JP2007004062A - Zoom optical system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a zoom optical system configured such that an image surface is fixed by zooming, and aberration fluctuation in zooming is reduced, and which has high optical performance all over the zoom range and whose optical entire length is shortened. <P>SOLUTION: In the zoom optical system configured by arranging a plurality of optical groups whose optical power is varied, one or more optical groups and a diaphragm in an optical axis direction and performing zooming by changing the power of the plurality of optical groups whose optical power is varied, the plurality of optical groups whose optical power is varied have a plurality of optical elements Ld respectively including rotationally asymmetric surfaces, moving in a direction different from an optical axis and changing the power in the optical group, and the one or more optical groups include an optical group where an optical element having positive refractive power and an optical element having negative refractive power are arranged from the diaphragm to an image side in order and which is immobile for zooming. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a zoom optical system suitable for an imaging device, a projection device, an exposure device, a reading device, and the like.

  In recent years, with the widespread use of digital cameras, camera-equipped mobile phones, and the like, compact and high-resolution zoom optical systems have been desired.

  In a compact and high-resolution zoom optical system, zooming is normally performed by moving a plurality of lens groups in the optical axis direction with respect to a light receiving surface (CCD or the like). At this time, in the zoom method in which the lens group is moved in the object direction, the total optical length (the length from the first lens surface to the image surface) becomes long, which is one cause that hinders the miniaturization of the entire lens system.

  On the other hand, an optical system using an optical element called an Alvarez lens that changes the power of the entire system by moving the optical element in a direction different from the optical axis direction has been proposed (Patent Documents 1 and 2). Non-patent document 1).

  Various zoom optical systems that perform zooming using these Alvarez lenses have been proposed (Patent Document 3).

  In the optical system disclosed in Patent Document 1, a curved surface represented by a cubic function is given to the lens, and the power is changed by shifting the two lenses relatively in a direction different from the optical axis direction. Since this optical system does not extend the lens group in the optical axis direction, the total lens length can be shortened by using it in the zoom optical system.

  The optical system disclosed in Patent Document 2 discloses an optical system in which aberrations are removed by providing a lens with a curved surface of not only a third-order but also a higher-order term, particularly a fifth-order term.

  Further, Patent Document 3 proposes an example in which this lens is used in a zoom optical system. The principle of changing the power while disposing at least two lenses and keeping the image point constant is disclosed.

  On the other hand, Non-Patent Document 1 discloses an optical system including a rotationally asymmetric optical element. Unlike the normal coaxial lens system, this optical system does not have a common axis (optical axis). Such a non-coaxial optical system is called an off-axial optical system, and the plane at the intersection of the component plane and the reference axis when the path along which the ray passing through the image center and the pupil center follows is used as the reference axis. It is defined as an optical system that includes a curved surface (Off-Axial curved surface) whose normal is not on the reference axis.

In this case, the reference axis has a bent shape. Therefore, paraxial theory based on Off-Axial theory must be used instead of the paraxial theory of paraxial systems, which is usually used to calculate paraxial quantities. The optical principle of this method is introduced in detail in Non-Patent Document 1, and is performed, for example, by calculating a 4 × 4 determinant based on the curvature of the surface.
U.S. Pat. No. 3,305,294 U.S. Pat. No. 3,583,790 Japanese Patent Application No. 01-248118 Optics Vol.29 No.3 (2000)

  Patent Document 1 and Patent Document 2 describe a method of changing power and correction of aberration using a set of rotationally asymmetric lenses, but the image plane cannot be made constant when the power is changed.

  Further, Patent Document 3 describes the principle of changing the power while keeping the image point constant, but it has not yet reached the design of a zoom optical system that can correct aberrations and actually obtain a good image.

  In order to construct a zoom optical system using an Alvarez lens, the zoom lens must be constructed so that the image plane is constant by zooming and aberration fluctuations due to zooming are reduced.

  According to the present invention, by appropriately using an optical group having a plurality of optical elements including a rotationally asymmetric surface and moving in a direction different from the optical axis and one or more optical groups, the image plane is constant by zooming, and in zooming An object of the present invention is to provide a zoom optical system having a short optical total length with little aberration variation, high optical performance over the entire zoom range.

The zoom optical system of the present invention is
A zoom optical system in which a plurality of optical groups with variable optical power, one or more optical groups and a diaphragm are arranged in the optical axis direction, and zooming is performed by changing the power of the plurality of optical groups with variable optical power. A plurality of optical groups having variable optical power, each having a plurality of optical elements Ld each including a rotationally asymmetric surface and moving in a direction different from the optical axis to change the power in the optical group; The one or more optical groups include a stationary optical group for zooming in which an optical element having a positive refractive power and an optical element having a negative refractive power are arranged in order from the stop toward the image side. It is a feature.

  According to the present invention, it is possible to obtain a zoom optical system having a short optical total length and having a high optical performance over the entire zoom range with a constant image plane by zooming and less aberration fluctuation during zooming.

  First, before describing the embodiments of the present invention, the rotationally asymmetric surfaces of the off-axial optical system constituting the zoom optical system of the present invention and how to express the structural specifications thereof will be described.

  The Off-Axial optical system is shown as Comparative Example 1 of the present invention described later. As shown in FIG. 2, an absolute coordinate system is set with the surface SO on the light incident side as the reference plane and the center PO of the reference plane SO as the origin. A path along which a ray passing through the origin PO and the pupil center follows is set as a reference axis.

  A straight line connecting the image center IPO and the origin of the absolute coordinate system which is the center PO of the reference surface SO is defined as the Z axis, and the direction is positive from the first surface toward the image center. This Z-axis is called an optical axis.

  Furthermore, the Y axis is a straight line that passes through the origin PO and forms 90 ° counterclockwise with respect to the Z axis according to the definition of the right-handed coordinate system, and the X axis is a straight line that passes through the origin and is perpendicular to the Z and Y axes. .

  The paraxial values shown below are the results of off-axial paraxial tracking. Unless otherwise noted, off-axis paraxial tracking is performed and the paraxial value is calculated.

The zoom optical system according to the present invention has a rotationally asymmetric aspheric surface, and the shape is expressed by the following equation.
(Equation 1)
^{z = C02y 2 + C20x 2 +} C03y 3 + C21x 2 y + C04y 4 + C22x 2 y 2 + C40x 4 + C05y 5 + C23x 2 y 3 + C41x 4 y + C06y 6 + C24x 2 y 4
+ C42x ⁴ y ² + C60x ⁶
Since Formula 1 is only an even-order term with respect to x, the curved surface defined by Formula 1 is a plane-symmetric shape with the yz plane (see FIG. 2) as the plane of symmetry.

Further, when the following conditions are satisfied, the shape is symmetrical with respect to the xz plane (see FIG. 2).
(Equation 2)
C03 = C21 = C05 = C23 = C41 = t = 0
Furthermore, when the following conditions are satisfied, it represents a rotationally symmetric shape.
(Equation 3)
C02 = C20
(Equation 4)
C04 = C40 = C22 / 2
(Equation 5)
C06 = C60 = C24 / 3 = C42 / 3
When the above conditions are not satisfied, the shape is rotationally asymmetric.

  The shapes of the rotationally symmetric surface and the rotationally asymmetric surface in the following examples and comparative examples are based on Equations 1 to 5.

  FIG. 1 is a lens cross-sectional view of Example 1 of the present invention.

  In FIG. 1, T, M, and W are lens cross-sectional views at the telephoto end (zoom position where the power of the entire system is the smallest), the intermediate zoom position, and the wide angle end (zoom position where the power of the entire system is the largest), respectively. It is.

  FIG. 5 is a lens cross-sectional view for explaining each element by selecting an intermediate zoom position (M in FIG. 1) of Example 1 in FIG. 1 as an example.

  The zoom optical system according to the first exemplary embodiment is a photographing lens system used in an imaging apparatus. In the lens cross-sectional view, the left side is the object side and the right side is the image side.

  The zoom optical system according to the first embodiment may be used as a projection device (projector). In this case, the left side is a screen and the right side is a projection surface.

  1 and 5, G1 and G2 are optical groups whose optical power is variable (in this embodiment, optical power and focal length change during zooming of the zoom optical system). G3 is an optical group whose optical power is unchanged (or substantially unchanged).

  SP is an aperture stop (stop), and is disposed between the optical group G2 and the optical group G2.

  Zooming is performed while changing the power of the two optical groups G1 and G2 whose optical power is variable to keep the image plane IP constant.

  The two optical groups G1, G2 having variable optical power each include a rotationally asymmetric surface, move in a direction different from the optical axis, and change the power in the optical groups G1, G2, two optical elements E1, E2, E3 and E4.

  Optical power refers to the power of the surface located on the optical axis. When an optical element having a rotationally asymmetric surface is decentered and the surface on the optical axis changes, the optical power also changes accordingly. To do.

  Examples of the present invention will be described. The imaging surface to be targeted in the embodiment is assumed to be a CCD, and the size thereof is 1/4 inch (2.7 mm length × 3.6 mm width). The brightness was F number 4 at the wide-angle end (zoom position where the optical power (power) of the entire system was the highest), and F number 6 at the telephoto end (zoom position where the optical power of the entire system was the lowest).

  In Example 1, the optical elements are composed of a total of six elements, and the optical elements E1, E2, E3, and E4 have a rotationally asymmetric shape in order from the object side (front) to the image side. Eccentric in the direction, and the amount of eccentricity changes continuously.

  Further, the amounts are positive and negative with respect to each other, and the absolute values are equal. The optical elements E5 and E6 have a rotationally symmetric aspherical shape. However, when an asymmetrical aberration remains on the optical axis, the optical elements E5 and E6 may have a rotationally asymmetrical shape in order to remove it. Further, the first group G1 is constituted by the optical elements E1 and E2.

  Similarly, the second group G2 is constituted by the optical elements E3 and E4. For the surface number, the reference surface, which is the origin of the absolute coordinate system, is defined as S0, the first surface of the optical element E1 is set to S1, and sequentially becomes S2, S3, S4, and the stop SP after the surface S4 (behind the optical element E2). Because there is, it is set as S5. The first surface of the optical element E3 is set to S6, and numbers are assigned in order, and the image surface IP is S17.

  Hereinafter, the rotationally asymmetric group (optical elements E1 to E4) and the rotationally symmetric group (optical elements E5 to E6) that are continuously decentered in the Y-axis direction are referred to as an eccentric movable block and an auxiliary block, respectively.

  Since only the eccentric movable blocks G1 and G2 increase power and make it difficult to correct aberrations, an auxiliary block G3 is provided. The rotationally asymmetric optical elements E1 to E4 have a rotationally asymmetric shape on both sides. The flat glass Ga placed immediately before the CCD surface is an infrared cut filter, a CCD cover glass, or the like.

Table 3 shows lens data of Example 1. The amount of deviation of each optical element (lens) from the Z-axis is as shown in Table 4, the coefficients of the rotationally symmetric aspheric surface expressed by Equation 12 are shown in Table 5, and the coefficients of the polynomial surface expressed by Equation 1 are as follows. Values are shown in Table 6. However, in Expression 12, h2 = X2 + Y2 is satisfied, c is a radius of curvature, and A and B are coefficients.
(Equation 12)

  FIG. 1 shows an optical path diagram at that time in the order of a telephoto end (a position where the optical power of the entire system is minimum), an intermediate zoom position, and a wide angle end (a position where the optical power of the entire system is maximum). Yes.

  The light beam incident on the reference surface S0 first enters the first group G1. The first group G1 includes two optical elements E1 and E2, and the surface numbers are S1 to S4.

  The optical elements E1 and E2 are decentered in the Y-axis direction, and their amounts are positive and negative with respect to each other as shown in Table 4, so that the absolute values are equal.

  Thereby, the power of the first lens group G1 is changed from positive to negative during zooming from the telephoto end to the wide angle end. The light beam emitted from the first group G1 then passes through the stop S5 (SP) and enters the second group G2. Similarly to the first group G1, the second group G2 includes two optical elements E3 and E4, and the surface numbers are S6 to S9. The optical elements E3 and E4 are decentered in the Y-axis direction, and their amounts are positive and negative with respect to each other as shown in Table 4, so that their absolute values are equal.

  Thereby, the power of the second lens group G2 is changed from negative to positive during zooming from the telephoto end to the wide-angle end. The light beam that has passed through these eccentric movable blocks G1 and G2 enters the next auxiliary block G3. The auxiliary block G3 supplements the insufficient power of the eccentric movable blocks G1, G2. The auxiliary block G3 includes two optical elements E5 and E6 each having a surface S10 to a surface S13 which are rotationally symmetric aspheric surfaces. Light rays that have passed through these optical elements E5 and E6 pass through an infrared cut filter and CCD cover glass Ga, and form an image without changing the image plane IP.

  Next, aberration diagrams at the telephoto end (T), the intermediate zoom position (M), and the wide-angle end (W) are shown in FIGS. 6 (A), 6 (B), and 6 (C), respectively. The horizontal axis represents the position of the ray on the pupil, and the vertical axis represents the deviation from the principal ray on the image plane. The range of the vertical axis is ± 20 μm. The numbers in FIGS. 6A, 6B, and 6C are angle-of-view numbers, which are as shown in FIG. 7 on the image plane (imaging device). Since it is symmetrical about the x axis, only the positive case needs to be considered in the x direction. It can be seen that the coma aberration is satisfactorily removed from the telephoto end to the wide-angle end when a light beam having an angle of view of 0 ° is viewed.

  FIG. 8 shows a distortion grating at the telephoto end T, the intermediate zoom position M, and the wide-angle end W. The vertical and horizontal dimensions of the lattice are 1/4 inch (2.7 mm length × 3.6 mm width). From this, it can be seen that the distortion is well suppressed.

  Since only the eccentric movable blocks G1 and G2 have high power and it is difficult to correct aberrations, the auxiliary block G3 is already described above. Furthermore, in this embodiment, the auxiliary block G3 composed of two coaxial optical elements (coaxial lenses) is arranged on the image plane side of the eccentric movable block G2. The powers of the two coaxial optical elements E5 and E6 are arranged in order of positive and negative refractive powers in order from the stop SP toward the image side (as they move away), and the eccentric movable blocks G1 and G2 are assisted. Yes.

  This is a generally known telephoto type, and the back focus (distance from the final optical surface to the image plane IP) can be shortened as compared with the case where the coaxial optical elements E5 and E6 are not provided. FIG. 9 shows a comparison between the lens data obtained by correcting the example implemented based on Patent Document 3 to a 1/4 inch size scale and Example 1. FIG. Comparing the total distance after the eccentric movable blocks G1 and G2, it can be seen that the former is 9.77 mm and the latter is 4.42 mm (the sum of S9 to S16), which is less than half. Therefore, the back focus can be shortened if the power of the two coaxial optical elements E5 and E6 constituting the auxiliary block G3 is arranged to have positive and negative refractive powers as the distance from the stop SP increases.

Further, when the power of the optical element E5 having a positive refractive power is φp and the power of the optical element E6 having a negative refractive power is φn, φn <φp
Is satisfied.

Also, at all zoom positions, the absolute value of the optical power of each of the optical elements E1 to E4 of the plurality of optical groups with variable optical power is | φd | max, and the optical values of the optical elements E5 and E6 of the optical group When the maximum absolute value of dynamic power is | φs | max | φd | max <| φs | max
Is satisfied.

  As a result, high-definition image quality that can reduce the power of the eccentric movable block can be achieved. FIG. 12 shows changes in power for each of the optical elements E1 to E6 with respect to the power of the entire system. From this, it can be seen that | φp |> | φn | and | φd | max <| φs | max.

  Also, at the wide-angle end, the power of the whole system becomes strong, and the power of each group tends to become strong. In general, an optical system with strong power tends to generate aberrations. Although it is inevitable that the power of the entire system becomes strong at the wide-angle end, aberrations can be suppressed if the power of each group can be reduced. Looking at the power change shown in FIG. 3, the power of the first group G1 changes linearly with respect to the power of the entire system, and the second group G2 changes in a curved line. It is clear from Equation 8 and Equation 9. However, there are various solutions depending on the curved surface shape of the rotationally asymmetric optical element and how to provide the amount of deviation. If this solution can reduce the power on the wide angle side, the aberration can be suppressed.

For that purpose, as shown in FIG. 10, the linear change G1 of the first group G1 is changed to a shape G1 ″ having a curvature in the positive power direction, and the second group G2 is changed to a negative power direction. With this configuration, the power is increased on the telephoto side, but the power can be decreased on the wide-angle side, that is, the intersection point where the power fluctuations coincide with each other may be on the wide-angle side. Compare the absolute values of the powers of the groups G1 and G2 at the wide angle end with | φgw | max, and compare the absolute values of the powers of the groups G1 and G2 at the telephoto end with a smaller value (G1). If | φgt | min, Equation 13 is satisfied.
(Equation 13)
| Φgw | max <| φgt | min (2)
Next, FIG. 11 shows the relationship between the power of the entire system and changes in the powers G1 + G2 of the combined systems of the groups G1, G2 and the first group G1 and the second group G2. Looking at this, as the power of the entire system increases, the power of the first group G1 of the eccentric movable block changes from positive to negative, and the power of the second group G2 changes from negative to positive.

  The intersection is on the wide angle side (W), and the absolute value of the power on the wide angle side is smaller than that on the telephoto side. On the wide angle side, the absolute value of the power of each group is reduced to suppress aberrations.

  Furthermore, aberration can also be suppressed by changing the power of the optical elements constituting the group to be positive or negative. Therefore, the power distribution within each group at the wide-angle end where aberration is likely to occur is made positive, negative, and reverse. Therefore, FIG. 12 shows changes in power for each of the optical elements E1 to E4 with respect to the power of the entire system. Looking at this, the power distribution within the group is in the order of positive, positive, negative, and negative power from the object side at the telephoto end, while in the order of positive, negative, positive, and negative power at the wide-angle end. It has become.

  The first group G1 and the second group G2 include optical elements whose optical powers have opposite signs at the wide angle end.

The eccentric movable blocks G1 and G2 change in power according to the change in the power of the entire system, while the auxiliary block G3 is constant. The power of the whole system can be obtained from the power of these two blocks and the principal point interval. In order to set the power of the entire system to a desired value, various combinations of the power arrangement of these two blocks are conceivable. However, when the power of the eccentric movable blocks G1 and G2 is increased, the aberration generated according to the eccentricity becomes large and it is difficult to suppress it. Therefore, the powers of the eccentric movable blocks G1 and G2 must be reduced or approximately equal by comparing the powers of the two. Therefore, at the telephoto end where the power of the entire system is weak, when the absolute value (sum) of the power of the eccentric movable block (G1 + G2) and the absolute value of the power of the auxiliary block G3 are | φdt | and | φst | To meet the requirements of
(Equation 14)
| Φdt | ≦ | φst | (1)
FIG. 13 shows the power (sum) of the eccentric movable block (G1 + G2) of Example 1 and the power of the auxiliary block G3. This shows that Formula 14 is established.

In general, it is known that when Petzval is large, the curvature of field increases and vice versa. Also in this embodiment, the curvature of field is kept small by reducing Petzval. Petzval is given by the following equation when the power at the optical element Ei (i = 1 to n) is φEi and the refractive index of the material is nEi.
(Equation 15)
PEi = φEi / nEi
In a normal zoom optical system using a coaxial optical element, this value is always constant. However, this is not constant in an optical system in which the optical elements are continuously decentered and the power changes as in this embodiment. Further, since the refractive index of the glass material is small from 1.45 to 1.9 with respect to the power change, the Petzval change may be referred to as the power change. Therefore, in order to suppress this Petzval, the maximum absolute value of the power at all zoom positions in the first group G1 and the second group G2 is | φ | max, and the total of the first group G1 and the second group G2 is φ12. Then, the range of power change is determined so as to satisfy the following equation.
(Equation 16)
− | Φ | max ≦ φ12 ≦ | φ | max (3)
From the power change diagram of FIG. 11, it can be seen that the total power G1 + G2 of the first group G1 and the second group G2 is within that range.

  Next, let's examine Petzval. Table 7 shows the groups E1 to E6, the eccentric movable block (G1 + G2), the auxiliary block G3, and the Petzval of the entire system. Similarly, Table 8 shows a conventional example as a comparative example.

  When the sum of the Petzval sums is compared between the two, the Petzval is smaller in Example 1. Since the sum of Petzval in the auxiliary block G3 is substantially equal to 0, the auxiliary block G3 removes curvature of field with two optical elements having positive and negative refractive powers.

  From this, the Petzval sum of the entire system can be considered to be almost all the contribution of the eccentric movable blocks G1 and G2. Therefore, paying attention to the Petzval of the eccentric movable blocks G1 and G2, it shows a low value of -0.13 at the telephoto end and 0.3 at the intermediate zoom position and the wide angle end, which is about a half of that of the conventional example. I understand that. This makes it possible to remove the curvature of field.

  Actually, when viewing the aberration diagram, curvature of field occurs at the intermediate zoom position and the wide-angle end, whereas at the telephoto end, it is not the curvature of field but only the inclination of the image plane. From this, the total Petzval sum corresponds to the aberration.

  Next, FIG. 14 shows changes in Petzval sum with respect to the power of the entire system of the first group G1, the second group G2, the first group G1, and the second group G2. The conventional example is also shown in FIG. 15 as a comparative example. When these are compared, the absolute values of the individual Petzval sums of the first group G1 and the second group G2 are almost equal in the conventional example and in the example 1, but the amount of change is smaller in the example 1 in terms of the amount of change. In the first embodiment, the positive Petzval in the first group G1 and the negative Petzval in the second group G2 cancel each other, and the sum thereof changes near 0, whereas in the conventional example, the first Petzval does not cancel out. The Petzval sum of the group G1 and the second group G2 is always positive. These two results are the results when the power φ12 is in the above range. As described above, in Example 1, field curvature was removed.

Ideally, if the positive and negative powers of the first group G1 and the second group G2 are opposite and the absolute values are equal, the refractive index of the glass material is in a limited range. And field curvature can be eliminated. The Petzval sum can be theoretically reduced if the powers of the first group G1 and the second group G2 are only positive, negative, and different and the absolute values are close. However, when such power is used in the first group G1 and the second group G2, there is a drawback that the power in the entire system is reduced and it is difficult to widen the angle. The method of gaining power in the entire system while suppressing the Petzval sum can be found from the following equation.
(Equation 17)
φ _total = φ ₁ + φ ₂ −eφ ₁ φ ₂
Formula 17 is a formula expressing the total power φtotal of the eccentric movable blocks G1 and G2 by the powers φ1 and φ2 of the first group G1 and the second group G2, and the principal point interval e therebetween. It can be seen that when the powers φ1 and φ2 are positive, negative, and different, the absolute values are close, and when the glass material is not changed, it is desirable to increase the principal point interval e as much as possible in order to increase the total power φtotal. Therefore, the principal point intervals were compared between Example 1 and the conventional example. Since the powers of the whole system are different from each other, the principal point intervals with respect to the power of the whole system are compared. These are shown in Tables 9 and 10.

  When these are compared, the distance between the principal points is substantially equal at the telephoto end and the wide-angle end, but the distance between the principal points in the first embodiment is larger in the middle. Accordingly, in the first embodiment, the power of the entire system is increased by increasing the distance between the principal points while keeping the absolute values of the powers of the first group G1 and the second group G2 the same.

  In this embodiment, the angle formed between the light beam and the optical axis is suppressed in order to reduce the deviation between the upper and lower lines of the axial light beam generated in the first embodiment (this will be referred to as axial coma aberration). went.

Therefore, attention is paid to the inclination of the tangent line at the point where the optical axis of each surface Si intersects as shown in FIG. If the tangent line of a surface has a positive inclination with respect to the optical axis, a surface with a negative inclination is similarly arranged to suppress the angle between the light beam and the optical axis and reduce the axial coma. be able to. As a result, each surface has an inclination as shown in the optical path diagram of FIG. 5. For example, when looking at the telephoto end surface S1, the tangent has a negative inclination with respect to the Y axis, whereas the tangent to the surface S3 is reversed. The axial coma aberration is reduced by making a positive tilt and a reverse correction. The axial coma can be suppressed by reducing this sum. Therefore, this range is set as the following range.
(Equation 18)

  Here, i is the surface number represented by Si, and Ki is the inclination at the surface Si.

  The maximum value of the inclination is Kmax, and the sum of the inclinations Ki of the optical surfaces of the entire system is ΣKi.

If the local curvature of the curved surface is C, the surface power φ is given by Non-Patent Document 1 as follows.
(Equation 19)
φ = 2 (N′cosθ′−Ncosθ) C
Here, N and θ are the refractive index and incident angle on the side where the light beam is incident, respectively, and N ′ and θ ′ are those on the side where the light beam is emitted.

From this equation, it can be seen that even if the local curvature is the same, the power varies depending on the refractive index before and after the curved surface, and the incident angle and exit angle. The following relation holds from Snell's law.
(Equation 20)
Nsinθ = N'sinθ '
By substituting this into Equation 19 and eliminating θ ′, the following equation is derived.
(Equation 21)

Here, when N = 1 and the equation is expressed in a graph with N ′ and θ, it is as shown in FIG.
θ was set in the range of −0.3 [rad] to 0.3 [rad] (approximately ± 17 ° range). From now on in this range

It can be seen that the amount of change varies more sensitively to changes in the refractive index N ′ than to changes in the incident angle θ.

  The value of power φ changes depending on the refractive index of the glass material when the front and back of the surface is air → glass material and glass material → air, and the power φ is larger when the front and back of the surface is glass material → air and the glass material is larger. growing. From the above, it can be seen that even if the radius of curvature is the same, the surface power varies depending on the refractive index.

On the other hand, in order to greatly change the power of the surface, it is necessary to increase the inclination of the curved surface and the change in curvature. Therefore, it can be said that the maximum value of the inclination tends to increase as the amount of change in power increases. From the above considerations, the power varies depending on the refractive index, so the range is defined as follows.
(Equation 22)
0 (degrees / mm) ≦ Kmax / Δφ ≦ 3 (degrees / mm) (5)
However, Δφ is the amount of change in power on each surface. FIG. 18 shows the relationship between Kmax and the power change amount Δφ for each surface in Example 1.

  Further, the straight line shown in FIG. 18 is a line of Kmax = Δφ, which is a relationship when θ and θ ′ are both 0 and the refractive index is 1. The surface above this straight line is the case of air → glass material, and the surface below the glass is glass material → air. It can also be seen that the power φ increases as the refractive index of the glass material increases, and approaches a straight line.

  Further, FIG. 19 shows the value of Kmax / Δφ for each surface. The slope Kmax is 0.16 to 2.5, which indicates that the range of Equation 22 is satisfied.

  Next, Table 11 shows the inclination of the tangent line of each surface. Even if the surfaces have the same inclination, the traveling direction of the light beam varies depending on the refractive index n as shown in FIG. 20, so that the inclination of the surface from which the light beam is emitted from the larger refractive index to the smaller one is set to -1. Multiply and judge. When the total is seen, it turns out that Numerical formula 18 is realized in all.

  Next, in order to confirm that the inclination of the tangent at the point intersecting the optical axis of each surface removes the axial coma, FIG. 21 shows the amount of axial coma on each surface. The horizontal axis is the surface number, and the vertical axis is the distance from the point to the surface Si when the underline of the light beam that has exited the surface Si reaches the optical axis as shown in FIG. ). The direction from the object plane to the image plane is positive.

  Since the surface S6 is a stop surface, it can be seen that the back focus of the underline of the surface S5 and the surface S6 has the same value in any figure in FIG. The absolute amount of the difference shown in FIG. 21 is the absolute amount of the back focus difference between the overline and the underline. If this is 0, the overline and underline are imaged at the same point, and no axial coma occurs. It will be.

  As can be seen from FIG. 21, the back focus of the tenth surface, which is the final surface of the rotationally asymmetric optical element, is about the same as the upper and lower lines. Therefore, the axial coma is eliminated, and after the rotationally asymmetric lens, the light beam becomes equivalent to the coaxial system, and no off-axial aberration remains. Therefore, the auxiliary lens arranged on the image plane side from the eccentric movable block does not need to be a rotationally asymmetric surface.

  Furthermore, in order to perform zooming while maintaining compactness, it is necessary to largely move the principal point position from the position of the optical element. In a conventional optical element in which a cubic curve is given on one side, the principal point position is only on the surface having the cubic coefficient, so that it does not vary greatly. As a method for greatly changing the principal point position, for example, one surface has a curvature and the shape of a rotationally asymmetric optical element is made to be a meniscus.

  Unlike biconvex lenses and biconcave lenses, meniscus-shaped lenses are lenses whose principal point can be outside the lens, and by adopting this shape for rotationally asymmetric lenses, the principal point can be greatly varied outside the lens. Can be made.

  However, if the rotationally asymmetric optical element is formed in a meniscus shape, a shift occurs between the upper and lower lines of the axial light beam at the telephoto end or the wide-angle end (when the light beam passes through the end of the optical element). Therefore, this must be corrected by another optical element. In a method for solving this, the optical element to be corrected is changed to a meniscus having a reverse inclination, and the deviation between the upper line and the lower line is canceled.

  When introducing a higher-order coefficient of the third order or higher into the surface, the coefficient is determined by paying attention to this. Further, it is desirable that the meniscus shape is shaped in the direction of reducing the mutual distance. This is because axial coma can be removed while minimizing on each surface by bringing optical elements closer to each other.

  As described above, axial coma is removed.

Assuming that the maximum amount when a rotationally asymmetric optical element having a surface shape given by Expression 10 is decentered by δ in the Y-axis direction is δmax, the power of the optical element can be obtained from Expression 11. Assuming that the thickness of the optical element is d, the following equation is established from the condition that the edge thickness of the optical element is 0 or more as shown in FIG.
(Equation 23)
a × δmax3 ≦ d
From Expression 23 and Expression 11, Expression 24 is obtained.
(Equation 24)

If Formula 24 is further modified, Formula 25 is obtained.
(Equation 25)

  Here, since 12 (n−1) is constant, the range of the product of the power φmax and the deviation amount δmax is determined by d / δmax.

That is, it is understood that the thickness is determined by the thickness of the optical element with respect to the shift amount. Increasing the thickness d increases power and increases aberrations. Conversely, increasing δmax increases the size of the optical element. Therefore, d / δmax is preferably set to 1 or less from the viewpoint of aberration correction and compactness. In Example 1, since the thickness d is 0.5 mm and the shift amount δmax is about 1.34 mm, d / δ = 0.37 <1. Summarizing the above, it can be said that it is appropriate if the following equation holds.
(Equation 26)
δ × φ <6 (6)
When the product of the deviation amount δ and the maximum value of the power φ is obtained in the first embodiment, δ = −1.34358 and φ = 0.238784 in the second group G2 at the wide angle end, and the product thereof is 0.32. ing. This fully satisfies the above range.

  In this embodiment and the following embodiments, focusing may be performed by moving the entire system or by moving one optical group in a direction perpendicular to the optical axis.

  FIG. 24 is an explanatory diagram of a digital still camera using the zoom optical system shown in Embodiment 1 as a photographing optical system. In FIG. 24, reference numeral 20 denotes a camera body, 21 denotes an imaging optical system constituted by the zoom optical system described with reference to FIG. 1, and 22 denotes a built-in camera body that receives a subject image formed by the imaging optical system. A solid-state imaging device (photoelectric conversion device) such as a CCD sensor or a CMOS sensor, a memory 23 for recording information corresponding to the subject image photoelectrically converted by the solid-state imaging device 22, and a liquid crystal display panel 24. It is a viewfinder for observing a subject image formed on the image sensor 22.

  Thus, by applying the zoom optical system of the present invention to an imaging apparatus such as a digital still camera, it is possible to realize a compact imaging apparatus having high optical performance.

  As described above, according to the present invention, it is possible to perform zooming while favorably removing aberrations by moving a rotationally asymmetric optical element in a direction different from the optical axis, and to be compact.

  The preferred embodiments of the present invention have been described above, but the present invention is not limited to these embodiments, and various modifications and changes can be made within the scope of the gist.

As described above, according to each embodiment of the present invention, it is possible to perform zooming while satisfactorily removing aberrations and obtain a compact zoom optical system.
[Comparative Example 1]
Next, Comparative Example 1 of the present invention is shown. Comparative Example 1 is designed with reference to Patent Document 3. FIG. 4 shows a lens cross-sectional view of Comparative Example 1.

As shown in FIG. 4, the zoom optical system of Comparative Example 1 includes two optical groups G1 and G2 each having two rotationally asymmetric optical elements E1 and E2 (E3 and E4). Group G1 and second group G2. First, these groups are approximated by one thin lens and paraxial calculation is performed. Next, the power of each thin lens is set to φ1 and φ2 respectively for the first group G1 and the second group G2, and the principal point interval and the back focus are set to e and Sk, respectively. Further, when the power of the entire system is φ and the focal length is f, the following equation is established (Expression 6).

Further, the following expression is established from the paraxial calculation for the back focus Sk.
(Equation 7)

Here, when the principal point interval e and the back focus Sk are determined, the powers φ1 and φ2 are expressed as a function of the power φ of the entire system from Expressions 6 and 7. That is, it is possible to represent the power change trajectory of the first group G1 and the second group G2 in the power change of the entire system. Therefore, if the principal point interval e = 3 and the back focus Sk = 15, the powers φ1 and φ2 are as follows.
(Equation 8)

(Equation 9)

The relationship between the powers φ1 and φ2 with respect to the power φ of the entire system is represented by a graph as shown in FIG. From this, it can be seen that the power of the first group G1 changes from positive to negative and the second group G2 changes from negative to positive as the power of the entire system increases. Here, the rotationally asymmetric curved surface is expressed by Expression 10, and the relationship between the coefficient a and the power is Expression 11.
(Equation 10)
z = ay ³ + 3ax ² y
(Equation 11)
φ = 12aδ (n-1)
x, y, and z are the axes shown above. δ is the amount of deviation of the two rotationally asymmetric optical elements E1, E2 (E3, E4) from the Z axis in the Y axis direction, and n is the refractive index of the lens. The coefficients a and n of the rotationally asymmetric optical elements E1 to E4 are shown in Table 1, and the deviation amount δ from the Z-axis is also measured at the telephoto end (tele end), intermediate zoom position (middle), and wide angle end (wide end). Shown in order. Table 2 shows the surface types and surface intervals of the surfaces S0 to S9.

  In FIG. 4, the light beam incident on the reference plane S0 first enters the first group G1. The first group G1 is composed of two optical elements (lenses) E1 and E2, and the surface numbers are S1 to S4 in order. The optical elements E1 and E2 are decentered in the Y-axis direction, and the amount of decentering changes continuously. Also, the amount is positive, negative, and reverse, and the absolute values are equal. As a result, the power φ1 of the first lens group G1 is changed from positive to negative during zooming from the telephoto end to the wide-angle end (hereinafter, the zoom direction is the same) as shown in FIG. The light beam emitted from the first group G1 then passes through the stop S5 and enters the second group G2. Similarly to the first group G1, the second group G2 includes two optical elements E3 and E4, and the surface numbers are S6 to S9. The optical elements E3 and E4 are decentered in the Y-axis direction, and the amount of decentering changes continuously. Further, the amounts are positive and negative with respect to each other, and the absolute values are equal. Accordingly, the power φ2 of the second group G2 is changed from negative to positive as shown in FIG.

  Light rays that have passed through these groups G1 and G2 are imaged without changing the image plane IP. However, when the image plane is viewed, it can be seen that although an image is formed, a large aberration occurs.

These occur regardless of the paraxial arrangement defined by Equations 8 and 9. For example, coma generated on the axis cannot be removed by the paraxial arrangement alone. From the above results, it can be seen that in the comparative example, the aberration cannot be corrected in the following points. this is,
(A) Since an optical system having a rotationally asymmetric optical element is asymmetric with respect to the optical axis, a shift occurs between the upper line and the lower line, and as a result, coma aberration also occurs in the axial ray.
(B) curvature of field occurs,
caused by.

  Therefore, in the embodiment of the present invention, by providing a stationary optical group for zooming in which an optical element having a positive refractive power and an optical element having a negative refractive power are arranged in this order from the stop to the image side, sufficient aberration can be obtained. A zoom optical system capable of eliminating the above has been achieved.

FIG. 2 is a lens cross-sectional view at a telephoto end, an intermediate position, and a wide-angle end according to the first exemplary embodiment. It is a figure explaining the Off-Axial optical system of the reference example 1. FIG. It is a figure which shows the power arrangement | positioning of the lens designed based on the reference example 1. FIG. It is sectional drawing of the lens designed based on the reference example 1. FIG. 5 is a lens cross-sectional view at an intermediate zoom position in Embodiment 1. FIG. FIG. 2 is an aberration diagram of Example 1 shown in FIG. 1. FIG. 2 is an aberration diagram of Example 1 shown in FIG. 1. FIG. 2 is an aberration diagram of Example 1 shown in FIG. 1. It is a figure which shows the number of the light ray in the image surface of Example 1 shown in FIG. FIG. 3 is a diagram illustrating a distortion grating at a telephoto end, an intermediate zoom position, and a wide-angle end according to the first exemplary embodiment. 6 is a back focus comparison diagram of the lens designed based on Reference Example 1 and Example 1 shown in FIG. 5. FIG. FIG. 6 is a diagram illustrating that the intersection of the powers of the optical groups G1 and G2 is shifted to the wide angle side in the first embodiment. It is a figure which shows the change of the power of the optical groups G1 and G2 of Example 1 shown in FIG. It is a figure which shows the change of the power of the optical elements E1, E2, E3, and E4 of Example 1 shown in FIG. It is a figure which shows the power change of the eccentric movable block of Example 1 shown in FIG. 1, and an auxiliary | assistant block. It is a figure which shows the change of the Petzval of Example 1 shown in FIG. It is a figure which shows the change of the Petzval of the lens designed based on the reference example 1. FIG. It is the figure which defined the tangent of the surface. It is a figure which shows the relationship of (Formula 21). It is a figure which shows the relationship between (DELTA) phi and K, max. It is a figure which shows the relationship of K, max / (DELTA) phi with respect to a surface number. It is a figure showing the difference in the direction to refract. It is a figure which shows the shift | offset | difference of the upper line and underline of embodiment shown in FIG. It is a figure explaining the back focus of surface Si. It is a figure showing the lens shape, the lens thickness, and the amount of deviation. FIG. 2 is an external perspective view of a digital still camera to which the zoom optical system shown in FIG. 1 is applied.

Explanation of symbols

G1-G3 optical group SP stop IP image surface E1-E6 optical group S1-S9 surface

Claims (16)

  Zoom optical system in which a plurality of optical groups with variable optical power, one or more optical groups and a diaphragm are arranged in the optical axis direction, and zooming is performed by changing the power of the plurality of optical groups with variable optical power The plurality of optical groups with variable optical power each include a plurality of optical elements Ld that include rotationally asymmetric surfaces and move in a direction different from the optical axis to change the power in the optical group. The above optical group includes a stationary optical group for zooming in which an optical element having a positive refractive power and an optical element having a negative refractive power are arranged in order from the stop toward the image side. Zoom optical system.   2. The zoom optical system according to claim 1, wherein the stop is arranged in a plurality of optical groups in which the optical power is variable. When the power of the optical element having the positive refractive power is φp and the power of the optical element having the negative refractive power is φn, φn <φp
The zoom optical system according to claim 1, wherein the following condition is satisfied. At all zoom positions, | φd | max is the maximum absolute value of the optical power of each optical element of the plurality of optical groups with variable optical power, and the optical power of each optical element of the one or more optical groups When the maximum absolute value of | φs | max is | φd | max <| φs | max
The zoom optical system according to claim 1, 2 or 3, wherein the following condition is satisfied. When the absolute value of the sum of the powers of the plurality of optical groups with variable optical power is | φdt | and the absolute value of the sum of the powers of the one or more optical groups is | φst | at the zoom position at the telephoto end. | Φdt | ≦ | φst |
The zoom optical system according to claim 1, wherein the following condition is satisfied. Of the plurality of optical groups having variable optical power, when the optical group A and the optical group on the image side of the optical group A are the optical group B,
When zooming from the telephoto end to the wide-angle end,
The power of the optical group A changes from positive to negative,
The power of the optical group B changes from negative to positive,
Within the entire zoom range, there is a zoom position where the optical powers of the optical group A and the optical group B match,
The zoom optical system according to any one of claims 1 to 5, wherein the coincident zoom position is located on a wider angle side than an intermediate zoom position within the entire zoom range. Of the plurality of optical groups having variable optical power, when the optical group A and the optical group on the image side of the optical group A are the optical group B, the absolute optical power at the zoom position at the wide-angle end. The larger value is | φgw | max, and the smaller absolute value of optical power at the zoom position at the telephoto end is | φgt | min. | Φgw | max <| φgt | min
The zoom optical system according to claim 1, wherein the following condition is satisfied. Of the plurality of optical groups having variable optical power, when the optical group A and the optical group on the image side of the optical group A are the optical group B, the maximum absolute value of the optical power in the entire zoom range When the value is | φ | max and the total value of the optical power at any zoom position in the optical group A and optical group B is φ _AB − | φ | max ≦ φ _AB ≦ | φ | max
The zoom optical system according to claim 1, wherein the following condition is satisfied. i represents the order of the optical surfaces S of the optical elements of the entire system in order from the object side to the image side, the inclination of the tangent to the optical axis at the point where the optical surface Si intersects the optical axis is Ki, and the maximum value of the inclination is K _max. , The total of the optical system tilt Ki

When

The zoom optical system according to claim 1, wherein the following condition is satisfied. When the change amount of the optical power accompanying the zooming of the optical surface of the optical element constituting the plurality of optical groups having the variable optical power is Δφ 0 (degree · mm) ≦ K _max / Δφ ≦ 3 (degree · mm)
The zoom optical system according to claim 1, wherein the following condition is satisfied. When the optical power of the optical group when a plurality of optical elements that change the power in the optical group are displaced by δ from the optical axis is δ × φ <6
The zoom optical system according to claim 1, wherein the following condition is satisfied.   12. The optical device according to claim 1, wherein among the plurality of optical groups having variable optical power, at least one optical group includes an optical element having an optical power having an opposite sign at a wide-angle end. 1 zoom optical system. Of the plurality of optical groups having variable optical power, when the optical group A and the optical group on the image side of the optical group A are the optical group B,
In order from the object side to the image side at the wide-angle end, the optical group A is composed of an optical element having a positive refractive power and an optical element having a negative refractive power.
13. The zoom optical system according to claim 1, wherein the optical group B includes an optical element having a positive refractive power and an optical element having a negative refractive power. Of the plurality of optical groups having variable optical power, when the optical group A and the optical group on the image side of the optical group A are the optical group B,
In order from the object side to the image side at the telephoto end, the optical group A is composed of an optical element having a positive refractive power and an optical element having a positive refractive power.
14. The zoom optical system according to claim 1, wherein the optical group B includes an optical element having a negative refractive power and an optical element having a negative refractive power.   The zoom optical system according to claim 1, wherein an image is formed on the photoelectric conversion element.   An image pickup apparatus comprising: the zoom optical system according to claim 1; and a photoelectric conversion element that receives an image formed by the zoom optical system.