JP2000284171A - Shooting lens for short-range shooting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a short-range photographing lens which has three lens groups as a whole and has good optical performance in a range from an object at infinity to a photographing magnification of about 0.5 times. To get SOLUTION: A first lens group having a positive refractive power from the object side,
A second lens group having a negative refractive power and a third lens group having a positive refractive power, wherein the second lens group is a 2a lens group having a negative refractive power as a whole from the object side with an air gap as a boundary When,
The second lens group is composed of a second lens group having a positive refractive power as a whole, and the second lens group is moved on the optical axis toward the image plane to perform focusing from an object at infinity to an object at a short distance, and The focal length F of the entire lens system at the time of an object, the focal length Fi of the i-th lens unit, the combined focal length F23 of the second and third lens units at infinity of the object, the focal length F2a of the 2a-th lens unit, The focal length F2b of the 2b lens group is set appropriately.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photographic lens suitable for a photographic camera, a video camera, a digital camera, etc., capable of photographing at a short distance, particularly in a high magnification range from an object at infinity to about 0.5 times. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photographic lens having high optical performance and capable of short-distance photography.

[0002]

2. Description of the Related Art Conventionally, in a photographic camera, a video camera, a digital camera, and the like, a macro lens or a micro lens (hereinafter referred to as a "macro lens") is used as an enlargement close-up photographing lens mainly for photographing an object at a short distance. There is something called. The macro lens is designed so that higher optical performance can be obtained, especially for a close object, as compared with other photographing lenses such as a general standard lens and a telephoto lens. Also, macro lenses are often used for a wide range of objects ranging from near objects to infinity objects.

Conventionally, as a focusing method of a macro lens, a method of moving the entire lens system on the optical axis, for example, Japanese Patent Application Laid-Open Nos. 5-142474 and 8-2016.
As proposed in Japanese Patent Application Publication No. 92-92, an optical system of a negative lens group (correction lens group) for correcting aberration fluctuation at the time of shooting a close-range object is arranged on the image plane side of an optical system of a positive lens group (photographing lens). And by moving the positive lens group on the optical axis.

[0004] In addition, a so-called floating photographing lens which performs focusing by moving each of a plurality of lens groups independently when photographing a short-distance object is disclosed in, for example, JP-A-2-19814 and JP-A-2-19814. No. 285313, and the like.

[0005]

Generally, in a photographing lens which enables close-up photographing, the amount of movement of a focus lens group for focusing increases. Generally, the lens group on the object side (first lens group) has a large outer diameter and is often heavy. For this reason, the method in which focusing is performed by the lens group on the object side has a problem that, for example, in a camera having an autofocus function, the driving torque becomes large and it is difficult to focus quickly.

Further, in order to enable a short-distance photographing with a maximum photographing magnification of about 0.5 times, fluctuations in aberrations during focusing become large, and it is very difficult to perform satisfactory aberration correction over the entire object distance. It becomes difficult.

The present invention focuses on a lens group other than the first lens group on the object side, and focuses on a wide range of objects from an object at infinity to a close object having a photographic magnification of about 0.5. It is an object of the present invention to provide a high-performance, short-range photographing lens capable of satisfactorily correcting aberration fluctuations at the time of photographing.

[0008]

According to the first aspect of the present invention, there is provided a photographing lens capable of photographing at a short distance, comprising a first lens having a positive refractive power from the object side.
A lens group, a second lens group having a negative refractive power, and a third lens group having a positive refractive power. The second lens group has a negative refractive power as a whole from the object side at the widest air gap. And a second lens unit having a positive refractive power as a whole.
The second lens group is focused on the object side at infinity to the object side at infinity by moving the second lens group on the optical axis to the image plane side, and the focal length of the entire lens system at the time of infinity object Is F, the focal length of the i-th lens unit is Fi, the combined focal length of the second and third lens units at infinity of the object is F23, the focal length of the 2a-th lens unit is F2a, and the focal point of the 2b-th lens unit is When the distance is F2b, 0.45 <
F1 / F <0.9 ... (1) 0.4 <| F2 / F | <2 ... (2) 0.55 <F3 / F <4.8 ... (3) | F / F23 | <1.1 (4) 0.15 <| F2a / F2b | <0.7 (F2a <0, F2b> 0) (5)

According to a second aspect of the present invention, in the first aspect of the present invention, the second lens group has a second a lens group having a negative refractive power as a whole with a wide air gap as a boundary, and a positive refractive power as a whole. And a second b lens group.

According to a third aspect of the present invention, in the first or second aspect, the second lens subunit includes a negative single lens having a concave surface facing the image plane side, and the second lens subunit includes a positive lens and a negative lens. It is characterized by.

In a fourth aspect of the present invention, in the first, second or third aspect of the present invention, the photographing magnification of the entire lens system when the object is at a close distance is β mod (<0), and the second lens is from an infinite object to a close object. When the amount of movement of the group is ΔX, the following conditional expression is satisfied: 0.35 <| ΔX / (F · βmod) | <0.8 (6)

[0012] The invention of claim 5 is the invention of claim 1, 2, 3, or 4.
When the focal length of the entire lens system is F, the focal length of the third lens unit is F3, and the lateral magnification of the third lens unit at infinity is β3, 0.28 <(F3 · (1 −β3)) / F <1 (7)

[0013]

FIG. 1, FIG. 5, FIG. 9, FIG. 13, FIG.
7 is a lens sectional view of each of Numerical Examples 1 to 5 of the present invention. 2, 3, and 4 are aberration diagrams of an object at infinity, a photographing magnification of 0.25 times, and a photographing magnification of 0.5 times according to Numerical Example 1 of the present invention. FIGS. 6, 7 and 8 are aberration diagrams of an object at infinity, a photographing magnification of 0.25 times, and a photographing magnification of 0.5 times in Numerical Example 2 of the present invention. FIGS. 10, 11, and 12 are aberration diagrams of an object at infinity, a photographing magnification of 0.25 times, and a photographing magnification of 0.5 times in Numerical Example 3 of the present invention. FIGS. 14, 15 and 16 show an object at infinity and a photographing magnification of 0.25 according to Numerical Embodiment 4 of the present invention.
FIG. 4 is an aberration diagram at a magnification of × 0.5 and a magnification of × 0.5. FIG. 18, FIG.
9 and FIG. 20 are aberration diagrams of an object at infinity, a photographing magnification of 0.25 times, and a photographing magnification of 0.5 times in Numerical Example 5 of the present invention.

In the figure, L1 is a first lens unit having a positive refractive power;
L2 is a second lens group having a negative refractive power for focusing, L3
Denotes a third lens group having a positive refractive power. Second lens unit L2
Has a second-a lens unit L2a having a negative refractive power and a second-b lens unit L2b having a positive refractive power with the widest air gap as a boundary. The arrow indicates the moving direction of the second lens group when focusing from an object at infinity to an object at a short distance (with an increase in imaging magnification). SP is an aperture. IP is an image plane.

In this embodiment, the first lens unit L1 having a positive refractive power and the second lens unit L1 having a negative refractive power are arranged in order from the object side.
2. It has a third lens unit L3 having a positive refractive power, and performs focusing from an object at infinity to an object at a close distance by moving the second lens unit L2 on the optical axis toward the image plane.

In this lens configuration, the lens outer diameter of the second lens group is reduced by the light beam converging action of the first lens group. Thereby, when the photographing lens of the present invention is applied to a camera having an auto-focusing mechanism, a problem arises in that the focusing lens group, which is a problem in the auto-focusing mechanism driven by the electric lens, becomes heavy and the moving distance is long. Has been resolved.

By providing a second lens unit for negative focusing between the first and third lens units having two positive refractive powers, a large image plane position can be corrected for a small lens movement on the optical axis. The effect has been obtained. Thus, when focusing from an object at infinity to an object at a certain finite distance, a small focus movement is performed. The second lens group is composed of a 2a lens group having a negative refractive power as a whole from the object side and a 2b lens group having a positive refractive power as a whole with an air gap therebetween. In particular, the correction of the axial aberration and the correction of the off-axis aberration in the second lens group are favorably performed.

In the second lens group, the second lens group and the second lens group are separated from each other by the widest air space of the second lens group so that the second lens group as a whole has a strong negative refractive power. This is effective for performing a focus movement with a small amount.

By satisfying the above-mentioned conditional expressions (1) to (5), high optical performance is obtained. It is not always necessary to satisfy all of the conditional expressions (1) to (5) at the same time. If only the intended optical performance is obtained, only the corresponding conditional expression may be satisfied when it is good.

Next, the technical meaning of the conditional expressions (1) to (5) will be described.

Conditional expression (1) relates to the refracting power of the first lens unit, which is a condition for ensuring a sufficient back focus while controlling mainly the entire length of the lens system.

When the refractive power of the first lens group becomes weaker than the upper limit, the refractive power of the second lens group needs to be weakened in order to maintain a constant focal length. As a result, the amount of focus movement increases. At the same time, the convergence effect of the first lens group is weakened, so that the lens diameter of the second lens group is increased, which is not good. On the other hand, if the refractive power of the first lens group is too strong beyond the lower limit, the image plane principal point position of the entire lens system shifts to the object side, and as a result, it becomes difficult to secure a sufficient back focus. come.

Conditional expression (2) is a condition for correcting the aberration fluctuation due to focus with good balance while controlling the amount of focus movement at a constant photographing magnification with respect to the refractive power of the second lens group which is the focus lens group. .

When the refractive power of the second lens group becomes weak and exceeds the upper limit of conditional expression (2), the focus movement amount of the second lens group with respect to a fixed object distance increases, and at the same time, the first lens It becomes difficult to correct the spherical aberration generated in the group. On the other hand, if the lower limit value is exceeded, the refracting power of the second lens group becomes too strong, causing large curvature of field, and it becomes difficult to correct this.

Conditional expression (3) relates to the refractive power of the third lens group, and is a condition for maintaining a proper balance between the back focus and the peripheral light amount.

If the refractive power of the third lens unit is too weak and exceeds the upper limit of conditional expression (3), the lens outer diameter of the third lens will be too large to secure a constant peripheral light amount. On the other hand, if the refractive power of the third lens group becomes too strong and exceeds the lower limit of conditional expression (3), it becomes difficult to secure a sufficient back focus.

Conditional expression (4) indicates that the second condition for an object at infinity
It relates to the combined refractive power of the lens group and the third lens group.

If the combined refracting power exceeds the upper limit and becomes too strong, a sufficient back focus is ensured, and the second and third lens units are moved on the optical axis in order to obtain a constant photographing magnification. It becomes difficult to keep the focus interval between the lens groups.

Conditional expression (5) indicates that the second lens group and the second lens group
It indicates the refractive power ratio of the lens group, and suppresses the fluctuation of aberration in focusing.

If the upper limit of conditional expression (5) is exceeded, the negative refractive power component of the second lens sub-unit in the second lens unit becomes weak, and the effect of correcting the spherical aberration generated by the first lens unit becomes weak. Not good. On the other hand, when the positive refractive power component of the second lens unit b is weakened beyond the lower limit, it becomes difficult to correct the off-axis aberration component mainly in a short distance range.

The photographic lens capable of taking a close-up image, which is the object of the present invention, is achieved by the above-described configuration. To further improve the optical performance in taking a close-up object, the following conditions must be satisfied. At least one should be satisfied.

(A-1) When the photographing magnification of the entire lens system when the object is at a close distance is β mod (<0), and the moving amount of the second lens unit from an object at infinity to an object at a close distance is ΔX, 35 <| ΔX / (F · βmod) | <0.8 (6)

Conditional expression (6) is obtained by normalizing the amount of movement of the second lens unit from the object at infinity to the object at the closest distance by the focal length of the entire lens system and the photographing magnification.

If the upper limit is exceeded, the amount of movement of the second lens group becomes too large, which increases the total length of the lens system, and at the same time increases the focus driving torque, which is not good.

(A-2) Assuming that the focal length of the entire lens system is F, the focal length of the third lens group is F3, and the lateral magnification of the third lens group at infinity is β3, 0.28 <( F3 · (1−β3)) / F <1 (7)

Conditional expression (7) is mainly for balancing the total lens length.

When the value exceeds the upper limit of conditional expression (7), the back focus becomes too long, and the total length of the lens increases. On the other hand, if the lower limit value is exceeded, the back focus becomes too short, and interference with the quick return mirror occurs when mounted on, for example, a single-lens reflex camera, which is not good.

(A-3) In the focal length of the quasi-telephoto system, the first lens unit is a positive lens unit from the object side, a negative lens with the concave surface facing the image surface side, and the concave lens is turned to the object side further. A Gaussian optical system having a negative lens unit and a positive lens unit is suitable for achieving a large-diameter optical system.

(A-4) At the focal length of the standard system, the first
It is desirable to use a retrofocus type optical system for the lens group in order to obtain a sufficient focus interval and back focus.

(A-5) The second lens group is a negative lens having the 2a lens group concave toward the image plane side, the second lens group being a positive lens having both convex lens surfaces, and a concave lens being the object side. It is good to constitute with the negative lens which aimed. According to this, it is possible to perform focusing with little aberration variation with a small number of lenses.

(A-6) The third lens group mainly corrects chromatic aberration of magnification, and for this purpose, it is preferable to have at least one negative lens and a positive lens whose both lens surfaces are convex.

(A-7) The iris diaphragm is preferably arranged in the first lens unit or in the air space between the first and second lens units.

(A-8) In order to improve the optical performance at the time of focusing, at the time of focusing, floating which slightly changes an arbitrary air space in the second lens group may be used.

(A-9) In order to further improve the optical performance, an aspherical surface, a diffractive optical element, or a refractive index distribution type optical material may be introduced into the optical system.

(A-10) An image may be displaced by decentering a part of the optical system to be used for image blur correction at the time of photographing.

Next, numerical examples of the present invention will be described. 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 spacing from the object side, and Ni and νi are the i-th lens surfaces in order from the object side. The refractive index and Abbe number of glass. Table 1 shows the relationship between the above-described conditional expressions and various numerical values in the numerical examples.

[0047]

[Outside 1]

[0048]

[Outside 2]

[0049]

[Outside 3]

[0050]

[Outside 4]

[0051]

[Outside 5]

[0052]

[Table 1]

[0053]

According to the present invention, by setting each element as described above, focusing is performed by a lens group other than the first lens group on the object side, and the photographing magnification is 0.5% from an object at infinity. It is possible to achieve a high-performance shooting lens capable of performing close-range shooting with good correction of aberration fluctuation when focusing on a wide range of objects up to a close-range object close to double magnification.

[Brief description of the drawings]

FIG. 1 is a sectional view of a lens according to a first embodiment of the present invention.

FIG. 2 is an aberration diagram for an object at infinity according to the first embodiment of the present invention.

FIG. 3 is an aberration diagram at a magnification of 0.25 × in the first embodiment of the present invention.

FIG. 4 is an aberrational diagram when the magnification is 0.5 × according to the first embodiment of the present invention.

FIG. 5 is a sectional view of a lens according to a second embodiment of the present invention.

FIG. 6 is an aberration diagram for an object at infinity according to the second embodiment of the present invention.

FIG. 7 is an aberration diagram at a magnification of 0.25 × in the second embodiment of the present invention.

FIG. 8 is an aberration diagram at a magnification of 0.5 × according to the second embodiment of the present invention.

FIG. 9 is a sectional view of a lens according to a third embodiment of the present invention.

FIG. 10 is an aberration diagram for an infinitely distant object according to the third embodiment of the present invention.

FIG. 11 is an aberration diagram for Embodiment 3 of the present invention when the magnification is 0.25 ×.

FIG. 12 is an aberration diagram at a magnification of 0.5 × in the third embodiment of the present invention.

FIG. 13 is a sectional view of a lens according to a fourth embodiment of the present invention.

FIG. 14 is an aberration diagram for an object at infinity according to the fourth embodiment of the present invention.

FIG. 15 is an aberration diagram at a magnification of 0.25 × in the fourth embodiment of the present invention.

FIG. 16 is an aberration diagram at a magnification of 0.5 × according to the fourth embodiment of the present invention.

FIG. 17 is a sectional view of a lens according to a fifth embodiment of the present invention.

FIG. 18 is an aberration diagram for an infinitely distant object according to the fifth embodiment of the present invention.

FIG. 19 is an aberration diagram at a magnification of 0.25 × in the fifth embodiment of the present invention.

FIG. 20 is an aberration diagram at a magnification of 0.5 × in the fifth embodiment of the present invention.

[Explanation of symbols]

 L1 1st lens group L2 2nd lens group L3 3rd lens group L2a 2a lens group L2b 2b lens group SP stop d d line gg line ΔS sagittal image plane ΔM meridional image plane

Claims (5)

[Claims] A first lens unit having a positive refractive power from the object side;
A second lens group having a negative refractive power and a third lens group having a positive refractive power, wherein the second lens group is a 2a lens group having a negative refractive power as a whole from the object side with an air gap as a boundary When,
The second lens group is composed of a second lens group having a positive refractive power as a whole, and the second lens group is moved on the optical axis toward the image plane to perform focusing from an object at infinity to an object at a short distance, and The focal length of the entire lens system at the time of an object is F,
The focal length of the lens unit is Fi, the combined focal length of the second lens unit and the third lens unit when the object is at infinity is F23, the focal length of the 2a lens unit is F2a, and the focal length of the 2b lens unit is F2b. Then, 0.45 <F1 / F <0.9 0.4 <| F2 / F | <2 0.55 <F3 / F <4.8 | F / F23 | <1.1 0.15 <| F2a /F2b|<0.7 (F2a <0, F2b> 0). 2. The second lens group includes a second lens group having negative refractive power as a whole at a boundary between the widest air gaps.
2. The photographic lens according to claim 1, wherein the photographic lens includes a second lens group having a positive refractive power as a whole. 3. The close-up photographable lens according to claim 1, wherein the second lens group includes a negative single lens having a concave surface facing the image surface side, and the second lens group includes a positive lens and a negative lens. Photographic lens. 4. When the photographing magnification of the entire lens system at a close distance object is β mod (<0), and the moving amount of the second lens unit from an infinity object to a close distance object is ΔX, 0.35 <| 4. The taking lens according to claim 1, 2, or 3, wherein the following conditional expression is satisfied: ΔX / (F · βmod) | <0.8. 5. When the focal length of the entire lens system is F, the focal length of the third lens unit is F3, and the lateral magnification of the third lens unit at the time of an object at infinity is β3, 0.28 <(F3 · ( The photographing lens according to claim 1, 2, 3, or 4, wherein the following conditional expression is satisfied: 1- [beta] 3)) / F <1.