JPH09281391A - Rear focus type zoom lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To secure space for a back focus by providing a zoom lens with 1st to 4th lens groups being positive, negative, positive and positive from an object side, performing zooming by the 2nd and the 4th lens groups and focusing by the 4th lens group, providing the 3rd lens group with a positive lens on an image side, whose lens surface on the image side is a strong refractive surface. SOLUTION: This zoom lens is provided with the 1st lens group L1 having positive refractive power, the 2nd lens group L2 having negative refractive power, the 3rd lens group L3 having the positive refractive power and the 4th lens group L4 having the positive refractive power from the object side. It is a rear focus type zoom lens where zooming is performed by the 2nd and the 4th lens groups L2 and L4 and focusing is performed by the 4th lens group L4. The 3rd lens group L3 is provided with the positive lens on the closest side to the image surface, whose lens surface on the image surface side is the stronger refractive surface than on the object side. Namely, by arranging the positive lens at the rear of the 3rd group L3 and specifying the shape of the lens, the zoom lens is approximate to a retrofocus type and the position of the principal point of the 3rd lens group L3 is arranged to be away from the 2nd lens group L2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rear focus type zoom lens, and more particularly to a front lens with a high zoom ratio while ensuring a long back focus such that a color separation prism is inserted between the lens and the CCD. The present invention relates to a rear focus type zoom lens having a small diameter and a large diameter.

[0002]

2. Description of the Related Art Recently, with the reduction in size and weight of home video cameras and the like, remarkable progress has been made in downsizing of zoom lenses for image pickup, and particularly, shortening of the total length, downsizing of front lens diameter, and simplification of configuration. Is focused on.

As one means for achieving these objects, there is known a so-called rear focus type zoom lens in which focusing is performed by moving a lens group other than the first lens group on the object side.

Generally, in a rear focus type zoom lens, the effective diameter of the first lens group is smaller than that of a zoom lens in which the first lens group is moved for focusing.
It is easy to downsize the entire lens system. In addition, close-up photography, especially extremely close-up photography, is possible, and since the relatively small and lightweight lens group is moved, the driving force of the lens group is small and quick focusing is possible.

As such a rear focus type zoom lens, for example, JP-A-62-206516, JP-A-62-215225, and JP-A-62-24213.
In the publication, etc., a positive first lens group, a negative second lens group, a positive third lens group, and a positive fourth lens group are provided in this order from the object side, and the second lens group is moved to change the magnification. A zoom lens for performing focusing by correcting the image plane variation due to zooming in the fourth lens group is disclosed.

Further, JP-A-4-43311, JP-A-4-153615, JP-A-5-19165, JP-A-5-27167, and JP-A-5-6.
Japanese Patent No. 0973 discloses an example in which the fourth lens group is configured by one convex lens or two convex lenses. Further, JP-A-5-60974 discloses a zoom lens in which the fourth lens group is composed of two concave and convex elements.

Further, JP-A-55-62419, JP-A-62-24213, and JP-A-62-21522.
5, JP-A-56-114920, JP-A-3
-200113, Japanese Patent Application Laid-Open No. 4-242707, Japanese Patent Application Laid-Open No. 4-343313, Japanese Patent Application Laid-Open No. 5-297.
In the publications such as Japanese Patent No. 275, the third group, the fourth group
It is disclosed that each group is composed of two lenses, a positive lens and a negative lens.

Further, as the performance of a video deck becomes higher (digitalization), the picture quality of a video camera is being improved.
As one of the methods, high image quality is achieved by separating an image with a color separation optical system. And as a lens suitable for it, Japanese Patent Application Laid-Open No. 5-72474 and Japanese Patent Application Laid-Open No. 6-5
1199, JP-A-6-337353, JP-A-6-347697, JP-A-7-199069 and JP-A-7-270684.

[0009]

As described above, in general, in a zoom lens, in order to achieve a reduction in the front lens diameter and the entire system, the so-called rear focus method is used rather than the distance adjustment by the first lens group. Is more suitable.

However, in JP-A-4-026811 and JP-A-4-88309, it was difficult to dispose the color separation prism in that structure.

Further, JP-A-4-43311, JP-A-4-153615, JP-A-5-19165, JP-A-5-27167, and JP-A-5-6.
In these zoom lenses disclosed in Japanese Patent Publication No. 0973, the zoom ratio is about 6 to 8 times, and in the case of a high-power zoom lens having a zoom ratio higher than this, the variation due to the variation of the chromatic aberration becomes too large to be corrected and sufficient optical performance is obtained. It was difficult to make the most of it. Further, even in the example disclosed in Japanese Patent Laid-Open No. 5-60974, the zoom ratio is still in the 8 × class, which is still not sufficiently high.

Furthermore, JP-A-55-62419, JP-A-56-114920, and JP-A-3-20011
In the example disclosed in Japanese Patent No. 3, the first group or the third group
Since the lens group also moves with zooming, the lens barrel structure becomes complicated, which is not suitable for achieving miniaturization. Further, in the examples disclosed in JP-A-4-242707, JP-A-4-343313, and JP-A-5-297275, the third group is configured to have a large air gap, and further, the third group. Since the negative lens in the group has a weak refractive power, it cannot be a type that can sufficiently correct the chromatic aberration generated in the third group in order to apply it to a high zoom lens. Furthermore,
In the example proposed in Japanese Patent Laid-Open No. 5-297275, the concave meniscus lens in the third lens group has a structure in which a strong concave surface is directed toward the image side, so that it is effective for telephoto conversion, but it occurs in the convex lens. Since it is not suitable for receiving a high-order flare component with a concave lens, it is a type unfavorable for large-diameter, high-magnification zoom lenses.

Further, JP-A-5-72474, JP-A-6-511199, JP-A-6-337353, JP-A-6-347697, and JP-A-7-199.
Even in the examples disclosed in Japanese Patent Laid-Open No. 069 and Japanese Patent Laid-Open No. 7-270684, the zoom ratio is about 10 to 12 times in all of the examples, and a sufficiently high magnification has not been achieved.

An object of the present invention is to improve the above-mentioned drawbacks of the conventional example, and particularly to improve the Japanese Patent Application Laid-Open No. 7-270684 proposed by the present applicant, and to protect an optical element such as a color separation prism and a zoom lens section. A rear focus type zoom that achieves a high zoom ratio of about 16 times with a large aperture while maintaining a sufficient back focus space for the target optical element and maintaining good optical performance over the entire zoom range and object distance. An object of the present invention is to provide a lens and a video camera to which the zoom lens can be attached and detached.

[0015]

To solve the above problems, the present invention has, in order from the object side, a first lens group having a positive refracting power, a second lens group having a negative refracting power, and a positive refracting power. And a fourth lens group having a positive refractive power, the second lens group and the fourth lens group are moved to perform zooming, and the fourth lens group is moved to focus. In the rear-focusing type zoom lens, the third lens group has a positive lens closest to the image surface side, and the lens surface on the image surface side of the positive lens has a stronger refracting surface than the object side. It is characterized by that.

That is, a positive lens is arranged behind the third lens group for making the light beam diverged in the third group substantially afocal, and by specifying this shape, it becomes closer to the retro type, By disposing the principal point position of the third lens group away from the second lens group, the principal point interval between the second lens group and the third lens group is further widened, and the axial ray height incident on the third lens group is high. To be higher. Therefore, the focal length of the fourth lens unit for keeping the focal length of the entire system at a predetermined amount can be lengthened, and the back focus as a working distance can be lengthened. That is, since the light flux leaving the third lens group is substantially afocal, the back focus length is approximately the same as the focal length of the fourth lens group when calculated using the principal point system. Therefore, in order to fix the focal length of the entire system and lengthen the focal length of the fourth lens unit, it is understood that the axial light height h in the third lens unit may be increased as shown in FIG.

[0017] Further, when the respective R _31r of the radius of curvature of the object side and the image side of the positive lens, and R _32r, the positive lens and the respective f _31, f ₃ a focal length of the third lens group By satisfying the conditional expression 1.0 <| R _31r / R _32r | <5.0 1.5 <f ₃ / f ₃₂ <5.0, the back focus is secured and good aberration correction is performed. .

In particular, it is desirable that the positive lens of the third lens group be a cemented lens of a positive refractive power lens and a negative refractive power lens.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION Next, specific examples of the present invention will be described.

1 to 9 are lens cross-sectional views of Numerical Embodiments 1 to 9 to be described later of the rear focus type zoom lens of the present invention, and FIGS. 10 to 18 are aberration diagrams of each embodiment.
In each aberration diagram, A is an aberration diagram at the wide-angle end, B is an intermediate aberration diagram, and C is various aberration diagrams at the telephoto end.

In the figure, L1 is the first lens unit having a positive refractive power, L1.
Reference numeral 2 is a second lens group having a negative refractive power, L3 is a third lens group having a positive refractive power, and L4 is a fourth lens group having a positive refractive power.
SP is an aperture stop, which is arranged immediately before the third lens unit L3. GA is a protective glass for protecting the zoom lens, and GB is a glass block such as a color separation prism, a CCD face plate, and a low-pass filter. The zoom lens unit L1 to GA is mounted on the camera body via the mount member C. Therefore, the image plane side after GB is included in the camera body.

In the present embodiment, the second lens group is moved to the image plane side as indicated by the arrow when the magnification is changed from the wide-angle end to the telephoto end, and the fourth lens group is moved due to the image plane variation due to the magnification change. Correcting. Further, a rear focus type in which focusing is performed by moving the fourth lens group on the optical axis is adopted. In particular, as shown by the curves 4a and 4b in FIG. 1, the lens is moved so as to have a convex locus toward the object side during zooming from the wide-angle end to the telephoto end. Thereby, the space between the third lens unit and the fourth lens unit is effectively used, and the overall length of the lens is effectively reduced. The solid curve 4a and the dotted curve 4b of the fourth lens group shown in the figure are image planes associated with zooming from the wide-angle end to the telephoto end when focusing on an object at infinity and a near object, respectively. The movement locus for correcting the fluctuation is shown. The first lens group and the third lens group are fixed during zooming and focusing.

In Numerical Examples 1 to 3 and 9, the third lens group is a cemented lens having negative refracting power composed of negative and positive,
It is composed of cemented lenses of positive and negative refracting power and constitutes a retro-type positive lens group as a whole. In the cemented lens having a negative refractive power, the lens surface on the object side has a concave surface facing the object side. In this way, the role of moving the principal point position of the third lens group away from the second lens group is given, which contributes to lengthening the back focus. In particular, a stronger negative power (having a shorter radius of curvature) is applied to the object side than to the image side, so that the principal point position is located further rearward.

On the other hand, in the cemented lens having the positive refractive power, the lens surface on the image side has a stronger refracting surface (having a shorter radius of curvature) than the object side surface of the cemented lens. In addition, it plays a role of moving the principal point position of the third lens group away from the second lens group, which contributes to making the focal length of the fourth lens group long and thus the back focus long.

Similarly, in Numerical Examples 4 to 8, the third lens group is composed of a negative single lens and a positive single lens, and constitutes a retro-type positive lens group as a whole. Further, the negative single lens has a strong concave surface on the object side and plays a role of moving the principal point position of the third lens group away from the second lens group, thereby increasing the focal length of the fourth lens group and thus the back focus. Is helping to lengthen.

On the other hand, in the positive single lens, the image side surface has a stronger refracting surface (having a shorter radius of curvature) than the object side surface, and the principal point position of the third lens group is moved away from the second lens group. It plays a role and contributes to lengthening the focal length of the fourth lens unit and thus lengthening the back focus.

As described above, in the present embodiment, the positive lens is arranged closest to the image surface side of the third lens group, and the image side lens surface of this positive lens is convex toward the image surface side, and the principal point position is arranged rearward. ,
The back focus is secured so that a color separation prism can be placed behind.

With the above construction, it is possible to provide a zoom lens having a sufficient back focus and a high zoom ratio, but it is more preferable to satisfy one of the following conditional expressions.

(I) The length when the distance from the final surface of the lens to the image plane of an object at infinity at the wide-angle end is converted to air is BF, the focal length of the entire system at the wide-angle end and the open F-number, half When the angle of view is f _W , F _NW , and ω, respectively,

[0030]

[Outside 2] Is a conditional expression.

If the F-number is made brighter than the lower limit, high-order spherical aberration and coma will occur, which makes correction difficult.

When the F number becomes darker than the upper limit value, the axial ray bundle becomes thin, whereby the color separation prism arranged between the final surface of the lens and the image plane can be downsized. become. That is, although it is not necessary to lengthen the back focus, it is necessary to lengthen the back focus, which leads to an increase in the total length of the lens.

(Ii) Further, the radiuses of curvature of the object-side and image-side lens surfaces of the positive lens closest to the image plane of the third lens group are R _31r , R _32r , the positive lens and the third lens, respectively. When the focal lengths of the groups are f ₃₂ and f ₃ , respectively, 1.0 <| R _31r / R _32r | <5.0 (2) 1.5 <f ₃ / f ₃₂ <5.0 (3 ) Is satisfied.

Both conditional expressions (2) and (3) are for limiting the curvature of the surface of the third lens group closest to the image surface. When the lower limit is exceeded, the curvature of the image surface and the focal length of the positive lens become loose. It becomes difficult to keep the back focus sufficiently long, which is the object of the present invention, and if the upper limit is exceeded, a higher-order spherical aberration that occurs when the third lens unit is emitted and enters the fourth lens unit having the focus function. Is difficult to correct, and it becomes impossible to achieve high performance.

(Iii) Further, the sum of the air distances from the first lens group to the second lens group and from the second lens group to the third lens group is L, the half angle of view at the wide angle end is ω, and the wide angle end is ω.
The focal length of the fourth lens group at the telephoto end and f
_W, f _t, f _4, when the air gap of the third lens group and the fourth lens group with respect to an object at infinity at the telephoto end was D, 0.66 <L / (f t · tanω) <1. 17 (4) 4.00 <f ₄ / _fw <7.00 (5) 0.10 <D / f _t <0.30 (6)

Conditional expression (4) optimizes the relationship between the moving space for zooming the second lens group and the zoom ratio. When the upper limit is exceeded, the space for zooming is too wide and the total length is too long. If the value exceeds the lower limit, the amount of zooming load of the second lens group is increased, so that the negative refracting power must be increased, and the negative Petzval sum indicating field curvature increases, which is not preferable.

Conditional expression (5) optimizes the length of the back focus. When the upper limit is exceeded, the back focus becomes longer than necessary and the total length becomes longer. When the lower limit is exceeded, a sufficiently long back focus is obtained. Will be difficult to secure.

Conditional expression (6) is for optimizing the relationship between the movable space of the fourth lens unit for focusing and the focal length at the telephoto end, and the larger the D exceeds the upper limit, the longer the overall length. However, if the lower limit is exceeded, sufficient space for focusing cannot be secured, and the operability of the zoom lens will be hindered.

Further, the radius of curvature of the lens surface closest to the object side of the third lens group is R _31f , the focal length of the lens closest to the object side of the third lens group is f ₃₁ , and the focal length of the third lens group is When f ₃ is set, the condition is −0.60 <R _31f / f ₃ <−0.10 (7) 0.30 <R ₃₁ f / f ₃₁ <0.90 (8).

Both the conditional expressions (7) and (8) are for limiting the curvature of the surface of the third lens unit closest to the object side. If the upper limit is exceeded, the curvature of the negative lens and the focal length will become loose. It becomes difficult to keep the back focus sufficiently long, and if the lower limit is exceeded, the higher-order spherical aberration that occurs when the ray bundle diverging from the second lens group enters the third lens group at the wide-angle end is corrected. It becomes difficult to achieve high performance.

Now, in order to sufficiently correct the chromatic aberration at the telephoto end, the second group may be composed of at least two negative lenses and at least one positive lens, but in the present embodiment, as described above. Further, in order to enlarge the principal point distance between the second lens group and the third lens group, a negative lens is arranged closest to the image surface side of the second lens group, which contributes to further lengthening the back focus.

In order to correct aberrations more satisfactorily, and particularly chromatic aberrations satisfactorily, it is necessary to have at least one cemented lens in the third lens group as shown in the first to third and ninth embodiments. As described above, with the improvement in image quality of video cameras, chromatic aberration, which has not been a serious problem in the past, in particular chromatic aberration of magnification, becomes a problem, and this is well corrected.

Further, in this embodiment, the aperture stop is arranged immediately before the third lens group in order to make the image of the first lens group small, but the present invention is not limited to this position, and the third lens group and the fourth lens group are not limited to this position. Or between the negative lens and the positive lens in the third lens group.

In this embodiment, the third lens group is made negative and positive in order, the exit pupil is made longer, and the state of the light rays emitted from the zoom lens is made substantially telecentric.
By making the angle of the light ray incident on the color separation prism arranged behind it, the change in the reflection characteristics due to the wavelength of the color separation system is eliminated, color separation is faithfully performed, and the color reproducibility of the image is made very good. ing.

Further, in a high-magnification lens such as this lens, the focal length at the tele end becomes very long, and the performance at the tele end and its vicinity is greatly affected by the second lens group. If an aspherical surface is introduced into this second lens group, it is possible to improve the optical performance.

Since the aspherical surface is basically for the purpose of correcting spherical aberration, it is desirable that the aspherical surface has a shape in which the positive refractive power becomes weaker toward the peripheral portion of the lens.

Further, in order to correct aberrations satisfactorily, and particularly chromatic aberrations satisfactorily, at least one positive lens in the fourth lens group is made of glass satisfying ν _d > 64.0. Where ν _d
Is the Abbe number of glass.

This conditional expression is a condition for satisfactorily correcting lateral chromatic aberration, and if the Abbe number is made smaller than the lower limit of the conditional expression, lateral chromatic aberration becomes underpreferable.

Examples of the present invention will be described below.

In the numerical examples, Ri is the radius of curvature of the i-th lens surface in order from the object side, Di is the i-th lens thickness and air gap in order from the object side, and Ni and νi are in order from the object side, respectively. It is the refractive index and Abbe number of the glass of the i-th lens.

Further, R28 to R2 in Numerical Embodiment 1
9, R26 to R27 in Numerical Examples 2, 3 and 9, R24 to R25 in Numerical Examples 4 to 8 are protective glass parts, R30 to R33 in Numerical Example 1, R28 in Numerical Examples 2, 3, and 9. -R31 Numerical Examples 4-8, R26-R29 and the like represent glass blocks such as a color separation prism, an optical filter and a face plate.

Table 1 shows the relationship between the above-mentioned conditional expressions and various numerical values in the numerical examples.

The aspherical shape has an X axis in the optical axis direction, an H axis perpendicular to the optical axis, a positive direction in which light travels, R is a paraxial radius of curvature, and each aspherical surface coefficient is K, B, C, D. , E,

[0054]

[Outside 3] It is represented by the following expression.

Further, for example, the display of "e-0 ^X " is "1
0- ^X "is meant.

[0056]

[Table 1]

[0057]

[Outside 4]

[0058]

[Outside 5]

[0059]

[Outside 6]

[0060]

[Outside 7]

[0061]

[Outside 8]

[0062]

[Outside 9]

[0063]

[Outside 10]

[0064]

[Outside 11]

[0065]

[Outside 12]

[0066]

With the configuration as described above, the FNo. While maintaining a large aperture of about 1.6, the entire zoom range and all objects are ensured while ensuring a sufficient back focus space in which optical elements such as prisms for color separation and optical elements for the purpose of protecting the zoom lens section enter. It is possible to provide a rear focus type zoom lens having good performance over a distance, and it is possible to realize a compact, lightweight and high performance lens detachable video camera by using this zoom lens.

[Brief description of drawings]

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

FIG. 2 is a lens cross-sectional view of Numerical Example 2 according to the present invention.

FIG. 3 is a sectional view of a lens according to a third numerical example of the present invention.

FIG. 4 is a sectional view of a lens according to a numerical example 4 of the present invention.

FIG. 5 is a lens cross-sectional view of Numerical Example 5 relating to the present invention.

FIG. 6 is a lens cross-sectional view of Numerical Example 6 relating to the present invention.

FIG. 7 is a lens cross-sectional view of Numerical Example 7 according to the present invention.

FIG. 8 is a lens cross-sectional view of Numerical Example 8 according to the present invention.

FIG. 9 is a lens cross-sectional view of Numerical Example 9 according to the present invention.

FIG. 10 is an aberration diagram of Numerical example 1 according to the present invention.

FIG. 11 is a diagram of various types of aberration of Numerical example 2 according to the present invention.

FIG. 12 is a diagram of various types of aberration of Numerical example 3 according to the present invention.

FIG. 13 is a diagram of various types of aberration of Numerical example 4 according to the present invention.

FIG. 14 is a diagram of various types of aberration of Numerical example 5 according to the present invention.

FIG. 15 is a diagram of various types of aberration of Numerical example 6 according to the present invention.

FIG. 16 is a diagram of various types of aberration of Numerical example 7 according to the present invention.

FIG. 17 is a diagram of various types of aberration of Numerical example 8 according to the present invention.

FIG. 18 is a diagram of various types of aberration of Numerical example 9 according to the present invention.

FIG. 19 is a principle diagram of a zoom lens according to the present invention.

[Explanation of symbols]

 L1 First lens group L2 Second lens group L3 Third lens group L4 Fourth lens group g g line d d line ΔM meridional image plane ΔS sagittal image plane

Claims (4)

[Claims] 1. A first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and a third lens group having a positive refractive power in order from the object side. A rear focus type zoom lens having four lens groups, wherein the second lens group and the fourth lens group are moved for zooming and the fourth lens group is moved for focusing. The rear focus type zoom lens is characterized in that the group has a positive lens closest to the image surface side, and the lens surface on the image surface side of the positive lens has a stronger refracting surface than the object side. 2. The length when the distance from the final surface of the zoom lens to the image surface of an object at infinity at the wide-angle end is converted into air, BF, the focal length and F number of the entire system at the wide-angle end, and the half field angle. Are f _W , F _{NW, and} ω, respectively. The rear focus type zoom lens according to claim 1, wherein the following conditional expression is satisfied. Wherein the positive lens on the object side and the image side curvature radius of each R _31r when, and R _32r, the positive lens and the respective f _31, f ₃ a focal length of the third lens group 2. The rear focus type zoom lens according to claim 1, wherein the conditional expression 1.0 <| R _31r / R _32r | <5.0 1.5 <f ₃ / f ₃₂ <5.0 is satisfied. 4. The rear focus type zoom lens according to claim 1, wherein the positive lens of the third lens group is a cemented lens of a positive refractive power lens and a negative refractive power lens.