently, the third lens group has been omitted, and the fourth lens group has been divided into a front lens group and a rear lens group. ) Type lenses have been proposed. As such a conventional example, Japanese Patent Application Laid-Open No. 62-24 / 1987
No. 213, JP-A-62-178917, JP-A-62-278.
206516, JP-A-62-215225, JP-A-2-53017, and JP-A-2-39011.

[0004]

SUMMARY OF THE INVENTION The above conventional example is 2 /
It is designed for a 3 inch, 1/2 inch size image sensor. The actual lens, besides the optical effective diameter,
It is necessary to secure a margin for the centering of the lens (a margin for centering) and a margin for holding the frame when it is placed in the mirror frame. Therefore, the border of the lens needs to have an optical effective diameter plus a diameter of about 2 mm and about 0.5 mm. 1/3 of the above conventional example
When applied to inch and quarter inch sizes, the margin to be secured is 0.5 mm or less, or even a negative value, making fabrication extremely difficult or impossible.

Further, those described in JP-A-62-206516 and JP-A-62-215225 are described in
The zoom ratio is about three times.

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and an object of the present invention is to provide a lens suitable for a 1/3 inch and 1/4 inch size image pickup device, and a zoom ratio of about 8 times. Another object of the present invention is to provide a compact zoom lens having a small number of components.

[0007]

According to the present invention, there is provided a zoom lens system comprising: a first lens unit having a positive refractive power; and a second lens unit having a negative refractive power and movable during zooming. A variable power system consisting of two groups, a third group having a positive refractive power
A fourth group having a positive refractive power and movable for zooming and for focusing, and an imaging system including two groups. An aspheric surface which is composed of two positive lenses and one negative lens whose surfaces are convex, and at least one of the lens surfaces of the third group has a positive refractive power that decreases as the distance from the optical axis increases. The fourth group includes only one positive lens.
At least one surface is an aspherical surface whose positive refractive power decreases as the distance from the optical axis increases, and further satisfies the following conditional expression.

(1) 0.9 <f ₃ / (f _W · f _T ) ^1/2 <1.3 (2) 0.5 <| f _3-3 | / f ₃ <0.9 (3) −0.1 <β _4S <0.3 (4) 0.7 <β _4T / β _4W <1.5 where f _W and f _T are the focal length of the entire system at the wide-angle end and the telephoto end, respectively, and f ₃ Is the composite focal length of the third lens unit, f _3-3 is the third
The focal length of the negative lens of the group, β _4S, is the focal length of the entire system (f
_W・ f _T ) ^1/2 , magnification of the 4th group when focusing on an object point at infinity, β
_4W and β _4T are the magnifications of the fourth lens group at the time of focusing on an object point at infinity when the focal length of the entire system is f _W and f _T , respectively.

[0009]

The reason why the configuration of the present invention is adopted will be described below together with the operation. In an optical system for a 1 / 3-inch or 1 / 4-inch image sensor, the absolute amount of the marginal or medium thickness (lens center thickness) to be secured is the same as that for 2/3 inches and 1/2 inches. The effect on the size of the optical system, such as the overall length of the lens, becomes relatively large, and this becomes more remarkable as the image pickup device becomes smaller.

Therefore, as the optical system of a 1/3 inch or 1/4 inch size image sensor, the conventional image sensor cannot be obtained simply by setting the thickness of the lens to be larger than the radius of curvature of the lens. The miniaturization of the optical system does not progress very much with the miniaturization, and the merit of miniaturizing the imaging device is diminished. In particular, as the number of constituent lenses increases, it becomes more difficult to reduce the size.

In the case of the lens of the present invention, in order to reduce the size and cost, it is desirable to eliminate the lens having little effect as much as possible and to configure the lens with the minimum necessary number of lenses.

For this purpose, the configuration of the imaging system composed of the third and fourth units requires the most contrivance. Therefore, in the present invention, the above configuration is adopted.

That is, in order to reduce the size of the lens system, it is important to provide a sufficient refractive power to the third lens unit and to arrange the principal point of the third lens unit as close to the object as possible. In order to arrange the principal point as close to the object side as possible, in the third group, two positive lenses having convex surfaces on the object side are arranged in order from the object side, and a negative lens is arranged on the most image side.

In order to satisfactorily correct chromatic aberration,
It is necessary to use at least one negative lens in the imaging system. In the present invention, the axial chromatic aberration and the lateral chromatic aberration are simultaneously corrected by the negative lens disposed closest to the image in the third lens unit.

When the third lens unit is configured as described above, the spherical aberration is insufficiently corrected. By introducing an aspheric surface in which the positive refractive power becomes weaker as the distance from the optical axis increases in the third group,
It is possible to achieve both miniaturization and correction of this aberration.

Furthermore, by introducing an aspheric surface whose positive refractive power becomes weaker as the distance from the optical axis increases in the fourth unit, coma and astigmatism of the imaging system can be corrected. It can be composed of one positive lens.

The conditional expressions (1) to (4) further define the lens configuration numerically, and are used to reduce the size of the lens.

(1) 0.9 <f ₃ / (f _W · f _T ) ^1/2 <1.3 (2) 0.5 <| f _3-3 | / f ₃ <0.9 (3) −0.1 <β _4S <0.3 (4) 0.7 <β _4T / β _4W <1.5 The above conditional expression (1) defines the focal length of the third lens unit, and the upper limit thereof. When the value exceeds the above, the conjugate of the imaging system becomes long, which is not suitable for the purpose of the present invention. On the other hand, when the value exceeds the lower limit, the second and third units easily mechanically interfere with each other, which is not preferable.

Conditional expression (2) defines the refractive power of the negative lens in the third lens unit. By giving the negative lens a strong refractive power, the principal point of the third lens unit is moved toward the object side. As a result, mechanical interference with the second group can be avoided, and the overall length can be reduced. If the upper limit of conditional expression (2) is exceeded, the second condition
Mechanical interference between the group and the third group is likely to occur, making it impossible to increase the zoom ratio. On the other hand, if the lower limit is exceeded, it becomes difficult to correct spherical aberration and coma even if an aspherical surface is introduced.

Conditional expression (3) indicates that the fourth condition
When the lower limit is exceeded, the distance between the principal points of the imaging system becomes large, so that the overall length is likely to be long. The amount of focusing and feeding of the fourth lens group is increased, and a large interval with the third lens group is required, which is not preferable.

When zooming from the wide-angle side to the telephoto side at the object point at infinity, the fourth lens unit keeps the image position constant by taking a locus that once moves to the object side and then returns to the image side. I have. By drawing such a locus, the space on the image side can be used more effectively than the third lens unit, and the size of the imaging system can be reduced.

Conditional expression (4) represents the fourth condition at the wide-angle end and the telephoto end.
This defines the ratio of the imaging magnification of the group. Conditional (4)
When the lower limit is exceeded, the fourth unit approaches the image side on the wide angle side, and when the upper limit is exceeded, the fourth unit approaches the image side on the telephoto side,
It becomes easy to mechanically interfere with an optical member such as an optical low-pass filter. In order to avoid mechanical interference, it is necessary to lengthen the back forks, which increases the overall length of the lens, which is not preferable.

For the above reasons, conditional expressions (1) to (4)
Needs to be satisfied.

As described above, since the conjugate of the imaging system can be shortened and mechanical interference with the variable power system can be prevented, a zoom lens having a short overall length can be obtained.

Further, it is more preferable to satisfy the following conditional expressions (5) to (7) regarding the imaging system so as to be advantageous for aberration correction. (5) -2 <SF _3-2 <-0.8 (6) 0.6 <SF _3-3 <1.8 (7) -3.5 <SF _3-2 / SF _3-3 <0 , SF _3-2 is the shape factor of the second positive lens from the object side of the third group ｛≡ (r ₁ + r ₂ ) / (r ₁ −
r ₂ )｝ (r ₁ is the object side, r ₂ is the radius of curvature of the image side surface), and SF _3-3 is the shape factor of the negative lens of the third group.

Conditional expressions (5) and (6) define the shapes of the adjacent positive lens and negative lens of the third lens unit, respectively, and conditional expression (7) defines the ratio of their shape factors. It is specified.

If the lower limits of conditional expressions (5), (6) and (7) are exceeded, spherical aberration will be overcorrected and the meridional image surface will be curved to the negative side. If the upper limits of conditional expressions (5), (6), and (7) are exceeded, conversely, spherical aberration will be insufficiently corrected, and the meridional image surface will be curved to the positive side, which is not preferable.

Also, regarding the variable power system, the overall length is shortened,
In addition, if conditions for favorably correcting aberrations are provided, a variable power lens having an even shorter overall length and excellent imaging performance can be obtained.

For this purpose, the first lens group includes 1 from the object side.
It is preferable to constitute a total of three negative meniscus lenses, a biconvex positive lens, and a positive meniscus lens having a convex surface facing the object side. If an aspherical surface is introduced, one negative meniscus lens and one It is also possible to use a total of two biconvex lenses.

Further, the second group is preferably composed of two negative lenses and one positive lens because it has a strong negative refractive power.

It is more preferable that the following conditional expression (8) is satisfied after the variable power system is configured as described above. (8) 0.9 <f ₁ / ｛f _T (f _W · f _T ) ^1/2 ｝ ^1/2 <1.4 where f ₁ is the composite focal length of the first lens unit.

Condition (8) defines the focal length of the first lens unit. If the upper limit is exceeded, the overall length of the variable power system becomes longer and the diameter of the front lens becomes larger. On the other hand, if the lower limit is exceeded, spherical aberration near the telephoto end tends to be insufficiently corrected.

[0033]

Next, embodiments 1-4 of the zoom lens of the present invention will be described.
Will be described. Although lens data of each embodiment will be described later, FIGS. 1 and 2 show lens cross sections of the first and third embodiments at the wide-angle end (W), the standard state (S), and the telephoto end (T), respectively. The lens section of the second embodiment is almost the same as that of the first embodiment, and the lens section of the fourth embodiment has a positive meniscus lens which is the second lens of the third group G3 and a negative lens which is the third lens. Except for the fact that the meniscus lenses are stuck, they are almost the same as those of the first embodiment, and therefore are not shown.

For the first lens group G1, Examples 1, 2, and 4
Is composed of a total of three elements from the object side: a cemented lens of a negative meniscus lens having a convex surface facing the object side and a biconvex positive lens, and a positive meniscus lens having a convex surface facing the object side. It consists of a total of two lenses: a negative meniscus lens with the convex surface facing the side and a cemented lens of a biconvex positive lens. With respect to the second group G2, in Examples 1, 2, and 4, the negative meniscus lens having the convex surface facing the object side from the object side, and the biconcave negative lens and the positive meniscus lens having the convex surface facing the object side are bonded. Example 3 consisting of a total of three lenses
Consists of a total of three lenses: a biconcave negative lens, and a cemented lens of a biconcave negative lens and a positive meniscus lens with the convex surface facing the object side. For the third group, Examples 1, 2, 4
Denotes a biconvex positive lens, a positive meniscus lens having a convex surface facing the object side, and a negative meniscus lens having a convex surface facing the object side.
Example 3 As described above, in Example 4, the second positive meniscus lens and the third negative meniscus lens are bonded together in Example 4.
Is composed of a biconvex positive lens, a positive meniscus lens having a convex surface facing the object side, and a biconcave negative lens. 4th group G
Reference numeral 4 denotes a single biconvex positive lens in each embodiment. Therefore, Examples 1, 2, and 4 were 1 in total.
Example 3 has nine lenses in total. In each embodiment, an optical member such as a filter is disposed on the image side of the fourth group G4.

In the first, second and fourth embodiments, two aspheric surfaces are used: the most object-side surface of the third lens unit G3 and the most object-side surface of the fourth lens unit G4. , Three surfaces, the most image-side surface of the first group G1, the most object-side surface of the third group G3, and the most object-side surface of the fourth group G4, are used.

The stop is disposed on the object side of the third lens unit G3. With this arrangement, the arrangement of the lens frame is easier to simplify than in the third group G3, and the relative eccentricity of the lenses in the third group G3 due to a manufacturing error of the lens frame is easily reduced.

The fourth embodiment is for a 1/3 inch size, and the first to third embodiments are for a 1/4 inch size. Obviously, the present invention can be applied to an optical system of an image pickup device of another size such as a device.

In the following, the symbols are the above, f is the focal length of the entire system, F _NO is the F number, ω is the half angle of view, r ₁ ,
r ₂ ... curvature radius of each lens surface, d _1, d ₂ ... the spacing between the lens surfaces, n _d1, n _d2 ... d-line refractive index of each lens, ν _d1, ν _d2 ... Each lens Is the Abbe number of Also,
The aspherical shape is represented by the following equation, where x is the optical axis direction and y is the direction orthogonal to the optical axis.

X = y ² / ｛r + (r ² −y ² ) ^1/2 ｙ + A ₄ y ⁴ + A ₆ y ⁶ + A ₈ y ⁸ + A ₁₀ y ¹⁰ where r is the radius of curvature on the optical axis, A ₄ , A ₆ , A ₈ and A ₁₀ are aspherical coefficients.

[0040] Example 1 f = 4.63~ 12.7 ~ 34.9 F NO = 1.40~ 1.54~ 2.30 ω = 24.4 ~ 9.4 ~ 3.4 ° r 1 = 41.6854 d 1 = 0.8000 n d1 = 1.84666 ν d1 = 23.78 r 2 = 20.0094 d _{2 = 3.5000 n d2 = 1.60311 ν} d2 = 60.70 r 3 = -77.0254 d 3 = 0.1500 r 4 = 15.6993 d 4 = 2.4000 n d3 = 1.60311 ν d3 = 60.70 r 5 = 49.7004 d 5 = ( variable) r ₆ = 186.9901 _{d 6 = 0.8000 n d4 = 1.69680} ν d4 = 55.52 r 7 = 5.4879 d 7 = 2.3418 r 8 = -7.6328 d 8 = 0.8000 n d5 = 1.60311 ν d5 = 60.70 r 9 = 7.7427 d 9 = 1.8000 n d6 = 1.84666 ν _{d6 = 23.78 r 10 = 70.1358 d} 10 = ( variable) r ₁₁ = ∞ _{(stop) d 11 = 1.5000 r 12 =} 8.3556 ( _{aspherical) d 12 = 3.7000 n d7 =} 1.59008 ν d7 = 61.20 r 13 = -26.1579 d _{13 = 0.1500 r 14 = 8.0340 d} 14 = 2.5000 n d8 = 1.60311 ν d8 = 60.70 r 15 = 55.4888 d 15 = 0.1400 r 16 = 84.9479 d 16 = 0.8000 n d9 = 1.84666 ν d9 = 23.78 r 17 = 5.9805 d 17 = (Variable) r ₁₈ = 8.6142 (aspherical surface) d ₁₈ = 2 _{.2000 n d10 = 1.59008 ν d10 =} 61.20 r 19 = -21.3236 d 19 = ( _{Variable) r 20 = ∞ d 20 =} 4.0000 n d11 = 1.51633 ν d11 = 64.15 r 21 = ∞ d 21 = 1.0000 r 22 = ∞ d _{22 = 0.7900 n d12 = 1.48749 ν} d12 = 70.20 r 23 = ∞ Aspheric surface twelfth surface A ₄ = -0.29708 × 10 ^-3 A ₆ = -0.96225 × 10 ^-6 A ₈ = -0.28575 × 10 ^-7 A ₁₀ = 0 18th surface A ₄ = -0.43387 × 10 ^-3 A ₆ = -0.10974 × 10 ^-4 A ₈ = 0.29774 × 10 ^-6 A ₁₀ = 0
.

[0041] Example 2 f = 4.63~ 12.7 ~ 34.9 F NO = 1.40~ 1.55~ 2.31 ω = 24.4 ~ 9.4 ~ 3.4 ° r 1 = 39.7855 d 1 = 0.8000 n d1 = 1.84666 ν d1 = 23.78 r 2 = 19.7060 d _{2 = 3.5000 n d2 = 1.60311 ν} d2 = 60.70 r 3 = -77.6727 d 3 = 0.1500 r 4 = 14.9490 d 4 = 2.4000 n d3 = 1.60311 ν d3 = 60.70 r 5 = 44.5359 d 5 = ( variable) r ₆ = 161.2246 _{d 6 = 0.8000 n d4 = 1.69680} ν d4 = 55.52 r 7 = 5.3175 d 7 = 2.2157 r 8 = -6.8922 d 8 = 0.8000 n d5 = 1.60311 ν d5 = 60.70 r 9 = 7.8130 d 9 = 1.8000 n d6 = 1.84666 ν _{d6 = 23.78 r 10 = 103.0782 d} 10 = ( variable) r ₁₁ = ∞ _{(stop) d 11 = 1.5000 r 12 =} 8.5218 ( _{aspherical) d 12 = 3.7000 n d7 =} 1.59008 ν d7 = 61.20 r 13 = -22.9567 d _{13 = 0.1500 r 14 = 10.5737 d} 14 = 2.5000 n d8 = 1.60311 ν d8 = 60.70 r 15 = 412.1577 d 15 = 0.1400 r 16 = 33.9090 d 16 = 0.8000 n d9 = 1.84666 ν d9 = 23.78 r 17 = 6.2415 d 17 = (Variable) r ₁₈ = 8.2930 (aspherical surface) d ₁₈ = _{2.2000 n d10 = 1.59008 ν d10 =} 61.20 r 19 = -30.7228 d 19 = ( _{Variable) r 20 = ∞ d 20 =} 4.0000 n d11 = 1.51633 ν d11 = 64.15 r 21 = ∞ d 21 = 1.0000 r 22 = ∞ d 22 _{= 0.7900 n d12 = 1.48749 ν d12} = 70.20 r 23 = ∞ Aspheric surface twelfth surface A ₄ = -0.37946 × 10 ^-3 A ₆ = -0.66806 × 10 ^-6 A ₈ = -0.34238 × 10 ^-7 A ₁₀ = 0 18th surface A ₄ = -0.21235 × 10 ^-3 A ₆ = -0.17772 × 10 ^-4 A ₈ = 0.82441 × 10 ^-6 A ₁₀ = 0
.

[0042] Example 3 f = 4.63~ 12.7 ~ 34.9 F NO = 1.40~ 1.48~ 2.18 ω = 24.4 ~ 9.4 ~ 3.4 ° r 1 = 15.9432 d 1 = 0.8000 n d1 = 1.84666 ν d1 = 23.78 r 2 = 11.3113 d _{2 = 5.7468 n d2 = 1.59008 ν} d2 = 61.20 r 3 = -62.9131 ( aspherical) d ₃ = _{(variable) r 4 = -148.5652 d 4 =} 0.8000 n d3 = 1.69680 ν d3 = 55.52 r 5 = 6.6022 d 5 = _{1.6717 r 6 = -8.6546 d 6 =} 0.8000 n d4 = 1.60311 ν d4 = 60.70 r 7 = 7.9368 d 7 = 1.8000 n d5 = 1.84666 ν d5 = 23.78 r 8 = 39.2330 d 8 = ( variable) r ₉ = ∞ (aperture _{) d 9 = 1.5000 r 10 =} 8.0439 ( _{aspherical) d 10 = 3.6000 n d6 =} 1.59008 ν d6 = 61.20 r 11 = -33.3988 d 11 = 0.1500 r 12 = 8.6043 d 12 = 1.9647 n d7 = 1.58913 ν d7 = 61.18 _{r 13 = 37.6452 d 13 = 0.2494} r 14 = -293.4076 d 14 = 0.8000 n d8 = 1.84666 ν d8 = 23.78 r 15 = 7.9697 d 15 = ( variable) r ₁₆ = 9.6081 (aspherical) d ₁₆ = 2.4000 n _d9 = 1.59008 ν _d9 = 61.20 r ₁₇ = -15.6067 d ₁₇ = (variable) r ₁₈ = _{∞ d 18 = 4.0000 n d10 =} 1.51633 ν d10 = 64.15 r 19 = ∞ d 19 = 1.0000 r 20 = ∞ d 20 = 0.7900 n d11 = 1.48749 ν d11 = 70.20 r 21 = ∞ Aspheric coefficient Third surface A ₄ = 0.25784 × 10 ^-4 A ₆ = -0.10511 × 10 ^-7 A ₈ = -0.43556 × 10 ^-9 A ₁₀ = 0 Tenth surface A ₄ = -0.23994 × 10 ^-3 A ₆ = -0.13012 × 10 ^-5 A ₈ = -0.33662 × 10 ^-7 A ₁₀ = 0 Surface 18 A ₄ = -0.66718 × 10 ^-3 A ₆ = -0.69577 × 10 ^-5 A ₈ = 0.43999 × 10 ^-7 A ₁₀ = 0
.

[0043] Example 4 f = 6.18~ 17.0 ~ 46.6 F NO = 1.23~ 1.35~ 2.04 ω = 27.0 ~ 10.5 ~ 3.9 ° r 1 = 59.6520 d 1 = 1.2000 n d1 = 1.84666 ν d1 = 23.78 r 2 = 28.1378 d _{2 = 5.1167 n d2 = 1.60311 ν} d2 = 60.70 r 3 = -98.6623 d 3 = 0.1500 r 4 = 20.5493 d 4 = 3.4943 n d3 = 1.60311 ν d3 = 60.70 r 5 = 60.5275 d 5 = ( variable) r ₆ = 677.3485 _{d 6 = 0.7500 n d4 = 1.69680} ν d4 = 55.52 r 7 = 7.2795 d 7 = 3.2501 r 8 = -10.0987 d 8 = 0.6000 n d5 = 1.48749 ν d5 = 70.20 r 9 = 10.4579 d 9 = 2.1651 n d6 = 1.80518 ν _{d6 = 25.43 r 10 = 54.2649 d} 10 = ( variable) r ₁₁ = ∞ _{(stop) d 11 = 1.5000 r 12 =} 11.6380 ( _{aspherical) d 12 = 4.8254 n d7 =} 1.66524 ν d7 = 55.10 r 13 = -61.1179 d _{13 = 0.3000 r 14 = 12.1328 d} 14 = 3.7831 n d8 = 1.56873 ν d8 = 63.16 r 15 = 231.9990 d 15 = 1.0000 n d9 = 1.84666 ν d9 = 23.78 r 16 = 8.4636 d 16 = ( variable) r ₁₇ = 9.8654 ( Aspheric surface) d ₁₇ = 3.6578 n _d10 = 1.58973 ν _{d10 = 60.78 r 18 = -28.2490 d 18} = ( _{Variable) r 19 = ∞ d 19 =} 5.5000 n d11 = 1.54771 ν d11 = 62.83 r 20 = ∞ d 20 = 1.2100 r 21 = ∞ d 21 = 0.6000 n d12 = 1.48749 ν _d12 = 70.20 r ₂₂ = ∞ Aspheric surface twelfth surface A ₄ = -0.88077 × 10 ^-4 A ₆ = -0.43861 × 10 ^-6 A ₈ = 0.21447 × 10 ^-8 A ₁₀ = -0.41615 × 10 ^-10 Seventeenth surface A ₄ = -0.22662 × 10 ⁻³ A ₆ = −0.16888 × 10 ⁻⁵ A ₈ = 0.24838 × 10 ⁻⁷ A ₁₀ = −0.65561 × 10 ⁻⁹ .

The spherical aberration, astigmatism, distortion, and lateral chromatic aberration at the wide-angle end (W), the standard state (S), and the telephoto end (T) of Examples 1 to 4 are respectively shown in FIGS. Shown in the figure.

In each embodiment, the above-mentioned conditional expressions (1) to (1) are used.
The value of (8) is shown in the following table.

[0046]

As described above, according to the present invention,
It is suitable for a small-sized image sensor having a size of 1/3 inch or 1/4 inch, and can provide a small zoom lens having a high zoom ratio of about 8 times and a small number of components.

[Brief description of the drawings]

FIG. 1 is a sectional view of a zoom lens according to a first embodiment of the present invention at a wide-angle end (W), a standard state (S), and a telephoto end (T).

FIG. 2 is a lens sectional view similar to FIG. 1 of a third embodiment.

FIG. 3 is an aberration diagram showing spherical aberration, astigmatism, distortion, and lateral chromatic aberration at the wide-angle end (W), the standard state (S), and the telephoto end (T) in Example 1.

FIG. 4 is an aberration diagram similar to FIG. 3 of the second embodiment.

FIG. 5 is an aberration diagram similar to FIG. 3 of the third embodiment.

FIG. 6 is an aberration diagram similar to FIG. 3 of the fourth embodiment.

[Explanation of symbols]

 G1 first group G2 second group G3 third group G4 fourth group

Continuation of the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) G02B 9/00-17/08 G02B 21/02-21/04 G02B 25/00-25/04

Claims (1)

(57) [Claims] 1. A zooming system comprising, in order from an object side, a first group having a positive refractive power, a second group having a negative refractive power and movable during zooming, and a positive refractive power. And a fourth group having a positive refractive power and movable for zooming and for focusing, and an imaging system consisting of two groups, wherein the third group is arranged from the object side. The positive refractive power becomes weaker as at least one of the lens surfaces of the third group is away from the optical axis, and comprises two positive lenses having a convex surface on the object side and one negative lens. The fourth group is composed of only one positive lens, and among the lens surfaces of the fourth group, at least one surface is an aspheric surface whose positive refractive power becomes weaker as the distance from the optical axis increases, Further, a zoom lens characterized by satisfying the following conditional expression: (1) 0.9 <f ₃ / (f _W · f _T ) ^1/2 <1. 3 (2) 0.5 <| f _3-3 | / f ₃ <0.9 (3) −0.1 <β _4S <0.3 (4) 0.7 <β _4T / β _4W <1. 5 where f _W and f _T are the focal lengths of the entire system at the wide-angle end and the telephoto end, respectively, f ₃ is the composite focal length of the third group, f _3-3 is the focal length of the negative lens of the third group, β _4S Is the focal length of the entire system is (f _W · f _T ) ^1/2 , the magnification of the fourth unit when focusing on an object point at infinity, and β _4W and β _4T are the focal lengths of the entire system are f _W and f _{T respectively.} Is the magnification of the fourth unit when focusing on an object point at infinity.