JP2851893B2 - Rear focus zoom lens - Google Patents

Description

Description: BACKGROUND OF THE INVENTION The present invention relates to a zoom lens suitable for a 35 mm film camera, a video camera, and the like, and more particularly, to a compact zoom lens having a high zoom ratio and a rear focus system. The present invention relates to a rear-focus zoom lens having good optical performance over a wide object range.

(Prior art)

Conventionally, a focusing method of a zoom lens is generally performed by moving a lens group constituting a lens closest to an object, and an amount of extension of the focus lens with respect to the same object is constant regardless of a focal length. This has the advantage that the mechanical structure of the lens barrel is relatively simple.

However, when focusing on a short-distance object, it is necessary to enlarge the image in order to prevent vignetting of the light beam. In recent years, an autofocus camera that automatically moves a focusing lens by a motor to perform a focusing operation is becoming mainstream, but the above-described focusing lens group, which is the heaviest in a photographic lens, has been used. Moving the motor has disadvantages in terms of motor energy consumption and focus speed.

On the other hand, in order to reduce such a problem, various so-called rear focus systems have been proposed in which a lens other than the above-described lens is moved to perform focusing. For example, JP-A-59-33418, JP-A-59-52215,
There is JP-A-63-195618 and the like.

[Problems to be solved by the invention]

In JP-A-59-33418, focusing is performed by moving a part of the fixed imaging lens during zooming. However, the focus sensitivity E (the amount of focus movement / the amount of movement of the focus lens) is completely reduced. Since they are the same in the focal region, the amount of extension at the telephoto end becomes large, and the size is not sufficient in terms of compactness.

Japanese Patent Application Laid-Open No. Sho 59-52215 discloses a zoom lens having a four-group configuration, and performs focusing by moving the third lens group toward the relay lens group. However, the overall length is relatively long and sufficient miniaturization has not yet been achieved.

Japanese Patent Application Laid-Open No. 63-195618 discloses a standard rear focus zoom lens including a wide angle range, and the number of moving lens groups of the moving lens group at the time of focusing is large. When a group is moved, it is difficult to perform a quick focus.

[Means for solving the problem]

The present invention has been made in view of such a problem, when using a rear focus method for a telephoto zoom lens having a high zoom ratio,
An object of the present invention is to provide a rear focus zoom lens that is compact and can perform quick focusing in consideration of lens controllability of a focus lens when applied to an auto focus camera.

To this end, the rear focus zoom lens according to the present invention includes, in order from the object side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit having a positive refractive power. And a rear group having a negative refractive power as a whole including a front 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. Zooming from the end to the telephoto end increases the distance between the first lens group and the second lens group so that the image magnifications of the front group and the rear group both increase, and the second lens group and the third lens The distance between the groups is reduced, the distance between the third lens group and the fourth lens group is increased, and the distance between the fourth lens group and the fifth lens group is reduced. Focusing is performed by moving the fifth lens group. The focal length of the fifth lens group is f ₅ , _E5T , _E5W ,
The focal length at the telephoto end of the entire system is f _T , and the amount of movement of the fifth lens group when zooming from the wide-angle end to the telephoto end is
M ₅ (However, the direction from the object side to the image plane side is positive.) When the zoom ratio is Z, It is characterized by satisfying the following conditional expression.

〔Example〕

 Hereinafter, embodiments of the present invention will be described with reference to the drawings.

1 to 3 show numerical examples 1 to 3 according to the present invention.
It is a lens sectional view of.

In order from the object side, I is a 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, and IV is a positive refractive power. V is a fifth lens group having a negative refractive power.

Under such a refractive power arrangement, the first lens unit I follows a movement locus indicated by a curve showing zooming from the wide-angle side to the telephoto side.
The distance between the second lens group II and the second lens group II is increased.
And the third lens group III, and the third lens group I
The distance between the second and fourth lens groups IV and IV is increased, and the fourth lens group
The distance between the IV and the fifth lens group V is reduced.
In other words, when zooming from the wide-angle end to the telephoto side, the first, second, and third lens groups are arranged, and the lateral magnification of the entire front group having a positive refractive power as a whole is increased. The zoom ratio is efficiently increased by changing the air spacing between the lens groups so that the lateral magnification of the entire rear group, which is composed of groups and has a negative refractive power as a whole, increases. or,
By making the refractive power of the front group and the rear group positive and negative, respectively, the telephoto type is made compact.

In the embodiment of the present invention, focusing from an object at infinity to a close object is performed by moving the fifth lens group toward the image side as shown by a solid line to improve controllability and optical performance over a wide range. I have.

 Next, the conditional expression will be described.

Conditional expression (I) defines the ratio of the feed amount to the same object at the wide-angle end and the telephoto end. Exceeding the lower limit of conditional expression (1) means that the sensitivity at the wide-angle end is too high or the sensitivity at the telephoto end is too small. In the former case, mechanical control of the focus group is not possible. In the latter case, the feeding amount at the telephoto end becomes too large, which is not preferable. Exceeding the upper limit of conditional expression (1) means that the sensitivity at the wide-angle end is too small or the sensitivity at the telephoto end is too large,
In the former case, it is necessary to increase the space for focusing at the wide-angle end, and the overall length of the lens becomes longer. In the latter case, performance degradation due to a manufacturing error increases, which is not preferable. Therefore, by setting E _5W and E _5T as in the conditional expression (1), the amount of extension for the same object is reduced at the wide-angle end, and is increased at the telephoto end. Less space,
Further, the overall length of the lens at the wide-angle end is reduced. On the other hand, at the telephoto end, focusing can be performed using the space expanded by zooming, so that the overall length of the lens can be reduced.

Conditional expression (2) is a conditional expression for making the sensitivity of the fifth lens group appropriate. If the focal length of the fifth lens group becomes shorter than the focal length of the entire system at the telephoto end beyond the lower limit value of conditional expression (2), it is advantageous for reducing the overall length of the lens. The sensitivity of the lens becomes too large, which is not preferable in terms of manufacturing and control of the focus lens. Exceeding the upper limit of (2), the focal length of the fifth lens group is longer than the focal length of the entire system at the telephoto end. Then, the compactness of the entire length of the lens becomes insufficient.

Conditional expression (3) is for making the ratio of the sensitivity at the wide-angle end to the sensitivity at the telephoto end an appropriate value based on the conditional expression (2). When the amount of movement of the fifth lens group exceeds the zoom ratio and the focal length of the fifth lens group, the ratio of sensitivity at the wide-angle end to the telephoto end becomes too small, and the amount of focus movement at the wide-angle end When the moving amount of the fifth lens group is increased beyond the upper limit of (3), the moving amount of the focus at the wide-angle end becomes large, and the total length of the lens becomes long to secure this space, which is not preferable. .

Next, the zoom lens composed of five groups according to the present embodiment includes:
The focal lengths of the entire system at the wide-angle end and the telephoto end are respectively f _W , f _T ,
The focal length of the i-th lens unit is f _i , the combined focal length of the first lens unit and the second lens unit at the wide-angle end is f _12W , and the third lens unit and the second lens unit during zooming from the wide-angle end to the telephoto end are zoomed in. When the amount of change in the principal point interval of the four lens groups is Δe ₃₄ and the length from the first lens surface to the final lens surface of the entire system at the wide-angle end is D _W , 0.05f _T / f _W <Δe ₃₄ / D _{W <0.15f T / f W ... (} 4) -3.5 <f 12W / f W <-1.3 ... (5) 0.05 <f 5 / f 12W <0.6 ...... (6) 0.35 <f 3 / f 4 <2.5 ... (7) The following conditional expression is satisfied.

In particular, in this embodiment, while miniaturizing the entire lens system,
In order to prevent an unnecessary long back focus, the entire lens system does not become a very strong reverse telephoto type at the wide-angle end, that is, the refractive power of each lens group and the near-reverse telephoto type become relatively weak. The axis arrangement is set.

 Next, the technical meaning of each of the above conditional expressions will be described.

Conditional expression (4) defines the amount of change in the principal point distance between the third lens unit and the fourth lens unit when zooming from the wide-angle end to the telephoto end with respect to the entire length of the lens at the wide-angle end. . If the principal point distance between the third lens unit and the fourth lens unit becomes too large at the telephoto end beyond the upper limit value of the conditional expression (4), the movement for zooming is performed so that the lens does not interfere at the wide-angle end. It is necessary to secure a large space, and as a result, the overall length of the lens at the wide-angle end becomes large, which is not preferable.

If the distance between the principal points between the third lens unit and the fourth lens unit becomes too small at the telephoto end beyond the lower limit value of the conditional expression (4), the total optical length at the wide-angle end becomes short while obtaining a desired zoom ratio. In order to achieve this, the zoom stroke of the first lens unit and the fifth lens unit must be increased, which is not preferable.

Conditional expression (6) defines the ratio of the focal length of the fifth lens unit to the combined focal length of the first lens unit and the second lens unit at the wide angle end. If the negative refractive power of the fifth lens unit becomes too weak beyond the upper limit value of the conditional expression (6), the distortion tends to decrease. However, in order to obtain a predetermined zoom ratio, the fourth lens unit and the It becomes necessary to increase the interval between the five lens groups at the wide-angle end, and the back focus at the wide-angle end becomes longer than necessary, making it difficult to reduce the size of the entire lens system.

If the negative refractive power of the fifth lens group is too strong below the lower limit of conditional expression (6), the thread-wafer-type distortion will increase and the Petzval sum will increase in the negative direction, resulting in an image surface characteristic. Is difficult to correct satisfactorily, which is not preferable.

Conditional expression (5) defines the ratio of the combined focal length of the first lens unit and the second lens unit at the wide-angle end to the focal length of the entire system at the wide-angle end. In particular, the back focus is made to have an optimum length to achieve compactness of the entire lens system.

If the combined negative refractive power of the first lens unit and the second lens unit becomes too weak below the lower limit value of the conditional expression (5), the lens outer diameter of the third lens unit and subsequent lens units is reduced, and If the stop is located closer to the image plane than the second lens group, if it is advantageous to reduce the outer diameter of the lens barrel by downsizing the stop system, the required back focus is secured and the entire lens system is compact. Therefore, it is necessary to increase the negative refracting power of the fifth lens group, and as a result, the thread-thread type distortion increases.

If the combined negative refracting power of the first lens unit and the second lens unit exceeds the upper limit value of the conditional expression (5), the back focus becomes too long, and the entire lens system becomes large. Fluctuations, especially fluctuations in spherical aberration and field curvature, increase. In addition, the diameter of the diaphragm for securing the required F-number increases, and the outer diameter of the lens barrel increases, which is not preferable.

Conditional expression (7) represents a third condition with respect to the focal length of the fourth lens unit.
This defines the ratio of the focal lengths of the lens groups. If the positive refractive power of the third lens unit becomes too weak beyond the upper limit of conditional expression (7), the distance between the fourth lens unit and the fifth lens unit at the wide-angle end is increased in order to obtain a predetermined zoom ratio. It is necessary to increase the size, and it becomes difficult to reduce the size of the lens system. In addition, in order to maintain the compactness of the lens system, it is necessary to increase the refractive power of the second lens unit or the fifth lens unit. As a result, it becomes difficult to satisfactorily correct various aberrations.

Also, if the refractive power of the third lens group becomes too strong beyond the lower limit of conditional expression (7), aberrations generated in the third lens group,
In particular, the spherical aberration increases, and it becomes difficult to correct the spherical aberration with another lens group in a well-balanced manner.

Next, numerical examples of the present invention will be described. In numerical examples
Ri is the radius of curvature of the i-th lens surface in order from the object side,
Is the i-th lens thickness and air gap from the object side, Ni and ν
i is the refractive index and the Abbe number of the glass of the i-th lens in order from the object side.

The relationship between each numerical example and each conditional expression is as follows.

[Effects] As described above, according to the present invention, it is possible to provide a rear focus zoom lens that is compact, has a high zoom ratio, and has good optical performance over a wide range up to a close object.

[Brief description of the drawings]

1 to 3 are lens cross-sectional views of a zoom lens according to the present invention. FIGS. 4A to 5C, 5A to 5C, and 6A to 6C show various numerical examples 1 to 3, respectively. It is an aberration figure. In the aberration diagrams, (A) -1, (B) -1, and (C) -1 are various aberration diagrams at the wide-angle end, the middle, and the telephoto end, respectively, and (A) -2, (B)-
2, (C) -2 shows various aberration diagrams at the wide-angle end, the middle, and the telephoto end when focusing is performed at an object distance of 3 m. d is a d-line, g is a g-line, SC is a sine condition, ΔS is a sagittal image plane, ΔM is a meridional image plane, and y is an image height.

Claims (2)

(57) [Claims] A first lens group having a positive refractive power, a second lens group having a negative refractive power, and a third lens group having a positive refractive power are arranged in order from the object side. And a fourth lens group having a positive refractive power and a fifth lens group having a negative refractive power. The zoom lens system further includes a rear group having a negative refractive power. The distance between the first lens group and the second lens group is increased, and the distance between the second lens group and the third lens group is reduced so that the image magnifications of the group and the rear group are both increased. The distance between the fourth lens group and the fourth lens group is increased, and the distance between the fourth lens group and the fifth lens group is reduced. Focusing is performed by moving the fifth lens group. The focal length is f ₅ , and the sensitivity at the telephoto end and the wide-angle end is E _5T , E _5W ,
The focal length at the telephoto end of the entire system is f _T , and the amount of movement of the fifth lens group when zooming from the wide-angle end to the telephoto end is
M ₅ (However, the direction from the object side to the image plane side is positive.) When the zoom ratio is Z, A rear focus zoom lens characterized by satisfying the following conditional expression. 2. The focal length of the entire system at the wide-angle end is represented by f _W ,
The focal length of the group is f _i , the combined focal length of the first and second lens groups at the wide-angle end is f _12W , and the third and fourth lens groups during zooming from the wide-angle end to the telephoto end. Where Δe _{34 is} the amount of change in the principal point interval of the lens and _DW is the length from the first lens surface to the final lens surface of the entire system at the wide-angle end, where 0.05 f _T / f _W <Δe ₃₄ / D _W <0.15 range of _{f T / f W -3.5 <f} 12W / f W <-1.3 0.05 <f 5 / f 12W <0.6 0.35 < claims, characterized by satisfying the f ₃ / f ₄ <2.5 the condition first 2. The rear focus zoom lens according to claim 1.