JP3028592B2 - Compact zoom lens - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a compact zoom lens, and more particularly to a zoom lens mounted on a camera such as a lens shutter camera with a built-in zoom lens.

2. Description of the Related Art At present, in order to achieve downsizing and cost reduction of a lens shutter camera with a built-in zoom lens, there is a demand for downsizing and cost reduction of a photographing lens. In order to make the lens system compact, including the amount of movement of the lens during zooming, it is necessary to increase the refractive power of each lens group. However, it can be said that increasing the refractive power while maintaining the performance is in the direction of increasing the number of lenses. On the other hand, for cost reduction, it is effective to reduce the number of lenses. Thus, the compactness and low cost of the lens system include many contradictory elements.

In addition, in the case where the lens system is wide-angled, in order to secure off-axis illuminance, the outer diameters of the lens closest to the object and the lens closest to the image in the optical system become large. In other words, when trying to widen the angle of the lens system, it is difficult to reduce the size of the camera while maintaining high performance.

Problems to be Solved by the Invention By the way, in recent years, technical progress in plastic molding, glass molding and the like has been remarkable, and it has become possible to produce aspherical surfaces at low cost.

In view of such circumstances, in the present invention, a zoom lens that achieves cost reduction, compactness, and wide angle with a small number of lenses while maintaining high optical performance by effectively using an aspheric surface is provided. The purpose is to provide.

Means for Solving the Problems In order to achieve the above object, a zoom lens according to the present invention comprises, in order from the object side, a front group having a positive refractive power and a rear group having a negative refractive power. In a zoom lens that changes magnification by changing the air gap between the rear lens group and the rear lens group, the most object side lens surface in the front lens group is concave and aspherical.

As described above, in general, in order to reduce the size of a zoom lens, it is necessary to shorten the overall length and to reduce the amount of movement of each group. In a positive / negative two-component zoom lens as in the present invention, if this is performed and a sufficient back focus is to be ensured, the refractive power of each group must be increased. As a result, the tendency for various aberrations to worsen becomes significant. In addition, in the case of widening the angle of the lens, the outer diameter of the lens closest to the object increases in order to secure sufficient off-axis illuminance.
The camera cannot be made compact. Further, it becomes difficult to secure the back focus. When the back focus becomes short, the outer diameter of the lens closest to the image becomes large. In this case, too, the camera cannot be made compact. Therefore, in order to ensure a sufficient back focus, it is necessary to increase the refractive power of each lens unit, and the performance of various aberrations and the like is deteriorated as described above.

In the present invention, the lens surface closest to the object is made concave to prevent the various aberrations from becoming worse.
Thus, the outer diameter of the lens closest to the object can be reduced while ensuring off-axis illuminance. Further, in this case, the positive distortion generated in the entire optical system by the concave surface can be returned to the negative direction, which is very effective.

Further, since an aspherical surface is used for this surface, it is possible to prevent the occurrence of coma at the peripheral portion of the screen.
It is also effective in correcting spherical aberration. If the lens on the object side is aspherical on both sides, coma at the periphery of the screen, which could not be corrected by the front surface alone, will be corrected by the rear surface. Higher-order spherical aberrations that could not be suppressed can also be corrected. In this way, by effectively using the aspherical surface, both high performance and compactness can be achieved at the same time.

In general, a positive / negative two-component zoom lens tends to generate positive distortion at the wide-angle end. In particular, when the first surface (the surface closest to the object) of the front group is a convex surface, the tendency is remarkable. However, when the first surface is concave as in the present invention, it is possible to prevent the distortion at the wide-angle end from increasing to the positive. In particular, when the curvature of the first surface is tight (when the radius of curvature is reduced), negative distortion may occur. This can be prevented by using an aspheric surface for the first surface. In order to prevent the negative distortion from increasing, it is desirable to use an aspheric surface in a direction in which the negative refractive power of the surface becomes weak.

Further, when the first surface is a concave surface, if the curvature is tight, the tendency of spherical aberration to fall to the over side becomes strong. Thus, by using an aspheric surface for the first surface as in the above case, it is possible to prevent the spherical aberration from falling to the over side. At this time, similarly to the above case, it is sufficient to introduce an aspheric surface in a direction in which the negative refractive power of the first surface becomes weaker. When such an aspherical surface is used, it is possible to prevent the lateral aberration of the off-axis light flux (especially, the lower light) from drooping downward (this causes inward coma to occur).

As described above, when the first surface is a concave surface, using an aspheric surface for the surface is effective for a plurality of aberrations, which is very preferable.

Furthermore, if at least two aspheric surfaces are provided in the front group,
Aberrations that could not be corrected by the first aspheric surface can be corrected by another aspheric surface.

It is preferable that the lens closest to the object in the front group be a negative lens, for example, a negative meniscus lens having a concave surface facing the object side.

When the lens closest to the object side is a negative lens, it is better that the principal point position of this lens is as far forward (object side) as possible.
The refractive power of the subsequent positive lens group (in the front group) can be relatively weakened, and the aberration occurring in that portion can be (relatively) reduced. When the lens surface closest to the object side is concave, the position of the principal point of the lens is located in front of the case where the lens surface is convex, and the refractive power of the positive lens group that follows can be weakened. This is advantageous for aberration correction.

Further, when the most object side lens is a negative meniscus lens, the rear surface of the lens is a positive lens surface, so that the spherical aberration generated by the front concave surface falls to the over side, or the lateral aberration occurs. Has the effect of returning downward droop of the lower light (light ray lower than the principal light ray), which is advantageous for aberration correction. Further, when the lens closest to the object side is a biconcave lens, the rear surface is also a negative lens surface, and the negative refractive power of the lens closest to the object side is stronger than that of the meniscus lens. Therefore, it is necessary to increase the positive refracting power of the lens group following the lens closest to the object side, which is disadvantageous in correcting aberration.

Each of the front group and the rear group can also be composed of two lenses. Since the present invention can be configured with such a small number of lenses, it is possible to reduce the cost. In order to correct chromatic aberration, at least two positive and negative lenses are required for each of the front and rear units.

By adopting a configuration that satisfies the following conditional expressions, it is possible to realize a higher-performance zoom lens.

It is preferable that at least one aspherical surface in the front group satisfies the following conditional expression.

The conditional expression is, when the maximum effective diameter of the aspheric surface is Y _max ,
For any height y in the vertical direction of the optical axis of 0.7Y _max <y <1.0Y _max , Here, φ ₁ : refractive power of the front group, N: refractive index of the object side medium from the aspherical surface, N ′: refractive index of the image side medium from the aspherical surface, X (y): surface shape of the aspherical surface, X ₀ (Y): aspherical reference surface shape, where r: reference radius of curvature of the aspherical surface, ε: quadratic curve parameter, A _i : aspherical surface coefficient,: paraxial radius of curvature of the aspherical surface ｛(1 /) = (1 / r) +2
A ₂ ｝,

The conditional expression means that the positive refractive power of the aspheric surface in the front group becomes weaker toward the periphery (the negative refractive power becomes stronger),
This is a condition for correcting spherical aberration. If the upper limit of the conditional expression is exceeded, spherical aberration will be undercorrected over the entire zoom range, and if the lower limit is exceeded, spherical aberration will be overcorrected over the entire zoom range.

It is preferable that the rear group has at least one aspherical surface. If an aspherical surface is used for the lens closest to the object in the rear group, distortion near the wide-angle end can be corrected. If an aspherical surface is used for the lens closest to the image plane in the rear group, field curvature can be corrected.

It is preferable that at least one aspheric surface in the rear group satisfies the following conditional expression.

The conditional expression is, when the maximum effective diameter of the aspheric surface is Y _max ,
For any height y in the vertical direction of the optical axis of 0.8Y _max <y <1.0Y _max , Where φ ₂ is the refractive power of the rear group.

The conditional expression means that the negative refractive power becomes weaker (positive refractive power becomes stronger) toward the periphery of the aspherical surface in the rear group, and is a condition for correcting distortion and field curvature in a well-balanced manner. is there. When the value exceeds the upper limit of the conditional expression, the distortion at the wide-angle end takes a large positive value. When the value exceeds the lower limit, the tendency of the image surface to curve in the negative direction over the entire zoom range becomes remarkable.

In the present invention, it is preferable that at least one double-sided aspherical lens is provided. When a double-sided aspheric lens is used, various aberrations that cannot be suppressed only by the object-side surface can be corrected by the image-side surface, so that the above-described effect of the aspheric surface can be further exhibited.

In order to reduce aberrations (improve performance) while reducing the size of the zoom lens, it is desirable to use at least three aspheric surfaces in the zoom lens.

In the present invention, it is desirable that all aspherical surfaces in the front group satisfy the following conditional expression.

The conditional expression is, when the maximum effective diameter of the aspheric surface is Y _max ,
For any height y in the vertical direction of the optical axis where 0 <y <0.7Y _max , It is.

When the value exceeds the upper limit of the conditional expression, the orbicular spherical aberration has a large negative value, and a shift of the focus position due to the stop-down becomes a problem. On the other hand, if the lower limit is exceeded, the spherical aberration correction effect on the annular light flux becomes excessive, and it becomes difficult to correct other aberrations and spherical aberration in a well-balanced manner. in this case,
The spherical aberration tends to be wavy.

When a lens having both aspheric surfaces is used in the front group, it is desirable that one surface satisfies the following conditional expression and the other surface satisfies the following conditional expression.

The conditional expression is, when the maximum effective diameter of the aspheric surface is Y _max ,
For any height y in the vertical direction of the optical axis where 0.7Y _max <y <Y _max , It is.

The conditional expression is, when the maximum effective diameter of the aspheric surface is Y _max ,
For any height y in the vertical direction of the optical axis where 0.7Y _max <y <Y _max , It is.

In the front group, an aspherical surface that satisfies the conditional expression means that the positive refractive power becomes weaker (the negative refractive power becomes stronger) toward the periphery. The conditional expression is a condition for correcting the spherical aberration from falling to the under side in the range of the third-order aberration region to the over side. At this time, on-axis light passing through a place far from the optical axis of the lens may be overcorrected and go to the over side. It is only necessary to introduce an aspheric surface having a higher positive refractive power (a lower negative refractive power) to the other surface.

Desirably, the deviation amount of the aspheric surface on the side satisfying the conditional expression from the reference spherical surface is larger than the deviation amount of the aspheric surface on the side satisfying the conditional expression from the reference spherical surface.

When the rear group includes an aspheric surface, it is desirable that all aspheric surfaces in the rear group satisfy the following conditional expression.

The conditional expression is, when the maximum effective diameter of the aspheric surface is Y _max ,
For any height y in the vertical direction of the optical axis where 0 <y <0.8Y _max , It is.

If the upper limit of the conditional expression is exceeded, the positive distortion and the positive shift of the field curvature become large in the intermediate angle of view band from the wide-angle end to the intermediate focal length region. If the lower limit is exceeded, negative distortion increases from the intermediate focal length region to the telephoto end, and the negative shift of the curvature of field becomes remarkable in the entire zoom range.

When a double-sided aspheric lens is used in the rear group, it is desirable that one surface satisfies the following conditional expression (which is the same as the conditional expression), and the other surface satisfies the following conditional expression.

The conditional expression is, when the maximum effective diameter of the aspheric surface is Y _max ,
For any height y in the vertical direction of the optical axis where 0.8Y _max <y <Y _max , It is.

The conditional expression is, when the maximum effective diameter of the aspheric surface is Y _max ,
For any height y in the vertical direction of the optical axis where 0.8Y _max <y <Y _max , It is.

In the rear group, an aspherical surface that satisfies the conditional expression means that the negative refractive power becomes weaker (positive refractive power becomes stronger) toward the periphery. This corrects distortion near the wide-angle end. Further, at this time, the curvature of field is favorably corrected by using an aspherical surface that satisfies the conditional expression.

It is desirable that the front group and the rear group are configured to satisfy the following conditional expressions.

Here, φ _W : refractive power of the entire system at the wide-angle end φ _T : refractive power of the entire system at the telephoto end β: zoom ratio where φ ₂ <0 β = φ _W / φ _T.

These are the total lens length, the amount of movement for zooming,
This is a condition for keeping the back focus and the correction state of various aberrations in a good balance.

If the lower limit of the conditional expression is exceeded, it becomes difficult to keep the back focus at an appropriate value (15% of the focal length at the wide-angle end) at the wide-angle end, which eventually leads to an increase in the lens diameter of the rear group. . If the upper limit is exceeded, the amount of movement of the front group and the rear group due to zooming becomes excessive, which is disadvantageous in terms of the lens barrel configuration.

When the lower limit of the conditional expression is exceeded, the Petzval sum takes a large negative value, the image plane falls significantly in the positive direction, and the distortion at the wide-angle end takes a large positive value. If the upper limit is exceeded, it is necessary to increase the change in the distance between the front and rear groups due to zooming, and the front and rear groups are greatly separated at the wide-angle end, thereby increasing the total lens length. .

Satisfying the following conditional expressions is also effective for maintaining a good balance between the entire length of the lens, the amount of movement for zooming, the back focus, and the state of correction of various aberrations.

However, φ ₂ <0.

The conditional expression defines the ratio between the refractive power of the entire system and the refractive power of the front group at the wide-angle end. If the upper limit of the conditional expression is exceeded, the refractive power of the front group becomes excessively large, and it becomes difficult to correct various aberrations generated in the front group, especially spherical aberration, even if an aspherical surface is used in the front group. If the lower limit is exceeded, the downward coma (inward coma) tends to occur around the screen.

The conditional expression defines a ratio between the refractive power of the entire system and the refractive power of the rear unit at the wide-angle end. If the upper limit of the conditional expression is exceeded, the rear group refractive power will be excessively large, and it will be difficult to correct various aberrations generated in the rear group, particularly the field curvature and distortion, even if an aspheric surface is used in the rear group. On the other hand, if the lower limit is exceeded, the tendency of downward coma to occur around the screen becomes remarkable, and it becomes difficult to secure a sufficient back focus.

When a glass material in the ν _d region (high dispersion) that satisfies the following conditional expressions and satisfies the conditional expressions is used for the concave lens of the front group, it is very advantageous in achromatization.

N _d ≦ 1.60... Ν _d ≦ 35.0 where N _d : refractive index of glass material for d-line ν _d : Abbe number of glass material for d-line

For example, when a lens that exceeds the ν _d region of the conditional expression is used for the concave lens of the front group, the ν _d of the convex lens and the concave lens in the front group becomes almost the same. Eventually, in order to achromatize, the convex and concave refractive powers must be increased, so that even if an aspherical surface is used, monochromatic aberration cannot be obtained.

When a glass material that satisfies the conditional expression and / or the following conditional expression is used for the convex lens, it is possible to prevent the image plane from falling in the positive direction due to the negatively large Petzval sum.

N _d ≦ 1.50 When a conditional expression and / or a glass material having a high refractive index exceeding the upper limit of the conditional expression is used, the Petzval sum tends to be a large negative value. In such a case, it becomes difficult to match the best positions of the on-axis and off-axis MTF values.

Those in the low refraction and high dispersion region as shown by the above conditional expression are mostly plastics. Therefore, when this is used for a lens, not only the entire lens system can be reduced in weight but also the cost can be reduced, and there is an advantage that it is suitable for mass production.

When a glass material in the ν _d region (high dispersion) that satisfies the above conditional expression is used for the convex lens in the rear group, it is very advantageous for achromatism.

For example, when a lens that exceeds the ν _d region of the conditional expression is used as the convex lens of the rear group, ν _d of the convex lens and the concave lens in the rear group becomes almost the same. Eventually, in order to achromatize, the convex and concave refractive powers must be increased, so that even if an aspherical surface is used, monochromatic aberration cannot be obtained.

When a glass material that satisfies the conditional expression and / or the conditional expression is used for the convex lens, it is possible to prevent the image plane from falling in the positive direction due to the negatively large Petzval sum.

If a high-refractive-index glass material exceeding the upper limit of the conditional expression and / or the conditional expression is used in that portion, the Petzval sum tends to have a large negative value. In such a case, it becomes difficult to match the best positions of the on-axis and off-axis MTF values.

Those in the low refraction and high dispersion region as shown by the above conditional expression are mostly plastics. Therefore, when this is used for a lens, not only the entire lens system can be reduced in weight but also the cost can be reduced, and there is an advantage that it is suitable for mass production.

EXAMPLES Examples of the compact zoom lens according to the present invention will be described below.

However, in each embodiment, r _i (i = 1, 2, 3,...) Is the radius of curvature of the i-th surface counted from the object side, d _i (i = 1, 2,
3, ...) indicate the i-th axial top surface distance counted from the object side, and N _i (i = 1,2,3, ...), ν _i (i = 1,2,3, ... .) Is the refractive index for the d-line of the i-th lens counted from the object side,
Indicates Abbe number. F indicates the focal length of the entire system, and F _NO indicates the open F number.

Note that, in the examples, a surface marked with an asterisk (*) indicates a surface constituted by an aspheric surface, and is defined by an expression representing the surface shape (X (y)) of the aspheric surface. .

<Example _{1> f = 29.0~39.3~53.3 F NO =} 4.1~5.6~7.6 Aspheric coefficient r ₁ : ε = 0.21656 × 10 A ₄ = 0.14925 × 10 ^-3 A ₆ = 0.127754 × 10 ^-5 A ₈ = −0.14269 × 10 ^-7 r ₂ : ε = 0.11927 × 10 A ₄ = 0.25952 × 10 ^-3 A ₆ = 0.23574 x 10 ^-5 A ₈ = 0.12146 x 10 ^-7 r ₅ : ε = 0.61354 A ₄ = 0.39082 x 10 ^-4 A ₆ = 0.17456 x 10 ^-6 A ₈ = 0.25816 x 10 ^-8 <Implementation example _{2> f = 29.0~39.3~53.3 F NO =} 4.1~5.6~7.6 Aspheric coefficient r ₁ : ε = 0.22038 × 10 A ₄ = 0.22559 × 10 ^-3 A ₆ = 0.47146 × 10 ^-6 A ₈ = −0.12054 × 10 ^-7 r ₂ : ε = 0.1202 × 10 A ₄ = 0.33655 × 10 ^-3 A ₆ = 0.22956 × 10 ^-5 A ₈ = −0.18420 × 10 ^-7 r ₅ : ε = 0.60620 A ₄ = 0.22371 × 10 ^-4 A ₆ = 0.12042 × 10 ^-6 A ₈ = 0.16891 × 10 ^-8 < example _{3> f = 29.0~39.3~53.3 F NO =} 3.6~5.0~6.7 Aspheric coefficient r ₁ : ε = -0.12243 A ₄ = 0.80992 × 10 ^-4 A ₆ = 0.15371 × 10 ^-5 A ₈ = -0.15720 × 10 ^-7 r ₂ : ε = -0.51536 × 10 A ₄ = 0.22100 × 10 ^{_{-3 A 6 = 0.23097 × 10 -5}} A 8 = 0.28762 × 10 -7 r 5: ε = -0.50602 × 10 A 4 = 0.31888 × 10 -4 A 6 = 0.21809 × 10 -6 A 8 = 0.44687 × 10 - ⁸ <example _{4> f = 29.0~39.3~53.3 F NO =} 4.1~5.6~7.6 Aspheric coefficient r ₁ : ε = 0.23111 × 10 A ₄ = 0.11200 × 10 ^-3 A ₆ = 0.84687 × 10 ^-6 A ₈ = −0.12491 × 10 ^-7 r ₂ : ε = 0.11805 × 10 A ₄ = 0.27319 × 10 ^-3 A ₆ = 0.28523 × 10 ^-5 A ₈ = 0.12017 × 10 ^-7 r ₅ : ε = 0.63667 A ₄ = 0.24033 × 10 ^-4 A ₆ = 0.11763 × 10 ^-6 A ₈ = 0.10013 × 10 ^-8 r ₇ : Ε = 0.84961 A ₄ = -0.40555 x 10 ^-5 A ₆ = -0.19877 x 10 ^-6 A ₈ = 0.81245 x 10 ^-9 <Example 5> f = 29.0 to 37.7 to 49.0 F _NO = 4.1 to 5.4 to 7.0 Aspheric coefficient r ₁ : ε = 0.10011 × 10 A ₄ = −0.10884 × 10 ⁻⁵ A ₆ = 0.30799 × 10 ⁻⁶ A ₈ = 0.17353 × 10 ⁻⁸ r ₂ : ε = 0.36082 A ₄ = 0.24372 × 10 ⁻³ A ₆ = −0.41879 × 10 ⁻⁶ A ₈ = 0.16047 × 10 ⁻⁶ r ₅ : ε = −0.22153 × 10 A ₄ = 0.96871 × 10 ⁻⁴ A ₆ = 0.22152 × 10 ⁻⁶ A ₈ = 0.47779 × 10 ⁻⁸ <example _{6> f = 29.0~39.3~53.3 F NO =} 4.1~5.8~7.6 Aspheric coefficient r ₁ : ε = 0.21680 × 10 A ₄ = 0.15318 × 10 ⁻³ A ₆ = 0.12171 × 10 ⁻⁵ A ₈ = −0.13072 × 10 ⁻⁷ r ₂ : ε = 0.13369 × 10 A ₄ = 0.26026 × 10 ^-3 A ₆ = 0.26458 × 10 ^-5 A ₈ = 0.73430 × 10 ^-8 r ₅ : ε = 0.62310 A ₄ = 0.45079 × 10 ^-4 A ₆ = 0.17834 × 10 ^-6 A ₈ = 0.26092 × 10 ^-8 r ₆ : Ε = 0.10000 × 10 A ₄ = 0.41364 × 10 ^-5 A ₆ = 0.14385 × 10 ^-7 A ₈ = 0.23560 × 10 ^-9 <Example 7> f = 24.7 to 33.3 to 38.8 F _NO = 3.6 to 4.9 to 5.7 Aspheric coefficient r ₁ : ε = 0.10357 × 10 A ₄ = 0.14588 × 10 ^-3 A ₆ = 0.72204 × 10 ^-5 A ₈ = −0.99988 × 10 ^-7 r ₂ : ε = 0.9000 A ₄ = 0.32870 × 10 ^-3 A ₆ = 0.76286 × 10 ^-5 A ₈ = 0.46323 × 10 ^-7 r ₅ : ε = -0.50386 × 10 A ₄ = 0.53724 × 10 ^-4 A ₆ = 0.39211 × 10 ^-6 A ₈ = 0.11732 × 10 ^-7 FIGS. 1 to 7 are lens configuration diagrams corresponding to Examples 1 to 7, wherein the arrows in the drawings indicate the positions from the widest end (W) to the most telephoto end (T) of the front and rear groups. The movement is schematically shown.

Each of Examples 1 to 7 includes, in order from the object side, a front lens group and a rear lens group. The front lens group includes a first lens including a negative meniscus lens concave to the object side and a biconvex positive second lens. The rear group consists of a third lens which is close to non-power and a fourth lens which is a negative meniscus lens concave on the object side.

In Examples 1 to 4, 6, and 7, the third lens is a positive meniscus lens convex on the image side, and in Example 5, the third lens is a negative meniscus lens concave on the object side.

In Examples 1 to 3, 5, and 7, both surfaces of the first lens and the object-side surface of the third lens are aspherical. In Example 4,
Both surfaces of the first lens, the object-side surface of the third lens, and the object-side surface of the fourth lens are aspherical. In the sixth embodiment, the first
Both surfaces of the lens and both surfaces of the third lens are aspheric.

8 to 14 are aberration diagrams respectively corresponding to Examples 1 to 7, in which (W) denotes the focal length at the wide-angle end,
(M) shows the aberration at the intermediate focal length, and (T) shows the aberration at the telephoto end focal length. The solid line (d) represents the aberration with respect to the d-line, and the broken line (SC) represents the sine condition. Further, a broken line (DM) and a solid line (DS) represent astigmatism on the meridional surface and the sagittal surface, respectively.

Table 1 shows the conditional expressions in Examples 1 to 7. And in the conditional expression Are shown respectively.

Table 2 shows that in the conditional expressions in Examples 1 to 7, And in the conditional expression Are shown respectively.

Tables 3 to 9 correspond to Examples 1 to 7, respectively.
In the conditional expression for each aspheric surface with respect to the value of y, Is represented by (I), and in the conditional expression Is represented by (II).

Effects of the Invention As described above, according to the present invention, a front group having a positive refractive power and a rear group having a negative refractive power are sequentially arranged from the object side, and the air between the front group and the rear group is formed. In a zoom lens that performs zooming by changing the interval, the lens surface closest to the object in the front group is made concave and aspheric, so that a high number of lenses can be used at low cost while maintaining high optical performance. Thus, it is possible to provide a zoom lens that realizes compactness, compactness, and wide angle.

Further, if the zoom lens according to the present invention is applied to a lens shutter camera with a built-in zoom lens, it is possible to achieve a reduction in size and cost of the camera.

[Brief description of the drawings]

FIG. 1, FIG. 2, FIG. 3, FIG. 4, FIG. 5, FIG. 6, and FIG. 7 are lens configuration diagrams corresponding to Examples 1 to 7 of the present invention, respectively. FIG. 8, FIG. 9, FIG. 10, FIG. 11, FIG. 12, FIG. 13, and FIG. 14 are aberration diagrams corresponding to Examples 1 to 7 of the present invention, respectively.

────────────────────────────────────────────────── 56 Continuation of the front page (56) References US Pat. No. 4,936,661 (US, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02B 9/00-17/08 G02B 21/02-21 / 04 G02B 25/00-25/04

Claims (5)

(57) [Claims] 1. A zoom lens system comprising: a front group having a positive refractive power and a rear group having a negative refractive power in order from the object side, wherein a magnification is changed by changing an air gap between the front group and the rear group. in the zoom lens to perform, lens surface of the front group in the most object side is a concave surface and is and aspherical, the maximum effective diameter of the aspherical surface when the Y _max, 0.7Y _max <
a zoom lens characterized by satisfying the following conditions for an arbitrary height y in the vertical direction of the optical axis where y <1.0Y _max ; Here, φ ₁ : refractive power of the front group, N: refractive index of the object side medium from the aspherical surface, N ′: refractive index of the image side medium from the aspherical surface, X (y): surface shape of the aspherical surface, X ₀ (Y): aspherical reference surface shape, where r: reference radius of curvature of the aspherical surface, ε: quadratic curve parameter, A _i : aspherical surface coefficient,: paraxial radius of curvature of the aspherical surface ｛(1 /) = (1 / r) +2
A ₂ ｝, 2. A zoom lens system comprising: a front group having a positive refractive power and a rear group having a negative refractive power in order from the object side, wherein a magnification is changed by changing an air gap between the front group and the rear group. In the zoom lens to be performed, when the lens surface closest to the object in the front group is concave and aspheric, and the maximum effective diameter of the aspheric surface is Y _max , 0 <y <0.
A zoom lens characterized by satisfying the following conditions for an arbitrary height y in the vertical direction of the optical axis of 7Y _max ; Here, φ ₁ : refractive power of the front group, N: refractive index of the object side medium from the aspherical surface, N ′: refractive index of the image side medium from the aspherical surface, X (y): surface shape of the aspherical surface, X ₀ (Y): aspherical reference surface shape, where r: reference radius of curvature of the aspherical surface, ε: quadratic curve parameter, A _i : aspherical surface coefficient,: paraxial radius of curvature of the aspherical surface ｛(1 /) = (1 / r) +2
A ₂ ｝, 3. The apparatus according to claim 1, wherein the front group includes, in order from the object side, a negative lens having a concave surface facing the object side and a positive lens. The zoom lens described. 4. The zoom lens according to claim 3, wherein said negative lens is a negative meniscus lens. 5. The zoom lens according to claim 1, wherein the following condition is further satisfied: Where φ _W : refractive power of the whole system at the wide-angle end, φ _T : refractive power of the whole system at the telephoto end, φ ₁ : refractive power of the front group, φ ₂ : refractive power of the rear group, β: zoom Ratio, where φ ₂ <0 β = φ _W / φ _T.