JPH0718974B2 - Variable focal length lens - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Technical Field The present invention comprises a variable focal length lens, in particular, a plurality of lens groups, in which the first lens group is a negative lens group and the second lens group is a positive lens group in order from the object side. And a variable focal length lens configured to change magnification by changing a distance between the first lens group and the second lens group.

(2) Description of Prior Art Generally, in a variable focal length lens, in addition to aberration correction in the reference state, aberration fluctuation during zooming must be corrected as small as possible. Therefore, the spherical aberration, coma aberration, and astigmatism of each lens group need to be individually corrected in each lens group, and each lens group is normally composed of several lens groups.

In recent years, there is an increasing demand for a compact variable focal length lens and a high magnification ratio, and for example, a first lens group is composed of a plurality of lens groups in order from the object side, a negative lens group, and a second lens group. In order to make a variable focal length lens of a type that is composed of a positive lens group and changes the distance between the first lens group and the second lens group to perform zooming compactly, the power of each lens group is paraxially arranged. Should be strengthened or the principal point spacing between the lens groups should be reduced. On the other hand, in order to increase the zoom ratio of the variable focal length lens, the power of each lens unit may be increased or the moving distance of the zoom lens unit may be increased paraxially. In this way, paraxially, the power of each lens group should be strengthened in order to make the variable focal length lens of the above type compact and to increase the magnification ratio, but in the actual lens system, A large number of constituent lenses are required to correct the occurrence of aberrations with the power of the lens units increased. Also, if the power per lens is strong, the curvature becomes tight, and the center lens thickness of the convex lens when the required edge thickness is taken, or the air distance between the concave surfaces when the marginal distance between adjacent lenses is taken is large. Is. Then, the total length of the lens group becomes large and the distance between the principal points must be made large, and as a result, the optical total length of the entire system cannot be shortened. Further, as 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.

In the zoom type as described above, the power of the second positive lens group is the strongest among all the lens groups and often makes a variable magnification. However, if the power of the second lens group is strengthened for compactness, The number of lenses required for the two lens groups has increased,
It was a win because the length of the lens group became long.

Further, when the length of the lens group near the image plane increases due to an increase in the number of lenses or the like, a lens back is provided because a rotating mirror or prism is provided between the final lens and the image plane. There is a case that the necessary lens back cannot be taken for a necessary length, and such a vicious circle limits the compactness and the high magnification in the ordinary homogeneous medium lens system.

Further, it is composed of a plurality of lens groups, and is composed of a first negative lens group and a second positive lens group in order from the object side, and the distance between the first negative lens group and the second positive lens group decreases from the wide-angle end to the telephoto end. With a two-group type variable focal length lens that performs zooming by
The optical total length of the entire system can be reduced by increasing the power of the second positive lens group and decreasing the principal point distance between the first negative lens group and the second positive lens group. If the power of the lens unit is tight, the number of lenses required for aberration correction of the second positive lens unit increases, and the total length of the lens unit becomes long. Therefore, the principal point interval must be increased, and the optical total length of the entire system cannot be reduced so much.

Further, when the length of the second positive lens group increases due to an increase in the number of lenses, a rotary mirror, a prism, etc. are provided between the final lens and the image plane, so that the lens back needs to have a certain length. Will not be able to get the necessary lens back.

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

For example, taking the two-group zoom composed of the first positive lens group and the second negative lens group from the object side as an example, in the case of the two-group zoom, the focal length f of the entire system is the focal length of the first negative lens group. Is f ₁ and the imaging magnification of the second positive lens group is β ₂ , f = f ₁
It can be represented by × β ₂ . Therefore, even if the image forming magnification is the same, the shorter the focal length of the second positive lens unit, the shorter the total optical length of the entire system can be made. In this case, in order to shorten the focal length of the second positive lens group while ensuring the lens distance between the first positive lens group and the second negative lens group, the lens group is a telephoto type and a telephoto type. Power must be set so that the main point moves to the front side by strengthening the tendency of, and if attempting to shorten the total length by this method, Petzval sum will be large with a negative value and field curvature will be in the over direction. Occur.

Furthermore, 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 a strong power are combined, spherical aberration or high-order aberration may occur remarkably. Cannot be corrected. In this way, the compactness of the zoom lens and the Petzval sum correction are in a conflicting relationship in the case of a homogeneous medium system.

(3) Summary of the Invention It is an object of the present invention to provide a compact, high-performance and high-magnification variable focal length lens that eliminates the above-mentioned conventional drawbacks.

A further object of the present invention is to provide a variable focal length lens that is easy to assemble and adjust.

In order to achieve the above object, a variable focal length lens according to the present invention comprises a plurality of lens groups, wherein the first lens group is a negative lens group and the second lens group is a positive lens group in order from the object side. In a variable focal length lens that performs zooming by changing the distance between the first lens group and the second lens group, the second lens group is the closest to the object side in the vicinity of the optical axis A refractive power distribution type lens having a lowering power is provided, the refractive index on the optical axis of the refractive power distribution type lens is N _o , the height from the optical axis is h, and the refractive index distribution of the lens is N (H) = N _o + N ₁ h ² + N ₂ h ⁴ + N ₃ h ⁶ ... (N ₁ , N ₂ , N ₃ ,
(... is a coefficient), N ₁ <0 and N ₂ <0 are satisfied, and the refractive index increases toward the outermost part from the optical axis in the direction most orthogonal to the optical axis on the image plane side. A refractive power distribution type lens, the refractive index on the optical axis of the refractive power distribution type lens is N ₀ , the height from the optical axis is h, and the refractive index distribution of the lens on the image side is N (H) = N ₀ + N ₁ h ² + N ₂ h ⁴ + N ₃ h ⁶ ... (N ₁ , N ₂ , N ₃ ,
Is characterized by satisfying N ₁ > 0 and N ₂ > 0.

Hereinafter, the present invention will be described in detail with reference to examples.

(4) Example FIG. 1 and FIG. 2 are a sectional view and an aberration diagram showing a configuration example of a variable focal length lens according to the present invention, in which Ri (i = 1, 2,
3, .........) is the i-th surface counted from the object side, and Di (i =
, 1, 2, 3 ..., Indicates the axial air distance or axial thickness between the i-th surface and the i + 1-th surface counted from the object side, A is the first lens group, and B is the second lens group. The arrows in the figure represent the approximate movement trajectory of the moving lens group.

Also, the aberration diagram shows spherical aberration, astigmatism, and distortion aberration when the focal length f is 100 mm, 138.9 mm, and 188.4 mm.
Is spherical aberration for the g-line, d is spherical aberration for the d-line,
S is astigmatism on the sagittal surface, and M is astigmatism on the meridional surface.

Tables 1-1 to 1-3 below show lens data of the present variable focal length lens and coefficients representing the refractive index distribution of the gradient index lens used. In Table 1-1, f is the focal length,
FNO is the F number, 2W is the angle of view, Ri (i = 1,2,3, ...) is the radius of curvature of the i-th surface counted from the object side, and positive and concave when convex on the object side. The case is negative. Also, Di (i
= 1,2,3, ...) is the axial air gap or axial thickness between the i-th surface and the i + 1-th surface counted from the object side.
i (i = 1, 2, 3, ...) Indicates the refractive index and Abbe number of the i-th lens counted from the object side. Further, Ni (h) represents the refractive index distribution of the radial type gradient index lens located at the i-th position from the object side, and this distribution can be expressed by the following equation (1).

Ni (h) = N ₀ ＋ N ₁ h ² ＋ N ₂ h ⁴ ＋ N ₃ h ⁶ ＋ N ₄ h ⁸ ＋ N ₅ h ¹⁰ ＋ ……
(1) Here, h is the distance from the optical axis in the radial direction, N ₀ is the refractive index on the axis, and N ₁ , N ₂ , N ₃ , ... Therefore, Table 1-3 shows the refractive index distribution of each gradient index lens for g-line and d-line. Table 1-2 shows the focal lengths and the axial air distances between the lens groups during zooming.

Hereinafter, the variable focal length lens will be described in detail.

This variable focal length lens is composed of a first negative lens group A and a second positive lens group B in order from the object side, and the distance between the first negative lens group A and the second positive lens group B from the wide-angle end to the telephoto end. Is to reduce and scale. The second positive lens unit B is a radial type gradient index lens having a positive transfer power and having curved surfaces R7 and R8 in order from the object side, and a radial type refraction having a negative transfer power having curved surfaces R9 and R10. It is composed of two rate distribution type lenses.

The positive lens composed of the object-side curved surfaces R7 and R8 of the second positive lens group B has a refractive index distribution such that the refractive index decreases from the optical axis to the outer peripheral portion in the direction orthogonal to the optical axis, The refractive index distribution coefficients N ₁ and N ₂ are N ₁ <0 and N ₂ <0, respectively. Because of having such a refractive index distribution, the spherical aberration of the positive lens composed of the curved surfaces R7 and R8 in which a large amount of spherical aberration normally remains is corrected. That is, by setting N ₁ <0, N ₂ <0, the refractive index distribution on the surface of the object-side convex surface R7 of the positive lens can be made such that the refractive index decreases from the optical axis to the outer peripheral portion, The spherical aberration generated in the under direction can be corrected in the over direction.
Furthermore, since N ₂ <0, the third-order spherical aberration due to transfer due to the refractive index distribution inside the lens is in the under direction, which can be balanced with the occurrence of the above-mentioned aberration on the surface. Corrects aberrations and coma.

When N ₁ <0, the power (= -2N ₁ D) due to the transfer inside the lens is> 0, and the positive power of the positive lens composed of the curved surfaces R7 and R8 is shared by the power due to the transfer. The curvature of the lens can be made gentle and the occurrence of aberrations will be small. Further, since the power of the positive lens can be strengthened, it is possible to strengthen the tendency of the second lens unit B of the telephoto type.

The lens composed of the image-side curved surfaces R9 and R10 of the second lens group B is
The shape is a convex lens, but it has a negative refractive power because it has a refractive index distribution in the direction orthogonal to the optical axis such that the refractive index increases from the optical axis to the outer peripheral portion, and the distribution coefficient is N ₁ >
Since 0, N ₂ > 0, distortion at the wide-angle end and astigmatism at the wide-angle end and the telephoto end are corrected. Further, when N ₁ > 0, the negative power of the rear group is strengthened, and the tendency of the second positive lens group B to be of the telephoto type is strengthened. Also, while the light rays are traveling inside the lens, spherical aberration at the telephoto end is generated in the over direction, and the front group of the second positive lens group B, that is, the positive lens including the surfaces R7 and R8, is in the under direction. It corrects the spherical aberration that occurs in the.

Further, the effect of the gradient index lens in the present variable focal length lens will be described in detail below.

In the lens of the same type as this variable focal length lens, since the second positive lens group, which is usually composed of five or more lenses, is composed of only two lenses, it is possible to achieve weight reduction and compactness. In addition, since the number of constituent elements is small, the lens spacing accuracy and the eccentricity accuracy between the lenses in the second positive lens group are severe,
Assembly and adjustment work has become significantly easier. The second positive lens unit B corrects spherical aberration at the telephoto end, distortion at the wide-angle end, and astigmatism over the entire range.

Since positive transfer power and negative transfer power exist inside the object-side and image-side lenses of the second positive lens group B, respectively, the positive refractive power on the object side of the second positive lens group B and The negative refracting power on the image side can be strengthened, and the telephoto type tendency of the second positive lens unit B can be strengthened to move the principal point position to the object side. Therefore, the principal point distance between the first negative lens group A and the second positive lens group B can be reduced, and the focal length of the second positive lens group B can be shortened, so that the optical total length of the entire system can be shortened. Further, since the number of constituent lenses is small and the total length of the second positive lens unit B can be kept small, a sufficient lens back can be secured at the wide-angle end.

Further, when the negative refractive index of the rear group in the second positive lens group is increased in order to reduce the size of a lens such as this variable focal length lens, the field curvature usually deteriorates significantly in the over direction.
By using a gradient index lens, it has become possible to prevent the curvature of field from falling too much.

FIG. 3 and FIG. 4 are a sectional view and an aberration diagram thereof showing another configuration example of the variable focal length lens according to the present invention. Symbols and arrows in the drawing have the same meanings as in the above-described embodiment, and C represents the third lens group. Also, the aberration diagrams show that the focal length f is 100 mm, 140 mm,
The one in the case of 200 mm is shown. Further, Tables 2-1 to 2-3 below show lens data of the present variable focal length lens and coefficients representing the refractive index distribution of the used gradient index lens. The symbols are the same as in the above embodiment.

Hereinafter, the variable focal length lens will be described in detail.

This variable focal length lens is composed of a first negative lens group A, a second positive lens group B, and a third negative lens group C in order from the object side, and the first negative lens group A and the second negative lens group A from the wide-angle end to the telephoto end.
The distance between the positive lens group B and the third positive lens group B is reduced, and the distance between the second positive lens group B and the third negative lens group C is increased to perform zooming.

The second positive lens unit B is a radial type gradient index lens having a positive transfer power and having curved surfaces R7 and R8 in order from the object side, and a radial type refraction having a negative transfer power having curved surfaces R9 and R10. It is composed of two rate distribution type lenses.

The positive lens composed of the object-side surfaces R7 and R8 of the second positive lens group B has a refractive index distribution such that the refractive index decreases from the optical axis to the outer peripheral portion in the direction orthogonal to the optical axis, The refractive index distribution coefficients N ₁ and N ₂ are N ₁ <0 and N ₂ <0, respectively. Because of having such a refractive index distribution, the spherical aberration of the positive lens composed of the curved surfaces R7 and R8 in which a large amount of spherical aberration normally remains is corrected. That is, by setting N ₁ <0, N ₂ <0, the refractive index distribution on the surface of the object-side convex surface R7 of the positive lens can be made such that the refractive index decreases from the optical axis to the outer peripheral portion, The spherical aberration generated in the under direction can be corrected in the over direction. Furthermore, since N ₂ <0, the third-order spherical aberration due to transfer due to the refractive index distribution inside the lens is in the under direction, which can be balanced with the occurrence of the above-mentioned aberration on the surface. Corrects aberrations and coma.

When N ₁ <0, the power (= -2N ₁ D) due to the transfer inside the lens is> 0, and the positive power of the positive lens composed of the curved surfaces R7 and R8 is shared by the power due to the transfer. The curvature of the lens can be made gentle and the occurrence of aberrations will be small. Further, since the power of the positive lens can be strengthened, it is possible to strengthen the tendency of the second lens unit B of the telephoto type.

The image-side lens of the second positive lens group B has a convex shape, but has a refractive index distribution such that the refractive index increases from the optical axis to the outer peripheral portion in the direction orthogonal to the optical axis, and thus is negative. It has a refracting power of, and corrects distortion at the wide-angle end and astigmatism at the wide-angle end and the telephoto end.
Also, while the light rays are traveling inside the lens, spherical aberration is generated in the over direction, particularly at the telephoto end, and the second positive lens unit B
The spherical aberration generated in the under direction is corrected by the front group of, that is, the positive lens including the curved surfaces R7 and R8.

Further, the effect of the gradient index lens in the present variable focal length lens will be described in detail below.

In the lens of the same type as this variable focal length lens, since the second positive lens group, which is usually composed of five or more lenses, is composed of only two lenses, it is possible to achieve weight reduction and compactness. In addition, since the number of constituent elements is small, the lens spacing accuracy and the eccentricity accuracy between the lenses in the second positive lens group are severe,
Assembly and adjustment work has become significantly easier. The second positive lens unit B corrects spherical aberration at the telephoto end, distortion at the wide-angle end, and astigmatism over the entire range.

Since positive transfer power and negative transfer power exist inside the object-side and image-side lenses of the second positive lens group B, respectively, the positive refractive power on the object side of the second positive lens group B and The negative refracting power on the image side can be strengthened, and the telephoto type tendency of the second positive lens unit B can be strengthened to move the principal point position to the object side. Therefore, the principal point distance between the first negative lens unit A and the second positive lens unit B can be reduced, and the focal length of the second positive lens unit B can be shortened, so that the optical total length of the entire system can be shortened.
Further, since the number of constituent lenses is small and the total length of the second positive lens unit B can be kept small, a sufficient lens back can be secured at the wide-angle end.

Further, like the present variable focal length lens, when the negative refractive index of the rear group in the second positive lens group is increased in order to reduce the size of the lens, normally the field curvature is significantly deteriorated in the over direction. It became possible to prevent the curvature of field from falling excessively by using a rate distribution type lens.

When at least one gradient index lens is used for at least the second positive lens group as in the present invention, spherical aberration is used when used in front of the lens group, and spherical aberration is used when used in the rear side of the lens group. Astigmatism and spherical aberration can be corrected. Further, since the Petzval sum is small in occurrence, the tendency of the second positive lens unit to be in the telephoto type can be strengthened, and a tight curved surface for Petzval correction and a tight power arrangement are not required, and the occurrence of high-order aberrations is reduced. That is, at least the second positive lens group can be configured with a small number of lenses to perform the above-mentioned aberration correction, so that the second positive lens group can be made compact, and further, the entire variable focal length lens system can be shortened and reduced in weight.

Also, by using a gradient index lens for a plurality of lens groups, the number of variable focal length lenses can be significantly reduced.
Ghosts can be significantly improved. Surface reflection,
The light amount loss of the entire system due to internal absorption is small, the T number can be made bright, and it becomes possible to secure a sufficient amount of transmitted light without a multilayer coating. As a matter of course, since the number of components is small, assembly and adjustment work is easy, and it can be mounted on ultra-small optical devices such as ultra-small cameras and gastroscopes that require a variable focal length lens to be installed. Becomes

(5) Effects of the Invention As described above, the variable focal length lens according to the present invention is a lightweight and compact lens, and further, the number of constituent lenses of each group is small and the lens can be easily assembled and adjusted.

[Brief description of drawings]

1 and 2 are a sectional view and an aberration diagram showing a configuration example of a variable focal length lens according to the present invention. FIG. 3 and FIG. 4 are a sectional view and an aberration diagram showing another configuration example of the variable focal length lens according to the present invention. A ... First lens group B ... Second lens group C ... Third lens group D ... Fourth lens group d ... Spherical aberration for d line g ... Spherical aberration for g line S ... On sagittal surface Astigmatism on M ... Astigmatism on meridional surface

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Jun Hattori 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Akinaga Horiuchi 770 Shimonoge, Takatsu-ku, Kawasaki-shi, Kanagawa Canon Inc. Company Tamagawa Plant (56) References JP-A-58-220115 (JP, A)

Claims (1)

[Claims] 1. A plurality of lens groups, wherein the first lens group is composed of a negative lens group and the second lens group is composed of a positive lens group in order from the object side, and the distance between the first lens group and the second lens group. In the variable focal length lens for varying the power by changing the distance, the second lens group has a refractive power distribution type lens that is highest in the vicinity of the optical axis on the object side and decreases in the outer peripheral portion, Let N _{0 be} the refractive index on the optical axis of the refractive power distribution type lens and h be the height from the optical axis, and the refractive index distribution of the lens is N (h) = N ₀ + N ₁ h ² + N ₂ h ⁴ + N ₃ h ⁶ ... (N ₁ , N ₂ , N ₃ ,
(... is a coefficient), N ₁ <0 and N ₂ <0 are satisfied, and the refractive index increases toward the outermost part from the optical axis in the direction most orthogonal to the optical axis on the image plane side. A refractive power distribution type lens, the refractive index on the optical axis of the refractive power distribution type lens is N ₀ , the height from the optical axis is h, and the refractive index distribution of the lens on the image plane side is N (h) = N ₀ + N ₁ h ² + N ₂ h ⁴ + N ₃ h ⁶ ... (N ₁ , N ₂ , N ₃ ,
The variable focal length lens is characterized by satisfying N ₁ > 0 and N ₂ > 0 when represented by a coefficient.