JPH0527167A - Zoom lens - Google Patents

Abstract

PURPOSE:To obtain the small-sized zoom lens which has about X8 variable power rate and consists of a small number of elements by using specific four groups of lens elements. CONSTITUTION:This zoom lens consists of a power variation system consisting of a 1st positive group G1 and a 2nd negative group G2 which is movable at the time of power variation and an image formation system consisting of a 3rd positive group G3 and a positive group G4 which is movable in the power variation and for focus adjustment. The G3 consists of two positive lenses which are convex to the object side and one negative lens and the G4 consists of one positive lens; and an aspherical surface which decreases in positive refracting power with the distance from the optical axis is used as at least one of the lens surfaces of the G3 and G4. Then the conditions shown by inequalities I-IV are satisfied. Here, fw and fr are the focal lengths of the whole system at the wide-angle end and telephoto end, f3 the composite focal length of the G3, f3-3 the focal length of the negative lens of the G3, beta45 the power of the G4 at the time of infinite-distance object point focusing when the focal length of the whole system is (fw,fT)1/2, and beta4w and beta4T the power of the G4 at the time of the infinite-distance object point focusing when the focal length of the whole system are fw and fT.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art In recent years, video cameras have been rapidly reduced in size and cost. In the image pickup section of a video camera, the size of the image pickup device (device) has been reduced from 2/3 inch, 1/2 inch size to 1/3 inch, 1/4 inch size. In line with this, as for a zoom lens for a video camera, a compact and low-cost lens suitable for 1/3 inch and 1/4 inch lenses is desired.

Conventionally, a zoom lens having a high zoom ratio of 6 times or more for a video camera has a four-group structure of positive, negative, negative, and positive from the object side. Was most often used to correct 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, and a new zoom (variable magnification) in which any one of them is provided with an image position correcting function and a focusing function. ) Type lenses have been proposed. As such a conventional example, JP-A-62-24
No. 213, JP-A No. 62-178917, JP-A No. 62-
No. 206516, JP-A-62-215225, JP-A-2-53017, and JP-A-2-39011.

[0004]

The above-mentioned conventional example is 2 /
It is designed for 3 inch and 1/2 inch size image pickup devices. In addition to the optical effective diameter, the actual lens is
It is necessary to secure a centering allowance for the lens (amount of allowance for centering) and a holding allowance for the frame when it is put in the lens frame. Therefore, the edge thickness of the lens needs to be about 0.5 mm with an optically effective diameter plus a diameter of about 2 mm. 1/3 of the above conventional example
When applied to the inch and 1/4 inch sizes, the margin to be secured is 0.5 mm or less, or even a negative value, which makes manufacturing extremely difficult or impossible.

Further, those described in JP-A-62-206516 and JP-A-62-215225 are:
The variable power ratio is about 3 times.

The present invention has been made in view of such a situation, and an object thereof is a lens suitable for a 1/3 inch or 1/4 inch size image pickup device, and a zoom ratio of about 8 times. It is to provide a zoom lens which has a small size and a small number of components.

[0007]

The zoom lens of the present invention which achieves the above object comprises, in order from the object side, a first group having a positive refracting power and a second group having a negative refracting power and movable during zooming. A variable power system consisting of two groups, and a third system having a positive refractive power
And an imaging system composed of two groups, a fourth group having a positive refracting power and movable during zooming and for focus adjustment, the third group from the object side to the object side. It is an aspherical surface composed of two positive lenses each having a convex surface and one negative lens, and at least one of the lens surfaces of the third lens group has a weak positive refractive power as the distance from the optical axis increases. Yes, 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 aspherical surface whose positive refracting power becomes weaker as it moves away from the optical axis, and further, the following conditional expression is satisfied.

(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 combined focal length of the third lens group, and f _3-3 is the third focal length.
The focal length of the negative lens of the group, β _4S is the focal length of the whole system is (f
_W · f _T ) ^1/2 , magnification of the 4th group when focusing on an object point at infinity, β
_4W, beta _4T is fourth group of magnification, the focal point at infinity in the focal length f _W, f _T of the whole system, respectively.

[0009]

The reason why the configuration of the present invention is adopted will be described below together with the operation. In the optical system for the 1/3 inch and 1/4 inch image pickup devices, the absolute amounts of the edge and medium thickness (lens center thickness) to be secured are the same as those for the 2/3 inch and 1/2 inch. The influence of the total lens length on the size of the optical system becomes relatively large, and this becomes more remarkable as the image pickup device becomes smaller.

Therefore, as the optical system of the 1/3 inch or 1/4 inch size image pickup element, the conventional image pickup element can be obtained by simply setting the medium thickness of the lens large with respect to the radius of curvature of the lens. The miniaturization of the optical system does not proceed so much with respect to the miniaturization, and the merit of miniaturizing the image pickup device is diminished. In particular, the more lenses the lens has, the more difficult it is to make the lens compact.

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

For that purpose, it is necessary to devise the structure of the image forming system composed of the third lens unit and the fourth lens unit. Therefore, in the present invention, the above-mentioned configuration is adopted.

That is, in order to reduce the size of the lens system, it is important to provide the third lens unit with a sufficient refractive power and to arrange the principal point of the third lens unit as close to the object side 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 the negative lens is arranged closest to the 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 negative lens arranged closest to the image side in the third lens group corrects the axial chromatic aberration and the lateral chromatic aberration at the same time.

If the third lens unit is constructed as described above, spherical aberration will be insufficiently corrected. By introducing an aspherical surface in which the positive refractive power becomes weaker with increasing distance from the optical axis,
It is possible to achieve both miniaturization and correction of this aberration.

Further, by introducing an aspherical surface whose positive refractive power becomes weaker as it moves away from the optical axis into the fourth lens unit, it becomes possible to correct coma and astigmatism of the image forming system. It can be composed of one positive lens.

The conditional expressions (1) to (4) further specify the lens configuration numerically, and are for miniaturization 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 conditional expression (1) defines the focal length of the third lens unit, and its upper limit. When the value exceeds the above range, 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 group and the third group tend to mechanically interfere with each other, which is not preferable.

Conditional expression (2) defines the refracting power of the negative lens in the third lens unit. By giving the negative lens a strong refracting power, the principal point position of the third lens unit is moved to the object side. By doing so, it is possible to avoid mechanical interference with the second group and to reduce the total length. If the upper limit of conditional expression (2) is exceeded, the second
Mechanical interference between the third lens group and the third lens group is likely to occur, and it becomes impossible to obtain a high zoom ratio. On the other hand, when the value goes below the lower limit, it becomes difficult to correct spherical aberration and coma even if an aspherical surface is introduced.

Conditional expression (3) is the fourth condition in the standard state.
It defines the magnification when focusing on an object point at infinity of the group.If the lower limit is exceeded, the principal point spacing of the imaging system will be large, and the overall length will tend to be long. The focusing extension amount of the fourth lens unit is large, and a large distance from the third lens unit 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 of once moving to the object side and then returning to the image side. There is. By drawing such a locus, the space on the image side of the third lens group can be effectively used, and the imaging system can be downsized.

Conditional expression (4) is the fourth condition at the wide-angle end and the telephoto end.
It defines the ratio of the imaging magnification of the group. Conditional (4)
If the lower limit of is exceeded, the fourth group approaches the image side on the wide-angle side, and if it exceeds the upper limit, the fourth group 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 total lens length, which is not preferable.

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

As described above, since the conjugate of the image forming system can be made short and mechanical interference with the variable power system can be prevented, a zoom lens having a short total length can be obtained.

Further, it is more preferable that the following conditional expressions (5) to (7) regarding the image forming system are satisfied 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 lens 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), SF _3-3 is the shape factor of the negative lens of the third group.

The conditional expressions (5) and (6) define the shapes of the positive lens and the negative lens adjacent to each other in the third group, and the conditional expression (7) defines the shape factor ratio thereof. It has been prescribed.

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

Also, regarding the variable power system, the total length is shortened,
In addition, if conditions are provided that can correct aberrations favorably, it is possible to obtain a variable power lens having an even shorter overall length and good imaging performance.

For that purpose, the first group is 1 from the object side.
It is preferable to compose 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 aspheric surface is introduced, one negative meniscus lens and one It is also possible to compose two biconvex positive lenses in total.

Since the second lens unit has a strong negative refractive power, it is preferable that the second lens unit be composed of two negative lenses and one positive lens.

It is more preferable that the following conditional expression (8) is satisfied after the variable power system is constructed 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 combined focal length of the first group.

Conditional expression (8) defines the focal length of the first lens unit. When the value exceeds the upper limit, the total length of the variable power system becomes long, and the diameter of the front lens also increases, while when the value goes below the lower limit, the spherical aberration near the telephoto end tends to be insufficiently corrected.

[0033]

EXAMPLES Next, Examples 1 to 4 of the zoom lens of the present invention
Will be described. Although the lens data of each example will be described later, FIGS. 1 and 2 show lens cross sections of Examples 1 and 3 at the wide-angle end (W), standard state (S), and telephoto end (T), respectively. The lens cross section of Example 2 is almost the same as that of Example 1, and the lens cross section of Example 4 is the positive meniscus lens which is the second lens of the third group G3 and the negative lens which is the third lens. Except for the fact that the meniscus lens is bonded, it is almost the same as that of the first embodiment, and therefore, illustration thereof is omitted.

For the first group G1, examples 1, 2, 4
Is a cemented lens of a negative meniscus lens having a convex surface facing the object side and a biconvex positive lens from the object side, and a positive meniscus lens having a convex surface facing the object side. It is composed of a total of two negative meniscus lenses each having a convex surface facing the side and a cemented lens having a biconvex positive lens. Regarding the second group G2, in Examples 1, 2, and 4, a negative meniscus lens having a convex surface facing the object side, a biconcave negative lens, and a positive meniscus lens having a convex surface facing the object side are cemented from the object side. Example 3 comprising a total of 3 lenses
Consists of a biconcave negative lens and a cemented lens including a biconcave negative lens and a positive meniscus lens having a convex surface facing the object side. For the third group, Examples 1, 2, 4
Is 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.
(However, as described above, in Example 4, the second positive meniscus lens and the third negative meniscus lens are bonded together.)
Consists of a biconvex positive lens, a positive meniscus lens having a convex surface on the object side, and a biconcave negative lens. 4th group G
In any of the embodiments, 4 is composed of one biconvex positive lens. Therefore, Examples 1, 2, and 4 have a total of 1
The third embodiment includes 0 lenses, and the third embodiment includes 9 lenses in total. In each of the embodiments, an optical member such as a filter is arranged on the image side of the fourth group G4.

As for the aspherical surface, in the first, second and fourth embodiments, the surface closest to the object in the third lens group G3 and the surface closest to the object in the fourth lens group G4 are used. In the above, three surfaces, that is, 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 diaphragm is arranged on the object side of the third lens group G3. With this arrangement, the configuration of the lens frame can be simplified more easily than with the third lens group G3, and the relative eccentricity of the lenses in the third lens group G3 due to a manufacturing error of the lens frame can be easily suppressed.

The fourth embodiment is intended for 1/3 inch size, and the first to third embodiments are intended for 1/4 inch size. Obviously, it can be applied to an optical system of an image pickup device of other 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. Also,
The aspherical shape is expressed 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 } + 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 and A ₄ , A ₆ , A ₈ and A ₁₀ are aspherical coefficients.

Example 1 f = 4.63 to 12.7 to 34.9 F _NO = 1.40 to 1.54 to 2.30 ω = 24.4 to 9.4 to 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 = ∞ (aperture) d ₁₁ = 1.5000 r ₁₂ = 8.3556 (aspherical surface) 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 = ∞ Aspherical surface 12th 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 = -0.109 74 × 10 ^-4 A ₈ = 0.29774 × 10 ^-6 A ₁₀ = 0
.

Example 2 f = 4.63 to 12.7 to 34.9 F _NO = 1.40 to 1.55 to 2.31 ω = 24.4 to 9.4 to 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 = ∞ (aperture) d ₁₁ = 1.5000 r ₁₂ = 8.5218 (aspherical surface) 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 coefficient 12th 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
.

Example 3 f = 4.63 to 12.7 to 34.9 F _NO = 1.40 to 1.48 to 2.18 ω = 24.4 to 9.4 to 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 surface) 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 surface) 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 surface) 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 3rd surface A ₄ = 0.25784 × 10 ^-4 A ₆ = -0.105 11 × 10 ^-7 A ₈ = -0.43556 × 10 ^-9 A ₁₀ = 0 10th surface A ₄ = -0.23994 × 10 ^-3 A ₆ = -0.13012 × 10 ^-5 A ₈ = -0.33662 × 10 ^-7 A ₁₀ = 0 18th surface A ₄ = -0.667 18 × 10 ^-3 A ₆ = -0.69577 × 10 ^-5 A ₈ = 0.43999 × 10 ^-7 A ₁₀ = 0
.

Example 4 f = 6.18 to 17.0 to 46.6 F _NO = 1.23 to 1.35 to 2.04 ω = 27.0 to 10.5 to 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 = ∞ (aperture) d ₁₁ = 1.5000 r ₁₂ = 11.6380 (aspherical surface) 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 ( Aspherical 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 coefficient 12th 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 17th surface A ₄ = -0.22662 × 10 ^-3 A ₆ = -0.168 88 x 10 ^-5 A ₈ = 0.24838 x 10 ^-7 A ₁₀ = -0.65561 x 10 ^-9 .

The spherical aberration, astigmatism, distortion, and chromatic aberration of magnification at the wide-angle end (W), the standard state (S), and the telephoto end (T) of Examples 1 to 4 are the aberrations of FIGS. 3 to 6, respectively. Shown in the figure.

The conditional expressions (1) to (1) to
The values of (8) are shown in the following table.

[0046]

As described above, according to the present invention,
It is possible to provide a zoom lens which is suitable for a small image pickup device such as 1/3 inch or 1/4 inch size, has a high zoom ratio of about 8 times, is small, and has a small number of constituent elements.

[Brief description of drawings]

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

2 is a lens cross-sectional view similar to FIG. 1 of Example 3. FIG.

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

FIG. 4 is an aberration diagram similar to FIG. 3 of Example 2.

FIG. 5 is an aberration diagram similar to FIG. 3 of Example 3;

FIG. 6 is an aberration diagram similar to FIG. 3 of Example 4.

[Explanation of symbols]

 G1 ... 1st group G2 ... 2nd group G3 ... 3rd group G4 ... 4th group

Claims (1)

Claim: What is claimed is: 1. A variable power system comprising, in order from the object side, a first lens group having a positive refractive power and a second lens group having a negative refractive power and movable during zooming. And a third lens group having a positive refractive power, and a fourth lens group having a positive refractive power and movable for zooming and for focus adjustment. The group is composed of two positive lenses having a convex surface on the object side and one negative lens from the object side, and as at least one of the lens surfaces of the third group is away from the optical axis. The fourth lens group is an aspherical surface having a weak positive refractive power, and the fourth group is composed of only one positive lens, and the positive refractive power becomes weaker as at least one of the lens surfaces of the fourth group becomes farther from the optical axis. 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, f ₃ is the combined focal length of the third lens group, and f _3-3 is the focal length 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 group at the time of focusing on an object point at infinity, β _4W and β _4T are respectively The focal lengths of the entire system are the magnifications of the fourth lens unit when focusing on an object point at infinity at f _W and f _T.