Wide angle lens and imaging apparatus

This application is a Continuation of PCT International Application No. PCT/JP2013/005292 filed on Sep. 6, 2013, which claims priority under 35 U.S.C. §119 (a) to Japanese Patent Application No. 2012-202227 filed on Sep. 14, 2012. The above application is hereby expressly incorporated by reference, in its entirety, into the present application.

The present invention relates to a wide angle lens and an imaging apparatus, and more specifically to a wide angle lens suitable for use with digital cameras and the like, and an imaging apparatus equipped with the wide angle lens.

Heretofore, in wide angle lenses for use with single-lens reflex cameras, a retrofocus type lens system in which a negative lens group and a positive lens group are disposed in order from the object side and is asymmetric with respect to the stop has been heavily used because a sufficient back focus needs to be ensured. As such a retrofocus type lens system, the lens system described, for example, in Japanese Unexamined Patent Publication No. 2004-219610 is known.

Technical Problem

Although, the retrofocus type lens system is suitable for ensuring the back focus, it causes difficulty in correcting aberrations related to the angle of view; in particular, satisfactory correction of lateral chromatic aberration is difficult, as it has an asymmetric structure with respect to the stop as described above. The lens system described in Japanese Unexamined Patent Publication No. 2004-219610 has relatively well corrected lateral chromatic aberration, but more satisfactorily corrected lateral chromatic aberration may sometimes be required when installing a digital back in place of film or the like, and the level of such requirements has recently been further raised.

The present invention has been developed in view of the circumstances described above, and it is an object of the present invention to provide a wide angle lens having excellent optical performance with well corrected lateral chromatic aberration, and an imaging apparatus equipped with the wide angle lens.

Solution to Problem

A wide angle lens of the present invention consists essentially of a front group, a stop, and a rear group having a positive refractive power, in order from the object side, in which a positive lens with a convex surface on the object side is disposed on the most object side in the front group, a negative meniscus lens with a convex surface on the object side is disposed second from the object side in the front group, and the wide angle lens satisfies conditional expressions (1) and (2) given below:

0.038<θ gFf− 0.6415+0.001618×ν df   (1)

ν df< 19  (2)

where

θgFf: partial dispersion ratio of the negative meniscus lens between the g-line and the F-line, and

νdf: Abbe number of the negative meniscus lens with respect to the d-line.

Preferably, in the wide angle lens of the present invention, the rear group includes one positive lens having a biconvex shape and the wide angle lens satisfies conditional expressions (3) and (4) given below:

0.020<θ gFr− 0.6415+0.001618×ν dr   (3)

ν dr> 75  (4)

where

θgFr: partial dispersion ratio of the positive lens having a biconvex shape between the g-line and the F-line, and

νdr: Abbe number of the positive lens having a biconvex shape with respect to the d-line.

Preferably, in the wide angle lens of the present invention, the front group includes a cemented lens and, of the cemented lens included in the front group, a cemented lens located closest to the stop includes a first positive lens having a positive refractive power and a second negative lens having a negative refractive power, and the wide angle lens satisfies conditional expressions (5) and (6) given below:

0.02< Ndp−Ndn< 0.25  (5)

0.2<ν dp−νdn< 5  (6)

where

Ndp: refractive index of the first positive lens with respect to the d-line,

Ndn: refractive index of the second negative lens with respect to the d-line,

νdp: Abbe number of the first positive lens with respect to the d-line, and

νdn: Abbe number of the second negative lens with respect to the d-line.

The wide angle lens of the present invention preferably satisfies a conditional expression (5′) given below and more preferably satisfies a conditional expression (5″) given below, instead of the conditional expression (5):

0.03< Ndp−Ndn< 0.12  (5′)

0.03< Ndp−Ndn< 0.07  (5″).

The wide angle lens of the present invention preferably satisfies a conditional expression (6′) given below and more preferably satisfies a conditional expression (6″) given below, instead of the conditional expression (6):

0.7<ν dp−νdn< 2.5  (6′)

0.7<ν dp−vdn< 1.5  (6″).

Preferably, in the wide angle lens of the present invention, the front group includes a cemented lens and, of the cemented lens included in the front group, a cemented lens located closest to the stop includes a first positive lens having a positive refractive power and a second negative lens having a negative refractive power, and the wide angle lens satisfies a conditional expression (7) given below:

ν dp< 33  (7)

where

νdp: Abbe number of the first positive lens with respect to the d-line.

The wide angle lens of the present invention preferably satisfies a conditional expression (7′) given below and more preferably satisfies a conditional expression (7″) given below, instead of the conditional expression (7):

ν dp< 30  (7′)

ν dp< 27  (7″).

Preferably, in the wide angle lens of the present invention, the front group includes a cemented lens and, of the cemented lens included in the front group, a cemented lens located closest to the stop has a configuration in which a positive lens and a negative lens are cemented in this order from the object side.

Preferably, in the wide angle lens of the present invention, the front group includes a cemented lens and, of the cemented lens included in the front group, a cemented lens located closest to the stop has a configuration in which a positive lens and a negative lens are cemented and the cemented surface of the positive lens and the negative lens has a concave shape on the object side.

The wide angle lens of the present invention preferably has a total angle of view of 80 degrees or more.

An imaging apparatus of the present invention is equipped with the wide angle lens of the present invention.

The foregoing “essentially” in the context of “consists essentially of” intends that the wide angle lens may include a lens having substantially no refractive power, an optical element other than a lens, such as a stop, a cover glass, a filter, and the like, a lens flange, a lens barrel, an image sensor, and a mechanical component, such as a camera shake correction mechanism, in addition to the constituent elements described above.

The partial dispersion ratio θgFf of a lens between the g-line and the F-line is defined, when the refractive indices of the lens with respect to the g-line, the F-line, and the C-line are taken as Ng, NF, and NC respectively, as θgFf=(Ng−Nf)/(NF−NC).

The signs of refractive powers of the lenses and the lens groups, and surface shapes of the lenses of the wide angel lens of the present invention are considered in the paraxial region for those having an aspherical surface.

Advantageous Effects of Invention

According to the present invention, the configuration of the first and the second lenses from the object side in the front group are set appropriately, in particular, the material of the second lens from the object side in the front group is set appropriately. This allows a wide angle lens having high optical performance with well corrected lateral chromatic aberration, while ensuring a wide angle of view, and an imaging apparatus equipped with the wide angle lens may be provided.

FIG. 1 is a cross-sectional view of a wide angle lens of Example 1 of the present invention, illustrating the configuration thereof.

FIG. 2 is a cross-sectional view of a wide angle lens of Example 2 of the present invention, illustrating the configuration thereof.

FIG. 3 is a cross-sectional view of a wide angle lens of Example 3 of the present invention, illustrating the configuration thereof.

A to D of FIG. 4 illustrate each aberration diagram of the wide angle lens of Example 1.

A to D of FIG. 5 illustrate each aberration diagram of the wide angle lens of Example 2.

A to D of FIG. 6 illustrate each aberration diagram of the wide angle lens of Example 3.

FIG. 7 is a perspective view of an imaging apparatus according to an embodiment of the present invention, illustrating the configuration thereof.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. FIGS. 1 to 3 are cross-sectional views of wide angle lenses according to embodiments of the present invention, illustrating configurations thereof and each corresponding to each of Examples 1 to 3 to be described later. In FIGS. 1 to 3 , the left side is the object side and the right side is the image side, and an axial light beam 2 and a maximum image height light beam 3 from an object at infinity are also illustrated. As the basic configurations and illustration methods of the examples shown in FIGS. 1 to 3 are identical, a description will be made, hereinafter, with reference mainly to the example illustrated in FIG. 1 , as a representative.

The wide angle lens according to an embodiment of the present invention consists essentially of a front group GF, an aperture stop St, and a rear group GR having a positive refractive power. Note that the aperture stop St shown in FIGS. 1 to 3 does not necessarily indicate the size or shape but the position on the optical axis Z.

As it is conceivable that, when the wide angle lens is installed in an imaging apparatus, a cover glass for protecting the image sensor, a various types of filters, such as a low-pass filter, an infrared cut filter, and the like are provided according to the specifications of the imaging apparatus, FIG. 1 illustrates an example in which a parallel plate optical member PP assuming these is disposed between the most image side lens surface and the image plane Sim. But the position of the optical member PP is not limited to that shown in FIG. 1 and a configuration without the optical member PP is also possible.

The lens configuration of each lens group of the example shown in FIG. 1 is as follows. That is, the front group GF is composed of a lens L 1 which is a positive meniscus lens with a convex surface on the object side, lenses L 2 , L 3 , and L 4 which are three negative meniscus lenses with convex surfaces on the object side, a positive lens L 5 , a negative lens L 6 , a lens L 7 which is a biconvex lens, and a lens L 8 which is a negative meniscus lens with a concave surface on the object side, in order from the object side. The rear group GR is composed of a negative lens L 9 , a lens L 10 which is a biconvex lens, a lens L 11 which is a negative meniscus lens with a convex surface on the object side, a lens L 12 which is a biconvex lens, a lens L 13 which is a negative meniscus lens with a convex surface on the image side, and a lens L 14 which is a positive meniscus lens with a convex surface on the image side, in order from the object side. The lenses L 7 and L 8 are cemented to form a cemented lens. The lenses L 11 , L 12 , and L 13 are cemented to form a three element cemented lens. The other lenses are uncemented single lenses. All lenses are spherical lenses.

The wide angle lens of the present embodiment is configured such that the lenses L 1 and L 2 disposed first and second from the object side in the front group GF are a positive lens with a convex surface on the object side and a negative meniscus lens with a convex surface on the object side. Such a combination of the first and the second lenses from the object side in the entire system allows distortion to be corrected satisfactorily while reducing the diameter of the lens disposed on the most object side in the entire system.

Further, the wide angle lens of the present embodiment is configured so as to satisfy conditional expressions (1) and (2) given below with respect to the lens L 2 disposed second from the object side in the front group G2:

0.038<θ gFf− 0.6415+0.001618×ν df   (1)

ν df< 19  (2)

where

θgFf: partial dispersion ratio of the lens located second from the object side in the front group between the g-line and the F-line, and

νdf: Abbe number of the lens located second from the object side in the front group with respect to the d-line.

Satisfaction of the conditional expression (1) allows satisfactory correction of higher order chromatic aberrations to be made easily. Satisfaction of the conditional expression (2) allows satisfactory correction of lateral chromatic aberration to be made easily. Simultaneous satisfaction of the conditional expressions (1) and (2) allows the correction of lateral chromatic aberration, mainly the correction of secondary spectrum of chromatic aberration, to be made easily. If either one of the conditional expressions (1) and (2) is not satisfied, the g-line tends to be over in lateral chromatic aberration.

Further, the wide angle lens of the present embodiment preferably includes one positive lens having a biconcave shape in the rear group GR and satisfies conditional expressions (3) and (4) given below with respect to the one positive lens having a biconcave shape. If the rear group GR includes two or more positive lenses having a biconcave shape, the following conditional expressions (3) and (4) are considered with respect to the same lens.

0.020<θ gFr− 0.6415+0.001618×ν dr   (3)

ν dr> 75  (4)

where

θgFr: partial dispersion ratio of the positive lens having a biconvex shape, included in the rear group, between the g-line and the F-line, and

νdr: Abbe number of the positive lens having a biconvex shape, included in the rear group, with respect to the d-line.

Simultaneous satisfaction of the conditional expressions (3) and (4) allows the correction of lateral chromatic aberration, mainly the correction of secondary spectrum of chromatic aberration, to be made easily. If either one of the conditional expressions (3) and (4) is not satisfied, the g-line tends to be over in lateral chromatic aberration.

For example, a cemented lens which includes a biconvex lens and a negative lens may be disposed in the rear group GR and the biconvex lens included in the cemented lens may be formed to satisfy the conditional expressions (3) and (4). Such arrangement allows the correction of the secondary spectrum of lateral chromatic aberration to be made more easily. In this case, for more satisfactory correction of lateral chromatic aberration, the cemented lens which includes the biconvex lens that satisfies the conditional expressions (3) and (4) is preferably a three element cemented lens and the three element cemented lens is more preferably composed of a negative lens, the biconvex lens, and a negative lens cemented together.

The wide angle lens of the present embodiment preferably includes a cemented lens in the front group GF and, of the cemented lens included in the front group GF, a cemented lens located closest to the aperture stop St preferably includes one positive lens and one negative lens. For example, in the example shown in FIG. 1 , the cemented lens located closest to the aperture stop St in the front group GF is the cemented lens in which the positive lens L 7 and the negative lens L 8 are cemented.

Then, when the one positive lens and the one negative lens included in the cemented lens located closest to the aperture stop in the front group GF are taken as first positive lens and second negative lens, conditional expressions (5) and (6) given blow are preferably satisfied with respect to these. If the cemented lens located closest to the aperture stop St in the front Group GF includes two or more positive lenses, the following conditional expressions (5) and (6) are considered with respect to the same positive lens. Likewise, if the foregoing cemented lens includes two or more negative lenses, the following conditional expressions (5) and (6) are considered with respect to the same negative lens.

0.02< Ndp−Ndn< 0.25  (5)

0.2<ν dp−vdn< 5  (6)

where

Ndp: refractive index of the first positive lens with respect to the d-line,

Ndn: refractive index of the second negative lens with respect to the d-line,

νdp: Abbe number of the first positive lens with respect to the d-line, and

νdn: Abbe number of the second negative lens with respect to the d-line.

Satisfaction of the conditional expressions (5) and (6) allows spherical aberration and field curvature to be corrected satisfactorily with respect to each color. If either one of the lower limits of the conditional expressions (5) and (6) is not satisfied, the chromatic aberration correction effect of the cemented surface of the cemented lens is reduced, whereby spherical aberration with respect to each color is not aligned and the field curvature of the g-line tends to be under. If either one of the upper limits of the conditional expressions (5) and (6) is not satisfied, the amount of aberrations generated at the cemented surface of the foregoing cemented lens is increased and the refractive power of the cemented surface needs to be reduced for suppressing the aberrations. Then, the chromatic aberration correction effect of the cemented surface of the cemented lens is reduced, whereby spherical aberration with respect to each color is not aligned and the field curvature of the g-line tends to be under.

Further, with respect to the foregoing νdp, the wide angle lens of the present embodiment preferably satisfies a conditional expression (7) given below:

ν dp< 33  (7)

Satisfaction of the conditional expression (7) allows a well-balanced correction of both longitudinal chromatic aberration and lateral chromatic aberration to be made easily. In a case in which the conditional expression (7) is not satisfied, an attempt to satisfactorily correct longitudinal chromatic aberration will result in that the g-line tends to be over in lateral chromatic aberration and the well-balanced correction of both longitudinal chromatic aberration and lateral chromatic aberration is difficult.

Simultaneous satisfaction of the conditional expressions (5), (6), and (7) allows spherical aberration and field curvature to be corrected more satisfactorily with respect to each color.

For more satisfactory aberration correction, the wide angle lens of the present embodiment preferably satisfies a conditional expression (5′) given below and more preferably satisfies a conditional expression (5″) given below, instead of the conditional expression (5):

0.03< Ndp−Ndn< 0.12  (5′)

0.03< Ndp−Ndn< 0.07  (5″).

For more satisfactory aberration correction, the wide angle lens of the present embodiment preferably satisfies a conditional expression (6′) given below and more preferably satisfies a conditional expression (6″) given below, instead of the conditional expression (6):

0.7<ν dp−νdn< 2.5  (6′)

0.7<ν dp−νdn< 1.5  (6″).

For more satisfactory aberration correction, the wide angle lens of the present embodiment preferably satisfies a conditional expression (7′) given below and more preferably satisfies a conditional expression (7″) given below, instead of the conditional expression (7):

ν dp< 30  (7′)

ν dp< 27  (7″).

In the wide angle lens of the present embodiment, of the cemented lens included in the front group GF, a cemented lens located closest to the aperture stop St preferably has a configuration in which a positive lens and a negative lens are cemented in this order from the object side. Such configuration allows satisfactory correction of field curvature with respect to each color, in particular, the correction of field curvature of the g-line. This has no relation to the number of lenses constituting the cemented lens. In the example shown in FIG. 1 , the cemented lens in the front group GF is composed of two lenses, the positive lens L 7 and the negative lens L 8 . But, even if a three element configuration is implemented, in which the negative lens L 6 , the positive lens L 7 , and the negative lens L 8 are cemented, the aforementioned advantageous effect of satisfactory correction of field curvature with respect to each color, in particular, the correction of field curvature of the g-line may be obtained.

In the case in which, of the cemented lens included in the front group GF, the cemented lens located closest to the aperture stop St has a configuration in which a positive lens and a negative lens are cemented, the cemented surface between the positive lens and the negative lens preferably has a concave shape on the object side. Formation of the cemented surface in a concave shape on the object side allows the correction of field curvature with respect to each color, in particular, the correction of field curvature of the g-line to be made more satisfactorily.

The wide angle lens of the present embodiment preferably has a total angle of view of 80 degrees or more for realizing an increased angle of view.

The foregoing preferable configurations may be combined in any manner and are preferably employed selectively, as appropriate, according to the specifications required of the wide angle lens. Appropriate employment of the preferable configurations allows an optical system having more favorable optical performance or an optical system that complies with higher specifications to be realized. The wide angle lens of the present embodiment is suitably applicable, for example, to an optical system that has a total angle of view of about 80 degrees or more and an F-number of about 4.8, and requires satisfactory correction of lateral chromatic aberration.

Next, numerical examples of the wide angle lens of the present invention will be described.