JPH071332B2 - Zoom lenses - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variable focal length lens, and more particularly, to changing the axial distance between a positive first lens group and a negative second lens group, and at the same time, changing the axial distance between the second lens group and the second lens group. The present invention relates to a zoom lens that performs zooming by changing an axial distance from a subsequent lens group.

(Prior Art) Generally, in a zoom lens, in addition to the aberration correction in the reference state, it is necessary to correct the aberration fluctuation during zooming as much as possible. Therefore, spherical aberration, coma aberration of each lens group,
And astigmatism must be corrected individually in each lens group, and each lens group is usually composed of several lenses.

In recent years, there has been an increasing demand for a compact variable focal length lens and a high magnification ratio, but like the present invention,
The first lens group is composed of a plurality of lens groups in order from the object side, and the second lens group is composed of a negative lens group.
In order to make a zoom lens of the type that performs zooming compact by changing the distance between the positive lens group and the second negative lens group and the distance between the second negative lens group and the third lens group, paraxially speaking, The power of each lens group may be strengthened or the principal point interval between each lens group may be reduced. Further, in order to increase the zoom ratio of the zoom lens, it is sufficient to paraxially increase the power of each lens unit or increase the moving distance of the zoom lens unit.

However, paraxially, it is better to strengthen the power of each lens group in order to make the zoom lens of the above type compact and to increase the zoom ratio, but in the actual lens system It is necessary to increase the number of constituent lenses in order to correct the occurrence of aberration to be small while increasing the power of.
Also, if the power per lens is strong, the curvature will be tight,
It is necessary to set a large air gap in contact with the central lens thickness of the convex lens when the required edge thickness is obtained or a concave surface when the marginal distance between adjacent lenses is taken. However, in that case, the total length of the lens group becomes large and the distance between the principal points must be made large. As a result, the optical total length of the entire system cannot be shortened. On the other hand, when the length of the lens unit becomes large, the moving space of the variable power lens unit becomes small, so that it becomes impossible to increase the zoom ratio.

Further, if the axial length of the first lens group or the second lens group becomes large, the effective diameter of the front lens required for the off-axis light beam becomes large, and the lens diameter cannot be made compact. Such a vicious circle occurs, and there is a limit to downsizing and high magnification in a normal spherical system.

In the zoom type according to the present invention, the second negative lens group that normally contributes most to zooming has the strongest power among all lens groups, so if the power of the second negative lens group is strengthened in order to achieve compactness, The number of lenses required for the negative lens group increased, and the length of the lens group became longer, which was a win.

Further, the first lens unit is composed of a plurality of lens units in order from the object side, the first lens unit is composed of a positive lens unit, the second lens unit is composed of a negative lens unit, and the third lens unit is composed of a positive lens unit having strong power. The distance between the first positive lens group and the second negative lens group increases toward the end, the distance between the second negative lens group and the third lens group decreases, and the distance between the second negative lens group and the third positive lens group increases. In a variable focal length lens of the type that performs zooming, the direction in which the principal point distance between the second negative lens unit and the third positive lens unit is reduced is in the direction in which the overall optical length of the entire system is reduced. When the power is tight, the number of lenses required for aberration correction of the second negative lens group increases, and the total length of the lens group becomes long. Therefore, the distance between the principal points must be increased, and the total optical length of the entire system cannot be reduced so much.

As described above, the conventional spherical system has a limit in reducing the total optical length.

On the other hand, if the power of each lens unit of the zoom lens is strengthened to reduce the total optical length of the entire system, it becomes difficult to correct the Petzval sum.

For example, taking a conventionally known four-group zoom as an example, it is composed of a first lens group positive from the object side, a second lens group negative, a third lens group positive or negative, and a fourth lens group positive, The first lens unit to the third lens unit constitute a zoom unit, and the fourth lens unit constitutes a relay unit.

In order to make the entire system compact with such a zoom lens, there are methods such as (1) increasing the power of each lens unit in the zoom unit and (2) decreasing the telephoto ratio of the relay unit. If the power of each group in the zoom section is strengthened by the method (1), the Petzval sum in the second negative lens group, which is usually the strongest, commonly called the variator, is large at a negative value, and the curvature of field becomes significantly over. In the method (2) as well, the direction of decreasing the telephoto ratio of the relay section is a direction in which the Petzval sum has a negative value, and if this method is attempted to be made compact, the field curvature will be over.

If the refractive index of the convex lens is lowered to correct the Petzval sum, or if a positive lens and a negative lens having strong power are combined, spherical aberration or high-order aberration will occur remarkably. become unable. In this way, the compactness of the zoom lens and the correction of the Petzval sum are in a conflicting relationship in the case of a spherical system.

Not limited to the zoom type, the zoom type is such that the first positive lens unit moves during zooming and the entire length extends from the wide-angle end to the telephoto end, or the fourth lens unit moves in the optical axis direction during zooming. However, the situation is the same when trying to further shorten the total length.

Incidentally, as one of proposals for making a four-group type zoom lens compact, there is one in JP-A-57-122413 which uses a compound eye optical system such as a self-focusing rod lens array in a relay lens portion. However, in this case, since the compound eye optical system is an erecting equal-magnification image forming system, it is unsuitable for use in a lens group such as a variator in which the image forming magnification of the lens group changes during zooming.

(Object of the Invention) An object of the present invention is to realize a zoom lens which is compact or has a higher zoom ratio. As a result, the weight is reduced, and the assembly and adjustment of the optical system is facilitated.

In order to achieve the above object, the first lens group having a positive refractive power, the second lens group having a negative refractive power, and the subsequent lens group are arranged in this order from the object side.
In a zoom lens in which zooming is performed by changing the distance between a lens group and the second lens and the distance between the second lens group and the subsequent lens group, the second lens group forms an optical axis with other lenses. It has a refractive index distribution type lens in which the refractive index is gradually increased from the optical axis to the outer circumference and the value of the refractive index gradient on the short wavelength side near the optical axis is smaller than the value of the refractive index gradient on the long wavelength side. Especially.

In addition, an example that realizes this is the numerical example 1, 4,
Although it corresponds to 7, other numerical examples are also shown as design examples in which the aberration level is corrected.

Further, in the case where the refractive index distribution type lens of the second negative lens group has a form in which the refractive index changes in the direction orthogonal to the optical axis, the refractive power of the lens is represented by h (distance from the optical axis) and Ni (h ) =
When NO + N ₁ h ² + N ₂ h ⁴ + ……, ・ It is desirable that the condition of N ₁ <0 …… (1) is satisfied. However, the refractive power mentioned above includes components of refraction and transfer.

By satisfying the condition (1), the refractive power is shared even inside the lens, so that the curvature of this lens surface can be reduced. Therefore, it is possible to suppress the generation of high-order aberrations to a minimum, and to reduce the lens thickness when the required edge thickness is taken or the air distance between adjacent lenses when the required marginal distance is taken to reduce the total length of the lens group. There is an effect that can be shortened.

(Explanation of Embodiments) FIG. 1 shows a zoom lens according to a first embodiment of the present invention, in which the first positive lens group 11 fixed during zooming moves from the object side, zooms by moving in the optical axis direction during zooming. Of the second positive lens group 12 that contributes to the zoom lens, the third positive lens group 13 that moves in the optical axis direction during zooming to correct the focus movement, and the fourth positive lens group 14 that is fixed during zooming The second negative lens group 12, commonly known as a variator, has a refractive index distribution in which the refractive index increases in the radial direction from the optical axis to the outer periphery, and the refractive index distribution shares the optical axis with other lenses (not the compound eye). It uses a mold lens.

See Numerical Example 1.

Unlike a normal lens of a homogeneous medium, the gradient index lens has a condensing action or a diverging action inside the lens and has power. Further, by controlling the refractive index distribution, an aberration correction effect is obtained on the lens surface and the lens middle part. In this embodiment, the power inside is 5 / the power of the variator section.
Since it is in charge of 6 and has excellent aberration correction capability, the variator unit, which is usually composed of 3 to 5 lenses, can be composed of a single lens and lenses of which the both surfaces have a gentle curvature. Therefore, the total length of the second negative lens unit becomes small, and it is possible to make it compact.

Although the gradient index lens has a single lens, it has the ability to correct aberrations, and is particularly excellent in the ability to correct Petzval sum and spherical aberration. The Petzval sum generated in the gradient index lens is the power due to the condensing and diverging effects inside the lens,
When the focal length of the entire system is expressed as standardized to 1, and the refractive index of the base is expressed as NO, it becomes a form inversely proportional to P = / No ² O and the square of NO, so the Petzval sum of the refractive surface is P. =
Compared to / NO, the occurrence is small. In the case of the present embodiment, the occurrence of negative Petzval sum is small. In the case of the spherical system, the Petzval sum generated from the variator is -1.25 if the power arrangement is the same as that of this embodiment.
Although it is about -1.3, the Petzval sum generated in the variator is as small as -1.025 in the present embodiment because of the above reason.

This means that the optical length of the entire system can be made smaller than that of the spherical system by increasing the power of the zoom unit or decreasing the telephoto ratio of the relay unit. In other words, the biggest drawback is that the Petzval sum that tries to increase the power of the zoom section or to reduce the telephoto ratio of the relay section in order to shorten the total length is large and becomes uncorrectable, which is the biggest drawback. In the case of the present embodiment in which the generation of the negative sum value is small, the possibility of shortening the optical total length of the entire system by the above method increases.

In this embodiment, the telephoto ratio of the relay section is reduced to make the optical total length of the entire system 254.8 mm, the ratio of the optical total length to the focal length at the telephoto end,
That is, the telephoto ratio could be made as small as 0.836. Furthermore, the spherical aberration could be corrected well by increasing the refractive index of the first positive lens in the relay portion by the amount that the Petzval sum has a margin. That is, usually 3 variators
The spherical aberration is corrected by the cemented surface of the cemented lens, which is composed of 5 to 5 lenses, but in the present embodiment, the spherical aberration can be corrected even though the number of constituent elements of the variator part is only one.

In the gradient index lens, the spherical aberration can be corrected while the light ray travels inside the lens by controlling the refractive index distribution shape inside. Specifically, it can be achieved by controlling the coefficient N ₂ when the distribution shape is expressed as Ni (h) = NO + N ₁ h ² + N ₂ h ⁴ + N ₃ h ⁶ + ...

On the other hand, in the radial type negative lens that is the second negative lens group 12, if the value of the refractive index gradient on the short wavelength side near the optical axis is smaller than the value of the refractive index gradient on the long wavelength side, The chromatic aberration generated by the variator is reduced, and fluctuations in axial chromatic aberration and lateral chromatic aberration due to zooming can be corrected to be small.

As described above, the zoom lens of this embodiment achieves compactness and high performance by using the gradient index lens in this way.

Normally, the variator of the zoom lens is composed of 3 to 5 lenses having a strong power, and the thickness of the lens, the air gap, and the eccentricity between the lenses must be suppressed very strictly, but one lens like this embodiment. If the lens is used, the assembly and adjustment work will be significantly facilitated.

FIG. 2 shows an example in which a radial type gradient index lens is used for the two lenses of the second negative lens group (corresponding to numerical example 2 described later). The zoom lens shown here comprises a first positive lens group 21, a second negative lens group 22, a third positive lens group 23, and a fourth positive lens group 24 in order, and a first positive lens group 21 and a second positive lens group 21 for zooming. The third positive lens group 23 moves independently, and the second negative lens group 22 and the fourth lens group 24 are fixed. The second negative lens group 22 is fixed, but has the largest zooming effect, and has an advantage that the lens barrel structure can be simplified as compared with the type in which the front three groups are moved.

This type of zoom lens increases the zoom ratio up to about 3 times despite increasing the angle of view at the wide-angle end.
The focal length of the lens group must be made quite short.
Therefore, since the radius of curvature of the concave shape of the lenses forming the second lens group becomes small, a large aberration tends to remain even if the number of lenses is increased.

By using the gradient index lens, in particular, the meniscus negative lens which is loaded with the negative power of the second negative lens group II can obtain the power as the negative lens component particularly depending on the distribution of the refractive index. Since it is possible, it is possible to loosen the radius of curvature of the surface having the concave surface toward the image side, and as a result, the occurrence of aberration is prevented.

FIG. 5 shows a third embodiment. Corresponding to Numerical Example 3. In the figure, 31 is the first positive lens group, 32 is the second negative lens group,
33 is a third positive lens group, and 34 is a fourth positive lens group.
This is a higher magnification zoom lens that moves in the same manner as.

In this embodiment, the second negative lens group 32 is composed of three negative lenses and one positive meniscus lens, and the positive refractive index of the meniscus positive lens increases from the optical axis to the outer peripheral portion in the direction orthogonal to the optical axis. It has a refractive index distribution that decreases.

In the zoom lens having the specifications like this example, it is desired to make the principal point interval between the first positive lens group 31 and the second negative lens group 32 as small as possible to reduce the total length and the front lens diameter. The group 32 is composed of two negative lenses and a positive lens from the object side, and the positive lens has a refractive index distribution such that a positive power is given, so that the positive lens having a gentle curvature is used as the second negative lens group 32. Move the front principal point forward to move the first positive lens group 31
By shortening the distance between the principal points of the second negative lens group 32, the overall length and the front lens diameter are reduced.

Also, by setting the coefficients N ₂ and N ₃ of the refractive index distribution to negative values, the second
The spherical aberration generated by the two negative lenses of the negative lens group is corrected over a high order.

As a result, the second negative lens group, which was usually composed of 5 to 6 lenses and the length of the lens group was large, could be composed of 3 lenses, thus achieving compactness. In addition, the assembly and adjustment of the second negative lens group, which is severely eccentric, is facilitated.

FIG. 7 shows Example 4 (corresponding to Numerical Example 4), in which a radial type gradient index lens is used for each lens group of the zoom lens. First positive lens group 41, second
The negative lens group 42, the third positive lens group 43, and the fourth positive lens group 44 are arranged in this order, and the first positive lens group 41, the second negative lens group 42, and the third
The positive lens group 43 moves independently to perform zooming.

Conventionally, this type of zoom lens uses three lenses for each of the first to third lens groups and four lenses for the fourth lens group to correct aberrations, but by using a gradient index lens It is possible to reduce the amount of aberration peculiar to the lens group,
The number of components could be reduced. Then, the aberration variation due to zooming is controlled by canceling the aberration associated with the power of each lens group.

In Example 5 (corresponding to Numerical Example 5) shown in FIG. 9, a first positive lens group 51, which is fixed during zooming, is moved in order from the object side, and a second negative lens group that moves in the optical axis direction during zooming and contributes to zooming. Lens group 52, third negative lens group that moves in the optical axis direction during zooming to correct focus movement
The image-side negative lens of the second negative lens group 52 of the second negative lens group 52 of the zoom lens composed of 53 and the fourth positive lens group 54 fixed during zooming has a refractive index such that the refractive index increases in the radial direction from the optical axis to the outer periphery. It uses a gradient index lens having a distribution. In addition to the above, a refractive index distribution type lens having a refractive index distribution in which the refractive index decreases in the radial direction from the optical axis to the outer periphery is provided in the first positive lens group 51, behind the fourth positive lens group fixed during zooming. As the positive lens, a gradient index lens having a refractive index distribution in which the refractive index becomes lower in the radial direction from the optical axis to the outer circumference was used.

The lens on the image side of the second negative lens unit is a negative lens, and the refractive power distribution such that the refractive index becomes higher toward the periphery of the lens as in the present embodiment can bear negative power inside the lens, so that the curvature is The amount becomes looser by that amount, and the occurrence of various aberrations becomes smaller. Also, the Petzval sum can be reduced. Further, if the refractive index distribution coefficient N ₂ is set to a negative value, the spherical aberration on the telephoto side can be effectively corrected by utilizing the refractive surface of the gradient index lens. Therefore, the second negative lens group, which is normally composed of 3 to 4 lenses, can be composed of one negative lens of homogeneous medium and one gradient index lens.

In addition, of the radial type negative lens in the second negative lens group,
If the value of the refractive index gradient on the short wavelength side near the coaxial axis is made smaller than the value of the refractive index gradient on the long wavelength side, the chromatic aberration generated in this lens group will be small, and fluctuations due to zooming in axial chromatic aberration and chromatic aberration of magnification will be reduced. Can be corrected small. On the other hand, if the first positive lens unit is a positive lens and a refractive index distribution is provided such that the refractive index becomes lower toward the periphery of the lens as in the present embodiment, it is effective for spherical aberration correction at the telephoto end, and other various Since the aberration can be corrected well, the first positive lens group can be composed of one gradient index lens.

Further, the rear convex lens located at a position far from the diaphragm of the fourth positive lens unit, which is fixed during zooming, and where the principal ray passing position of the off-axis light beam is high, has a refractive index that decreases from the optical axis to the outer circumference in the radial direction. By providing a refractive index distribution having a refractive index distribution and controlling a higher-order distribution coefficient, astigmatism and distortion can be favorably corrected over the entire screen.

FIG. 11 shows the sixth embodiment. Corresponding to Numerical Example 6.

In this embodiment, the first positive lens group 61 moves in the optical axis direction during zooming in order from the object side, the second negative lens group 62 moves in the optical axis direction during zooming and contributes to zooming, and fixed during zooming. Third positive lens group
The second negative lens group 62 of the zoom lens composed of 63 uses a gradient index lens having a refractive index distribution in which the refractive index increases in the radial direction from the optical axis to the outer periphery. Further, a refractive index distribution type lens having a refractive index distribution in which the refractive index is lowered in the radial direction from the optical axis to the outer periphery is provided in the first positive lens group 61, which is the most object in front of the third positive lens group fixed during zooming. Side of the positive lens, the gradient index type lens having a refractive index distribution in which the refractive index gradually increases from the optical axis to the outer periphery in the direction orthogonal to the optical axis, and the positive lens of the cemented lens adjacent to it has the optical axis direction A gradient index lens having a refractive index distribution such that the refractive index decreases from the object side to the image side, in the negative lens on the object side of the two negative lenses in the rear group, in the direction orthogonal to the optical axis. In addition, a gradient index lens having a refractive index distribution in which the refractive index increases from the optical axis to the outer circumference is used. The negative lens of the cemented lens in the front lens group of the third positive lens group, which is fixed during zooming, and the negative lens closest to the image side in the rear lens group are made of homogeneous optical glass.

If the second negative lens group is provided with a refractive index distribution such that the refractive index increases toward the periphery of the lens, the negative power can be shared inside the lens and the curvature of the lens can be remarkably relaxed. It is effective for correcting spherical aberration at the edge and can reduce the Petzval sum.

For example, if the power distribution is the same, the Petzval sum generated in the second negative lens group composed only of homogeneity is about -1.45 to -1.6 when the focal length of the entire system is normalized to 1. In this embodiment, the refractive index distribution type lens of the radial type is used, and the value is -1.06, which is extremely small.

By using the gradient index lens in this way, it is possible to reduce the Petzval sum in the second negative lens group, and thus it is possible that the telephoto ratio of the relay section can be reduced and the total optical length of the entire system can be shortened. To have.

On the other hand, if the first positive lens unit is a positive lens and a refractive index distribution is provided such that the refractive index becomes lower toward the periphery of the lens as in the present embodiment, it is effective for spherical aberration correction at the telephoto end, and other various Since the aberration can be corrected well, the first positive lens group can be composed of one gradient index lens.

Further, in the present embodiment, the third positive lens group, which is the relay section, is of the strong telephoto type, but the gradient index type lens is also used for the third positive lens group as described above to further strengthen the tendency of the telephoto type. The telephoto ratio was reduced, and the total optical length of the entire system was significantly shortened.

As in the present embodiment, a negative radial type gradient index lens is used for the second negative lens group which is a variator, or a negative radial type gradient index distribution is used for the rear group of the third positive lens group which is a relay. When a radial lens or an axial type gradient index lens is used for the front lens group of the positive lens group and the third positive lens group, the portability is improved when a lens having a uniform refractive index with the same specifications as this embodiment is used. The value shown, that is, the total length at the time of storage (the total length at the wide-angle end) divided by the focal length at the telephoto end is usually 0.85 to 1, but it can be made as small as 0.577. This is the first achievement achieved by using a gradient index lens.

The seventh embodiment illustrated in FIG. 13 is the first from the object side, which is fixed during zooming.
A positive lens group, a second negative lens group that moves in the optical axis direction during zooming and contributes to zooming, a third positive lens group that moves in the optical axis direction during zooming to correct focus movement, and a fixed lens during zooming. The second negative lens group of the zoom lens including the fourth positive lens group, an axial type having a refractive index distribution such that the refractive index decreases toward the object side of the so-called variator in the optical axis direction from the object side toward the image side. The gradient index lens is used, and a radial type gradient index lens having a refractive index distribution in which the refractive index increases in the radial direction from the optical axis to the outer periphery on the image side is used.

Since the negative lens on the image side also shares the power inside the lens, the curvature of the refracting surface is much gentler than when a lens made of a homogeneous material is used.

If the negative lens on the object side has the above-mentioned refractive index distribution, the strong concave surface on the image side decreases the refractive index toward the outer circumference, so spherical aberration in the over direction is small and effective for correcting spherical aberration. Is. And this feature and the effect of correcting the spherical aberration that tends to occur in the over direction to the under direction while the lens is traveling by controlling the refractive index distribution inside the negative lens (radial type) on the image side. Since it has such a structure, the joining surface, which is always provided in a normal variator, becomes unnecessary, and the number of sheets and the total length of the variator can be shortened.

As described above in the description of each example, the use of the gradient index lens in the second negative lens group can achieve excellent effects. However, the overall description is as follows. is there.

1) Since the number of lens groups can be reduced, weight and size can be reduced. 2) Since the number of constituent lenses can be reduced and the length of the lens group can be reduced, a space is provided between the second negative lens group and another lens group, and the moving range of the moving group can be increased. High magnification can be easily achieved. 3) Since the number of constituent lenses can be reduced, the principal point distance between the second negative lens group and the other lens groups can be reduced, and the power arrangement can be shortened to significantly reduce the overall length. 4) Further, since the principal point distance between the first and second lens groups can be made small, the effective diameter of the front lens necessary for the off-axis light beam can be made small, and the lens outer diameter and the filter diameter can be made small. 5) Since the aberration can be corrected to be smaller in each lens group, it is possible to achieve a variable focal length lens in which variation in aberration due to zooming is small. 6) Since the occurrence of Petzval sum is small, the power of the second negative lens unit can be increased and the total length can be shortened.

As described above, in the present invention, the first lens group is composed of the positive lens group and the second lens group is composed of the negative lens group in order from the object side.
An inhomogeneous medium is included in at least the second negative lens group of the variable focal length lens that performs zooming by changing the distance between the positive lens group and the second negative lens group and the distance between the second negative lens group and the third lens group. We have achieved a high-performance, lightweight, compact, and high-magnification variable focal length lens by using the graded index lens. Further, in the variable focal length lens of the above type, a variable focal length lens having a simple structure and easy to assemble and adjust has been achieved.

[Brief description of drawings]

FIG. 1 is a lens sectional view showing a first embodiment of the present invention. Second
The figure is an aberration curve diagram. FIG. 3 is a lens cross-sectional view showing a second embodiment. FIG. 4 is an aberration curve diagram. FIG. 5 is a lens sectional view showing a third embodiment. FIG. 6 is an aberration curve diagram. FIG. 7 is a lens sectional view showing a fourth embodiment. FIG. 8 is an aberration curve diagram. FIG. 9 is a lens sectional view showing a fifth embodiment. Fig. 10 is an aberration curve diagram. First
FIG. 11 is a lens cross section showing a sixth embodiment. Figure 12 is an aberration curve diagram. FIG. 13 is a lens sectional view showing a seventh embodiment. Figure 14 is an aberration curve diagram. In the figure 11,12,31,41,51,61,71; first lens group 12,22,32,42,52,62,72; second lens group 13,23,33,43,53,63, 73; third lens group 14,24,41,44,54,74; fourth lens group Z ₁ , Z ₂ , Z ₃ ; locus of expansion during movement M; meridional focal line S; sagittal Focal line

Front page continuation (72) Inventor Jun Hattori 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Shigeyuki Suda 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (56) References JP-A-58-202420 (JP, A) JP-A-58-220115 (JP, A)

Claims (1)

[Claims] 1. A first lens group having a positive refractive power in order from the object side,
The second lens group having negative refracting power and the subsequent lens group, and varying the distance between the first lens group and the second lens and the distance between the second lens group and the subsequent lens group In the zoom lens for performing the above, the second lens group shares the optical axis with other lenses, the refractive index is gradually increased from the optical axis to the outer periphery, and the refractive index gradient on the short wavelength side near the optical axis is A zoom lens having a gradient index lens having a value smaller than the value of the refractive index gradient on the long wavelength side.