Zoom lens and optical apparatus including zoom lens

1. Field of the Invention

The present invention relates to a retrofocus type zoom lens including a negative lens unit located on a magnifying side.

2. Description of the Related Art

In an image projection apparatus such as a liquid crystal projector, an optical system with a wide field angle is required to project a large image at a short distance. In contrast to this, an optical system having a relatively long back focus is required to provide a color combining optical system (such as a prism) between a final surface of a projection optical system and an image display element such as a liquid crystal panel.

An interchangeable lens for a single-lens reflex camera requires a space for providing a mirror box on the rear side (image pickup element side or film surface side) of the interchangeable lens, and thus is required to have a back focus longer than a focal length.

Therefore, as such an optical system, there is used a retrofocus type optical system in which a negative refractive power is arranged on a magnifying side and a positive refractive power is arranged on a reduction side. However, the refractive powers are asymmetrically arranged, and hence it is difficult to correct a distortion or a lateral chromatic aberration.

In recent years the number of pixels of the image display element used in image projection apparatus and the number of pixels of an image pickup element used in single-lens reflex camera have rapidly increased. For that reason, an optical system having high optical performance over the entire magnification range is required. In particular, it is important to correct distortion corresponding to distortion of an image, or a chromatic aberration which influences color blurring or sharpness of an image in a case of a white light source to a high degree.

U.S. Pat. No. 7,292,398 and U.S. Pat. No. 7,480,102 each disclose a structure, in which an optical element having large extraordinary dispersion is provided as a concave lens of a first lens unit (negative lens unit) of the retrofocus type optical system, in which the negative lens unit is provided on the magnifying side, to thereby reduce the lateral chromatic aberration.

In the structure described above, a material having large extraordinary dispersion is influenced by a small negative refractive power to correct a lateral chromatic aberration of the g-line in the vicinity of a wide-angle region. However, when an image obtained by a digital image pickup apparatus is to be greatly enlarged or when the image is to be enlarged and projected onto a screen by the image projection apparatus, it is necessary to further reduce the chromatic aberration. However, in order to correct the chromatic aberration, when the refractive power of an element made of an extraordinary dispersion material is increased to strongly correct the chromatic aberration, the element needs to have a biconcave shape, and hence it is difficult to correct the distortion to a high standard.

Japanese Patent Application Laid-Open No. 2007-178894 discloses a structure in which a concave lens having a partial dispersion characteristic larger than a reference line (θ=−0.001682*νd+0.6438) is provided for the first lens unit (negative lens unit) of the retrofocus type optical system, in which the negative lens unit is provided on the magnifying side.

In the above-mentioned Japanese Patent Application Laid-Open No. 2007-178894, the extraordinary dispersion characteristic and the refractive power of the negative lens are insufficient, and hence a secondary chromatic aberration correction effect is not sufficiently obtained. Therefore, it is difficult to sufficiently correct the lateral chromatic aberration of the g-line in the vicinity of an image at the wide-angle end.

According to the present invention, there is provided a zoom lens, including, in order from a magnifying side: a first lens unit having a negative refractive power; a second lens unit having a positive refractive power; and a rear lens group, which includes multiple lens units and has a positive refractive power as a whole, in which at least two lens units are moved for magnification, in which the first lens unit includes a first lens having a negative refractive power, and in which the following conditions are satisfied:

0.79 <θgF NL −(1.0 E− 4 ×νd NL 2 −9.1 E− 3 ×νd NL )<1.00,

1.5<( R 1+ R 2)/( R 1− R 2)<10.0, and

0.0005 <|ΔθgF NL ·fw )/( νd NL ·f NL )|<0.0020,

where R 1 and R 2 indicate a magnifying side curvature radius and a reduction side curvature radius of the first lens, θgF NL indicates a partial dispersion ratio of the first lens with respect to a g-line and an F-line, νd NL indicates an Abbe number of the first lens, N NL indicates a refractive index of the first lens, and f NL indicates a focal length of the first lens, and

where Δθ gF NL =θgF NL −(−0.001682 ×νd NL +0.6438).

According to the present invention, even when a retrofocus type optical system is provided as a zoom lens, both a distortion and a lateral chromatic aberration may be excellently reduced.

Further features of the present invention become apparent from the following description of exemplary embodiments with reference to the attached drawings.

FIG. 1 is a lens cross sectional view illustrating Embodiment 1 of the present invention;

FIG. 2 is a lens cross sectional view illustrating Embodiment 2 of the present invention;

FIG. 3 is a lens cross sectional view illustrating Embodiment 3 of the present invention;

FIGS. 4A and 4B are aberration graphs in Embodiment 1 of the present invention;

FIGS. 5A and 5B are aberration graphs in Embodiment 2 of the present invention;

FIGS. 6A and 6B are aberration graphs in Embodiment 3 of the present invention; and

FIG. 7 illustrates an image projection apparatus according to the present invention.

Hereinafter, a zoom lens according to each of exemplary embodiments of the present invention and an optical apparatus (such as image pickup apparatus or image projection apparatus) including the zoom lens are described in detail with reference to the attached drawings.

Note that the image pickup apparatus includes: an image pickup element such as a charge coupled device (CCD), for photoelectrically converting an image (light) of a subject into an electrical signal; and a zoom lens for forming the image of the subject on the image pickup element (CCD). The image projection apparatus includes: an image display element such as a liquid crystal panel or a micro-mirror device having multiple minute mirrors; and a zoom lens for projecting light from the image display element to a projection surface such as a screen. In other words, according to the zoom lens, the image display element and the projection surface are brought into a conjugate relationship (relationship for forming image from image display element on projection surface).

In the image pickup apparatus described above, a reduction side (reduction conjugate side or side on which conjugate length is short) corresponds to an image pickup element side (CCD side), and a magnifying side corresponds to a subject side (magnifying conjugate side, side on which conjugate length is long, or object side). In the image projection apparatus, the reduction side corresponds to an image display element side (liquid crystal panel side or LCD (Liquid Crystal Display) side), and the magnifying side corresponds to a projection surface side such as a screen side (light traveling direction).

In each of zoom lenses according to Embodiments 1 to 3 of the present invention, at least two lens units are moved during magnification, to thereby ensure a desired magnification ratio. A wide-angle end and a telephoto end correspond to zoom positions in a case where a lens unit for zooming is located in each of both ends of a range in which the lens unit is moveable on an optical axis in view of mechanism.

Next, the zoom lenses according to the exemplary embodiments of the present invention are described.

Each of the zoom lenses according to the present invention includes, in order from the magnifying side, a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, and the rear lens group, which includes multiple lens units and has a positive refractive power as a whole. The rear lens group includes a third lens unit, a fourth lens unit, a fifth lens unit, and a sixth lens unit in Embodiment 1 described later, and includes a third lens unit and a fourth lens unit in each of Embodiments 2 and 3 described later. Multiple lens units are moved in an optical axis direction during magnification (during zooming). The first lens unit which is located closest to the magnifying side and has the negative refractive power is not moved during magnification, but moved during focusing.

According to a feature of the zoom lens described above, the first lens unit includes a first lens having a negative refractive power, and the first lens satisfies the following Conditional Expressions (1), (2), and (3).

0.79 <θgF NL −(1.0 E− 4 ×νd NL 2 −9.1 E− 3 ×νd NL )<1.00  (1)

1.5<( R 1+ R 2)/( R 1− R 2)<10.0  (2)

0.0005 <|ΔθgF NL ·fw )/( νd NL ·f NL )|<0.0020  (3)

where θgF NL indicates a partial dispersion ratio of the first lens with respect to a g-line and an F-line, νd NL indicates an Abbe number of the first lens, N NL indicates a refractive index of the first lens, and f NL indicates a focal length of the first lens. In addition, assume that ΔθgF NL is defined by

ΔθgF NL =θgF NL −(−0.001682 ×νd NL +0.6438),

and R 1 and R 2 indicate a magnifying side curvature radius and a reduction side curvature radius of the first lens.

The Abbe number νd (νd NL in this case) and θgF (θgF NL in this case) of the first lens which are defined as described above are normally used. When refractive indexes with respect to a g-line, an F-line, a d-line, and a C-line are denoted by Ng, NF, Nd, and NC, νd and θgF are expressed by the following equations.

ν d =( Nd− 1)/( NF−NC )

θ gF =( Ng−NF )/( NF−NC )

In the present invention, a solid material refers to a material which is solid in a case where the optical system is used, and thus a state before the optical system is used, such as a manufacturing state is not mentioned. For example, a solid material obtained by curing a material which is liquid during manufacturing corresponds to the solid material in the present invention.

Conditional Expression (1) specifies a relationship between the Abbe number and the partial dispersion ratio of the first lens. When the condition of Conditional Expression (1) exceeds any one of the upper limit and lower limit thereof, the extraordinary dispersion of the first lens becomes excessive or insufficient, and hence it is difficult to correct a secondary chromatic aberration.

It is more desirable to satisfy the following Conditional Expression (1a), and hence a zoom lens may be realized in which a lateral chromatic aberration in a short-wavelength region is excellently reduced.

0.79 <θgF NL −(1.0 E− 4 ×νd NL 2 −9.1 E− 3 ×νd NL )<0.95  (1a)

Conditional Expression (2) specifies a shape of the first lens. When the condition of Conditional Expression (2) exceeds the lower limit thereof, the magnifying side curvature radius becomes smaller, and hence it is difficult to correct a distortion. In contrast to this, when the condition of Conditional Expression (2) exceeds the upper limit thereof, a refractive power provided for the solid material becomes smaller, and hence secondary chromatic aberration correction performance reduces.

It is more desirable to satisfy the following Conditional Expression (2a), and hence a zoom lens may be realized in which the distortion and the secondary chromatic aberration are excellently reduced.

2.6<( R 1+ R 2)/( R 1− R 2)<7.0  (2a)

Conditional Expression (3) specifies secondary chromatic aberration reduction performance of the first lens. This conditional expression specifies a relationship with respect to not only the extraordinary dispersion but also high dispersion and the refractive power of the first lens. When |ΔθgF NL ·fw)/(νd NL ·f NL )| is equal to or smaller than the lower limit of the Conditional Expression (3), the correction of the secondary chromatic aberration using the first lens is insufficient, and hence there remains color blurring in the vicinity of a wide-angle region. In contrast to this, when |ΔθgF NL ·fw)/(νd NL ·f NL )| is equal to or greater than the upper limit of the Conditional Expression (3), the chromatic aberration is excessively corrected. This is not desirable.

It is more desirable to satisfy the following Conditional Expression (3a), and hence a zoom lens may be obtained in which the secondary chromatic aberration is excellently corrected.

0.0005 <|ΔθgF NL ·fw )/( νd NL ·f NL )|<0.0015  (3a)

In the zoom lens according to the present invention, the Abbe number νd NL of the first lens desirably satisfies the following conditional Expression.

10.0<νd NL <23.5  (4)

When νd NL is equal to or smaller than the lower limit of the Conditional Expression (4), it is difficult to perform primary achromatism by the first lens unit. In contrast to this, when νd NL is equal to or greater than the upper limit of the Conditional Expression (4), it is difficult to correct a lateral chromatic aberration in a short-wavelength region close to the wide-angle region.

When the Abbe number νd NL of the first lens more desirably satisfies the following Conditional Expression (4a), the primary chromatic aberration may be more excellently corrected.

14.0<νd NL <23.5  (4a)

In the zoom lens according to the present invention, when an Abbe number of a positive lens located closest to the first lens is denoted by νp, it is further preferred that the following condition be satisfied.

10.0<νp<38.0  (5)

Conditional Expression (5) is for excellently reducing a primary chromatic aberration caused by a high-dispersion solid material. When the Conditional Expression (5) is not satisfied, the chromatic aberration is excessively corrected or insufficiently corrected. This is not desirable.

When the Abbe number νp of the positive lens more desirably satisfies the following Conditional Expression, a more excellent effect is obtained.

18.5<νp<38.0  (5a)

Further, the refractive index N NL of the first lens more desirably satisfies the following conditional expression.

1.70<N NL <2.30  (6)

Conditional Expression (6) specifies the refractive index of the first lens. When N NL is equal to or smaller than the lower limit of the Conditional Expression (6), not only the distortion caused by the first lens having the strong refractive power but also the lateral chromatic aberration become larger. In contrast to this, when N NL is equal to or greater than the upper limit of the Conditional Expression (6), a Petzval sum becomes larger, and hence it is difficult to reduce a field curvature.

When the refractive index N NL of the first lens more desirably satisfies the following conditional expression,

1.75<N NL <2.00  (6a),

a zoom lens may be realized in which various aberrations including the distortion are excellently reduced.

FIG. 1 is a lens cross sectional view illustrating the zoom lens according to Embodiment 1 of the present invention. The detailed optical data are described below in Numerical Embodiment 1. The zoom lens according to Embodiment 1 of the present invention is applied to a projection optical system used for an image projection apparatus. The zoom lens may be applied to an image pickup optical system.

The zoom lens according to Embodiment 1 includes, in order from the magnifying side, a first lens unit L 1 having a negative refractive power, a second lens unit L 2 having a positive refractive power, a third lens unit L 3 having a positive refractive power, a fourth lens unit L 4 having a negative refractive power, a fifth lens unit L 5 having a positive refractive power, and a sixth lens unit L 6 having a positive refractive power. Each of the first to sixth lens units includes at least one lens (or diffraction grating or mirror having curvature).

During magnification from the wide-angle end to the telephoto end, the second, third, fourth, and fifth lens units, i.e. lens units other than the first and sixth lens units, are moved to the magnifying side to ensure a magnification ratio of approximately 1.7 times. When a projection distance is changed, a focus operation (focusing) is performed by moving the first lens unit.

In this embodiment, a high-dispersion material is used for a negative lens (first lens) located closest to the magnifying side, in the first lens unit having the negative refractive power. Therefore, a high-dispersion material is used for a positive lens of the first lens unit and a suitable refractive power is provided for the positive lens, to thereby reduce a primary chromatic aberration caused by constituting the negative lens by the high-dispersion material.

The negative lens of the first lens unit is normally made of a low-dispersion material. In this embodiment, the negative lens is made of the high-dispersion material (νd=23.0). When the retrofocus type optical system has the structure described above, a function for returning the lateral chromatic aberration on the short-wavelength side of the wide-angle region to an under side is obtained.

When a high-extraordinary dispersion material (θgF=0.6454) is employed as a material of the first lens having the negative refractive power and a strong refractive power is provided for the first lens, a secondary lateral chromatic aberration in the vicinity of the wide-angle region, in particular, the degree of curve of a g-line may be corrected. The strong refractive power may be provided for the first lens having the negative refractive power, and hence a refractive index of the material of the first lens may be increased (Nd=1.77). Therefore, the amount of occurrence of distortion may be reduced. The first lens is provided as a negative meniscus lens having a convex surface facing toward the magnifying side so as to prevent mainly the distortion from significantly occurring.

The first lens is desirably provided in a magnifying side position far from a stop. In this case, the first lens may be provided in a position in which an on-axis beam height is low and an off-axis beam height is high, and hence the lateral chromatic aberration may be significantly corrected with little influence on an on-axis chromatic aberration.

Hereinafter, the zoom lens according to Embodiment 2 of the present invention is described with reference to FIG. 2 . The zoom lens according to this embodiment is applied to a projection optical system for an image projection apparatus. The detailed optical data of the zoom lens according to Embodiment 2 are described below in Numerical Embodiment 2.

The zoom lens according to Embodiment 2 includes, in order from the magnifying side, a first lens unit L 1 having a negative refractive power, a second lens unit L 2 having a positive refractive power, a third lens unit L 3 having a positive refractive power, and a fourth lens unit L 4 having a positive refractive power.

During magnification from the wide-angle end to the telephoto end, the first and fourth lens units are not moved, the second lens unit is moved to the reduction side, and the third lens unit is moved to the magnifying side, to thereby ensure a magnification ratio of approximately 1.5 times. A focus operation (focusing) is performed by moving the first lens unit.

In this embodiment, a material having extraordinary dispersion (θgF=0.7200) higher than that in Embodiment 1 is employed for a first lens having negative refractive power located closest to the magnifying side, in the first lens unit, and a stronger refractive power than that in Embodiment 1 is provided for the first lens. Therefore, a lateral chromatic aberration in the vicinity of the wide-angle region, in particular, the degree of curve of a g-line may be suppressed (reduced). The other structures are the same as that in Embodiment 1 and thus the detained description is omitted.

Hereinafter, the zoom lens according to Embodiment 3 of the present invention is described with reference to FIG. 3 . The zoom lens according to this embodiment is applied to an optical system for a single-lens reflex camera. The detailed optical data of the zoom lens according to Embodiment 3 are described below in Numerical Embodiment 3.

The zoom lens according to Embodiment 3 includes, in order from the magnifying side (subject side), a first lens unit L 1 having a negative refractive power, a second lens unit L 2 having a positive refractive power, a third lens unit L 3 having a positive refractive power, and a fourth lens unit L 4 having a positive refractive power.

During magnification from the wide-angle end to the telephoto end, the first lens unit is not moved and the second, third, and fourth lens units are moved to the magnifying side.

In Embodiment 3, a material having dispersion (νd=17.4) higher than that in Embodiment 1 is used as a material of a negative lens (first lens) located closest to the magnifying side, in the first lens unit. Therefore, the lateral chromatic aberration on the short-wavelength side of the wide-angle region, which is caused in a retrofocus type optical system, may be excellently reduced. When a high-extraordinary dispersion material (θgF=0.6780) is employed as a material of the first lens having the negative refractive power and a strong refractive power is provided for the first lens, a lateral chromatic aberration in the vicinity of the wide-angle region, in particular, the degree of curve of a g-line may be lowered (reduced).

The first lens has the strong refractive power as in Embodiment 2, and hence a material having a higher refractive index (Nd=1.94) may be used as the material of the first lens, to thereby easily reduce the occurrence of distortion. The first lens is provided as a concave meniscus lens having a convex surface facing the magnifying side so as to prevent mainly the distortion from significantly occurring.

The other points which are not specially described are the same as in Embodiment 1.

Hereinafter, the detailed optical data of the zoom lenses according to Embodiments 1, 2, and 3 of the present invention are described. A surface number “i” is counted from the object side. In addition, Ri indicates a curvature radius of an i-th lens surface and Di indicates a total of a lens thickness and an air interval between the i-th surface and an (i+1)-th surface. Further, Ndi, νdi, and θgFi indicate a refractive index with respect to the d-line, an Abbe number with respect to the d-line, and a partial dispersion ratio between the g-line and the F-line.

Glass blocks GB located closest to the image side correspond to color combining prisms.

Assume that k, A, B, C, D, and E indicate aspherical coefficients.

When a displacement amount from a surface vertex in the optical axis direction at a position corresponding to a height h from the optical axis is expressed by x, an aspherical surface shape is defined using the following equation.

x =( h 2 /R )/[1+{1−(1 +k )( h/R ) 2 } 1/2 ]+Ah 4 +Bh 6 +Ch 8 +Dh 10 +Eh 12 , where R indicates a curvature radius.

Results obtained by calculation using Conditional Expressions (1) to (6) on the zoom lenses according to Embodiments 1, 2, and 3 are illustrated in the following Table 1.

Embodiments 1, 2, and 3 are described in detail, but the present invention is not limited to the embodiments and various modifications and changes may be made without departing from the scope of the present invention. For example, the fourth lens unit and the fifth lens unit as described in Embodiment 1 may be moved as a single lens unit. That is, a zoom lens may include, in order from the magnifying side, five lens units having negative, positive, positive, positive, and positive refractive powers.

Each of the zoom lenses according to Embodiments 1, 2, and 3 described above may be applied to an optical apparatus such as an image pickup apparatus or an image projection apparatus.

FIG. 7 illustrates a three-plate type color liquid crystal projector using the zoom lens according to the present invention as a projection lens, which is an image projection apparatus in which multiple color beams from multiple liquid crystal display elements are combined by a color combining unit and enlarged and projected onto a screen by the projection lens. In a color liquid crystal projector 100 illustrated in FIG. 7 , respective light beams from three panels 5 R, 5 G, and 5 B for R (red light), G (green light), and B (blue light) are combined by a prism 200 serving as the color combining unit, to thereby form a single optical path. In FIG. 7 , the single prism 200 is illustrated as the color combining unit. However, the present invention is not limited to such a prism. Multiple prisms (dichroic prism or polarization beam splitter) may be used. A combined color beam obtained by the color combining unit is projected to a screen 400 by a zoom lens 300 .

An image pickup apparatus according to the present invention desirably includes: an image pickup element (photoelectric transducer) such as a CCD; and a zoom lens (which may be any of zoom lenses according to Embodiments 1, 2, and 3) for forming an image of a subject on a light incident surface of the image pickup element.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments.

This application claims the benefit of Japanese Patent Application No. 2008-316039, filed Dec. 11, 2008, which is hereby incorporated by reference herein in its entirety.