JPH08327904A - Small-sized zoom lens - Google Patents

Abstract

PURPOSE: To shorten the overall length of a zoom lens while imparting a high variable power ratio to a rear focus type zoom lens of five-group constitution by moving second and fourth lens groups to vary the powder, executing focusing with a fourth lens group and disposing a full-aperture diaphragm between a third lens group and fourth lens group. CONSTITUTION: This zoom lens consists of the first lens group I having positive refracting power, second lens group II having negative refracting power, third lens group III having positive refracting power, fourth lens group IV having positive refracting power and fifth lens group V having negative refracting power. The image plane fluctuation accompanying power variation is corrected by moving the fourth lens group IV and the focusing is executed by moving the fourth lens group IV. The full-aperture diaphragm SP is arranged between the third lens group III and the fourth lens group IV. The relation between the air spacing D23 at the telephoto end for the infinite object at the lens face on the most image face side of the second lens group II and the lens face on the most object side of the third lens group III and the focal length fw at the wide angle end is so set as to satisfy 0.04<D23 /fw<0.21.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention has a high zoom ratio of about 10 and an F number of about 1.8 which is suitable for video cameras and photographic cameras, a large aperture ratio, and maintains good optical performance. The present invention also relates to a zoom lens that is downsized.

[0002]

2. Description of the Related Art Hitherto, various zoom lenses for photographic cameras, video cameras, etc. have been proposed which employ a so-called rear focus type in which focusing is performed by moving a lens unit other than the first lens unit on the object side. Has been done.

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 to perform focusing, and it is easy to downsize the entire lens system. Close-up photography, especially extremely close-up photography, is facilitated, and since relatively small and lightweight lens groups are moved, the driving force of the lens groups is small and quick focusing is possible.

As such a rear focus type zoom lens, for example, in Japanese Patent Laid-Open No. 63-247316, a positive first lens group, a negative second lens group, a positive third lens group, and a positive lens group are sequentially arranged from the object side. Has a fourth lens group of
Disclosed is a zoom lens that moves a lens group to perform zooming, and moves a fourth lens group to perform image plane variation and focusing due to zooming.

In Japanese Patent Laid-Open No. 4-43311, the first lens group having a positive refractive power, the second lens group having a negative refractive power, the third lens group having a positive refractive power, and the third lens group having a positive refractive power are arranged in this order from the object side. Fourth
The third lens group is composed of a positive lens and a negative lens, and the third lens group is a telephoto type lens, whereby the total lens length after the third lens group is shortened.

Further, in JP-A-4-301612, from the object side, 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 positive refractive power are arranged in this order. And the fifth lens group having negative refracting power,
A zoom lens in which the second and fourth lens groups perform zooming and the fourth lens group performs focusing is proposed, and the negative fifth lens group aims to shorten the total lens length after the third lens group.

[0007]

Generally, when a rear focus system is adopted in a zoom lens, the entire lens system is downsized as described above, quick focusing is possible, and further close-up photography is facilitated. .

It is composed of 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 fourth lens group having a positive refractive power in order from the object side. In the zoom lens, if the refracting power of the third lens group is strengthened and the total lens length after the third lens group is shortened, the movement amount of the fourth lens group during zooming or focusing becomes too large, and the zoom intermediate region is zoomed. It is said that the third lens group and the fourth lens group mechanically interfere with a close object, or the air gap between the third lens group and the fourth lens group has to be widened, and the total length becomes longer. There was a problem.

In the 5-group zoom disclosed in Japanese Patent Laid-Open No. 4-301612, in which a fixed negative lens is arranged further on the image side, the third lens group and thereafter are made into a telephoto type to shorten the total lens length. As shown in the drawing, since the aperture stop is arranged between the second lens group and the third lens group, it is necessary to extend the area behind the second lens group where the luminous flux is strongly diverging.

Further, at this time, variation in aberration during zooming and focusing becomes large, and it is very difficult to maintain high optical performance while achieving miniaturization.

The present invention, while adopting the rear focus system, is intended to achieve a large aperture ratio and a high zoom ratio, while preventing further enlargement of the entire lens system, and further, achieving a total zoom range from the wide-angle end to the telephoto end. An object of the present invention is to provide a rear-focus type zoom lens having a simple structure, which has good optical performance over the object range from infinity objects to short-distance objects over a double range.

[0012]

The features of the zoom lens of the present invention are, in order from the object side, a first lens group having a positive refractive power, a second lens group having a negative refractive power, and a positive refractive power. A third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power.
A lens unit and a fifth lens unit having negative refracting power are provided, and at least the second and fourth lens units are moved to perform zooming, focusing is performed by the fourth lens unit, and the aperture stop is set to the first lens unit. It is arranged between the third lens group and the fourth lens group.

[0013]

Embodiments FIG. 1 shows Embodiments 1 and 2 of the zoom lens of the present invention.
FIG. In the figure, I is the first lens group having a positive refractive power,
II is a second lens group having a negative refractive power, III is a third lens group having a positive refractive power, IV is a fourth lens group having a positive refractive power, V
Is a fifth lens unit having a negative refractive power. In the aberration diagrams, (A), (B), and (C) show various aberration diagrams at the wide-angle end, the middle, and the telephoto end, respectively.

At the time of zooming from the wide-angle end to the telephoto end, at least the second lens group is moved to the image plane side as indicated by an arrow, and the image plane variation due to zooming is corrected by moving the fourth lens group. There is.

Further, a rear focus system is adopted in which the fourth lens group is moved on the optical axis for focusing. A practical curve 4a and a dotted curve 4b of the fourth lens group shown in the figure correct the image plane variation due to 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 doing is shown.

In this embodiment, the first lens group, the third lens group and the fifth lens group are fixed during zooming and focusing.

It is also possible to move at least the first lens group in order to reduce the variable magnification share of the second lens group.

In the present embodiment, the fourth lens group is moved to correct the image plane variation due to zooming, and the fourth lens group is moved to perform focusing. In particular, as shown by the curves 4a and 4b in the figure, the zoom 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. As a result, the space between the third lens group and the fourth lens group is effectively used, and the total lens length is effectively shortened.

In the present embodiment, when focusing from an object at infinity to a near object at the telephoto end, for example, the fourth lens group is moved forward as shown by a straight line 4c in the figure.

In the present invention, the aperture stop SP for determining the F number, which is conventionally arranged between the second lens group and the third lens group, is arranged between the third lens group and the fourth lens group. Thus, the space is effectively used, and the occurrence of curvature of field due to the increase of Petzval sum when the total lens length of the third lens group and thereafter is shortened is reduced.

This will be described in more detail below. In order to achieve downsizing of the zoom lens, it is necessary to increase the negative refractive power of the second lens group within a range where there is no problem of aberration correction and reduce the movement amount of the second lens group for zooming. At this time, the divergence of the light flux from the second lens group becomes stronger. Therefore, in order to shorten the total lens length after the third lens group, it is effective to reduce the principal point distance between the second lens group and the third lens group. However, conventionally, since the aperture stop is arranged between the second lens group and the third lens group, it is necessary to secure the mechanical space. At this time, if it is attempted to forcibly reduce the total lens length after the third lens group, it is necessary to increase the telephoto ratio by strengthening the refractive power of the negative fifth lens group, and thus refracting the second lens group. The Petzval sum of the entire system, which became negative by increasing the force, also increases negatively, and it becomes difficult to correct the sagittal field curvature in particular.

On the other hand, in the present invention, the aperture stop is arranged between the third lens group and the fourth lens group to reduce the distance between the second lens group and the third lens group, and conversely to the third lens group. By increasing the distance between the fourth lens group, the negative Petzval sum increase when the distance from the third lens group to the image plane is reduced, and the field curvature can be satisfactorily corrected when the total length is shortened. There is.

In particular, in the present invention, the air distance D ₂₃ at the telephoto end and the entire system at the wide-angle end between the most image-side lens surface of the second lens unit and the most object-side lens surface of the third lens at an infinite object. Of the focal length f _W of 0.04 <D ₂₃ / f _W <0.21 (1) is satisfied, the distance between the second and third lens groups is appropriately set to shorten the total lens length. Is effectively achieved.

When the upper limit of conditional expression (1) is exceeded and the distance between the second and third lens groups becomes large, it becomes difficult to shorten the total lens length of the third lens group and thereafter. On the other hand, if the value goes below the lower limit, the lenses may collide with each other during tracking adjustment, which is not preferable. Also to satisfy the third distance between the most image plane side surface and the aperture stop of the lens group D _S of _{0.11 <D S / f W <} 0.45 (2) The condition at the wide-angle end in the present invention By setting it, the total length is effectively shortened within the range that does not interfere with the operation of the diaphragm blades.

If the distance between the third lens group and the diaphragm becomes too large beyond the upper limit of conditional expression (2), the distance from the first surface of the front lens to the entrance cavity becomes long and the diameter of the front lens becomes large. Not preferable.

On the other hand, when the value goes below the lower limit, the distance between the lens surface and the diaphragm becomes too short and there is a possibility that the lens may come into contact with the diaphragm blade when the diaphragm blade is warped.

More preferably, in this embodiment, the optical power is set by setting the magnification β ₅ of the fifth lens group for an infinite speed object so as to satisfy the condition 1.2 <β ₅ <2.0 (3). The overall lens length is shortened while maintaining performance.

If the lower limit of conditional expression (3) is exceeded and the magnification of the fifth lens group becomes small, the effect of sufficiently shortening the total length cannot be obtained.

On the contrary, if the magnification exceeds the upper limit and the magnification becomes large, it is advantageous for shortening the total length of the lens, but the Petzval sum becomes large negatively, and it becomes difficult to correct the field curvature and the telecentricity is considerably deteriorated. Difficult to apply.

To shorten the lens length of the third lens group and thereafter while maintaining a predetermined optical performance, the following conditions should be satisfied.

When the focal length of the third lens group and the focal length at the wide-angle end of the entire system are f ₃ and f _W , respectively 2.1 <f ₃ / f _W <3.4 (4) Conditional expression (4) Relates to the focal length of the third lens group. If the lower limit value is exceeded and the refracting power of the third lens group becomes too strong, correction of spherical aberration and coma becomes insufficient, and it is difficult to secure back focus. It becomes.

On the contrary, if the upper limit is exceeded, the total length of the lens cannot be shortened sufficiently.

Further, in the present embodiment, in order to further reduce the movement amount of the fourth lens unit to shorten the total lens length after the third lens unit, when the focal length of the fourth lens unit is f ₄ , It is better to satisfy the conditional expression of. 1.5 <f ₄ / f _W <3.1 (5) Conditional expression (5) relates to the focal length of the fourth lens group, and the refractive power of the fourth lens group becomes too strong beyond the lower limit value. Therefore, the variation of spherical aberration during zooming becomes large. On the other hand, if the upper limit is exceeded, the movement amount of the fourth lens unit becomes too large, and it is necessary to widen the air space between the third and fourth lens units, and a sufficient effect of shortening the overall length cannot be obtained.

Further, in the zoom lens of the present embodiment, in order to shorten the lens length of the zoom portion and further shorten the total lens length, the following conditional expression should be satisfied.

That is, when the focal length of the second lens group and the focal length at the telephoto end of the entire system are f ₂ and f _r , respectively.

[0036]

[Outside 1]

Conditional expression (6) relates to the refracting power of the second lens group, and is for effectively obtaining a predetermined zoom ratio while reducing aberration fluctuations due to zooming. If the lower limit value is exceeded and the refractive power of the second lens group becomes too strong, it becomes easier to downsize the entire lens system, but the Petzval sum increases in the negative direction, the field curvature increases, and the aberration associated with zooming increases. Fluctuation increases. Further, when the refractive power of the second lens group becomes too weak beyond the upper limit value, the fluctuation of aberration due to zooming decreases, but the movement amount of the second lens group for obtaining a predetermined zoom ratio increases, and It is not good because the total length is getting longer.

The zoom lens of this embodiment is achieved by satisfying the above conditions, but it is effective to reduce the thickness of each lens group itself in order to shorten the total lens length.

In this embodiment, in order to reduce the number of lenses, it is preferable to introduce an aspherical surface into each lens group, especially the third lens group and the fourth lens group.

Particularly, it is preferable that the third lens group is composed of a single lens having at least one aspherical surface, and the fourth lens group is composed of two lenses, a negative meniscus lens and a positive lens.

Further, by using a glass material which satisfies the condition that the Abbe number ν _1N is ν _1N <23 for the negative lens of the first lens group, the achromatic effect is enhanced and the lens thickness of the positive lens is made thin. It becomes possible to do.

The first lens group is usually composed of a positive and negative cemented lens and a positive meniscus lens, but as shown in Example 3, the cemented lens is separated and an air gap is provided between the positive lens and the negative lens. Even if the rear principal point of the first lens group is moved to the second lens group side to reduce the principal point spacing of the 1-2 lens group, the actual spacing between the 1-2 lens groups is kept the same. ,
It is also possible to suppress the expansion of the front lens diameter when widening the angle.

Further, as shown in Example 4, the negative cemented lens of the second lens group may be separated into two lenses, a negative lens and a positive lens, to increase the degree of freedom in aberration correction.

Next, numerical examples of the present invention will be shown. 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 from the object side, and Ni and υi are the values of the i-th lens in order from the object side, respectively. The refractive index of glass and the Abbe number.

Table 1 shows the relationship with each conditional expression in each numerical example. Incidentally, the two surfaces closest to the image plane in Numerical Examples 1 to 4 are glass materials such as a face plate.

The aspherical shape has an X axis in the optical axis direction, an H axis in the direction perpendicular to the optical axis, a positive light traveling direction, and R ₀ as aspherical curvature radii K, B, C, D and E, respectively. As a coefficient,

[0047]

[Outside 2] It is expressed by In addition, in the aspherical surface coefficient, "e
“ ^X ” indicates “10 ^X ”.

[0048]

[Outside 3]

[0049]

[Outside 4]

[0050]

[Outside 5]

[0051]

[Outside 6]

[0052]

[Table 1]

[0053]

As described above, according to the present invention, as described above, the refracting power of the five lens groups and the moving conditions of the second and fourth groups and the position of the aperture stop at the time of zooming are set and the focus is adjusted. In this case, by adopting a lens configuration that moves the fourth lens group, it is possible to reduce the size of the entire lens system and achieve a zoom ratio of 1
A rear focus type zoom lens with an F number of 1.8 and a large aperture ratio, which has excellent optical performance with little aberration fluctuation during focusing, while achieving good aberration correction over the entire zoom range of about 0. can do.

[Brief description of drawings]

FIG. 1 is a lens cross-sectional view of Numerical Embodiments 1 and 2 according to the present invention.

FIG. 2 is a lens cross-sectional view of Numerical Example 3 according to the present invention.

FIG. 3 is a lens cross-sectional view of Numerical Example 4 according to the present invention.

FIG. 4 is a diagram of various types of aberration of the zoom lens of the numerical value example 1.

FIG. 5 is a diagram of various types of aberration of the zoom lens of Numerical Example 2.

FIG. 6 is a diagram of various types of aberration of the zoom lens of the numerical value example 3.

FIG. 7 is a diagram of various types of aberration of the zoom lens of the numerical value example 4.

[Description of Reference Signs] I First lens group II Second lens group III Third lens group IV Fourth lens group V Fifth lens group ΔM Meridional image surface ΔS Sagittal image surface d d line g g line

Claims (3)

[Claims] 1. A first lens element having a positive refractive power in order from the object side.
A lens group, a second lens group having a negative refractive power, a third lens group having a positive refractive power, a fourth lens group having a positive refractive power, and a fifth lens group having a negative refractive power, At least the second lens group and the fourth lens group are moved to perform zooming, focusing is performed by moving the fourth lens group, and an aperture stop is provided between the third lens group and the fourth lens group. A zoom lens characterized by being placed. 2. The air gap at the telephoto end between the lens surface of the second lens group closest to the image side and the lens surface of the third lens group closest to the object side at the telephoto end is D ₂₃ , and the entire system at the wide angle end is The zoom lens according to claim 1, wherein the conditional expression 0.04 <D ₂₃ / f _W <0.21 is satisfied when the focal length is f _W. 3. A conditional expression of 0.11 <D _S / f _W <0.45 is satisfied, where D _S is the distance between the most image-side surface of the third lens unit at the wide-angle end and the aperture stop. The method according to claim 1 or 2, wherein
The described zoom lens.