Zoom lens

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2010-040456, filed on Feb. 25, 2010, the entire contents of which are incorporated herein by reference.

1. Field of the Invention

The present invention relates to a zoom lens ideal for video cameras and in particular, surveillance cameras.

2. Description of the Related Art

Conventionally, surveillance cameras, such as those for closed circuit television (CCTV) have been used to monitor unmanned facilities. Surveillance cameras capture images during the day using visible light and at night using near-infrared light. Therefore, a lens system that can be used day or night, i.e., a lens system that can handle both visible and near-infrared light is demanded for surveillance cameras.

Typically, in a lens system designed for the visible light range, chromatic aberration occurs in the near-infrared light range and images captured at night using near-infrared light are out of focus. Thus, a lens system that can correct chromatic aberration over a wide spectrum (from the visible light range to the near-infrared light range) such that the focal points of the spectrum become uniform, is preferable for use in a surveillance camera. A lens that is capable of magnification, is compact, and has a large focal ratio and high optical performance is yet more preferable.

Conventionally, zoom lenses have been proposed that are capable of handling light in the visible range to the near-infrared range and are mountable to a surveillance camera (see, for example, Japanese Patent Application Laid-Open Publication No. 2005-134887). The zoom lens disclosed in Japanese Patent Application Laid-Open Publication No. 2005-134887 includes sequentially from an object side, a first lens group having a negative refractive power, a diaphragm, and a second lens group having a positive refractive power. The first lens group includes sequentially from the object side, 2 negative meniscus lenses, and a cemented lens that includes a biconcave lens and a positive lens. Further, the second lens group includes 2 simple lenses disposed farthest on the object side.

Conventionally, in addition to being able to handle wavelengths over a wide spectrum, ranging from the visible light range to the near-infrared light range, a high focal ratio enabling sharp images to be captured even in dimly lit places is also demanded of lens systems for surveillance cameras. Further, with rapid advances in increasing the pixels of imaging elements (CCD, CMOS, etc.), lens systems capable of capturing even finer details of an object, i.e., megapixel lens systems, have also come to be demanded. Therefore, in particular, a megapixel lens system for a surveillance camera is demanded that over the entire zoom range, can favorably correct various types of aberration with respect to light in the visible range to the near-infrared range and that further has extremely high optical performance.

However, with conventional arts, including the lens system disclosed in Japanese Patent Application Laid-Open Publication No 2005-134887, it is difficult to maintain high optical performance while satisfying demand for a higher focal ratio and a smaller size. In other words, if a higher focal ratio and a smaller size are achieved, the correction of various types of aberration occurring with respect to light in the visible range to the near-infrared range becomes difficult, arising in a problem that pixels cannot be increased on the megapixel order.

To solve the problems associated with the conventional arts above, an object of the present invention is to provide a megapixel zoom lens that over the entire zoom range, favorably corrects various types of aberration occurring with respect to light in the visible range to the near-infrared range.

It is an object of the present invention to at least solve the above problems in the conventional technologies.

A zoom lens according to one aspect of the present invention includes sequentially from an object side a first lens group having a negative refractive power; a diaphragm; and a second lens group having a positive refractive power. Zoom from a wide angle edge to a telephoto edge is performed by displacement of the second lens group along an optical axis, toward the object side. Correction of imaging plane variation accompanying the zoom, is performed by displacement of the first lens group along the optical axis. The second lens group includes sequentially from the object side, a positive first lens having at least on aspheric surface and a positive second lens. Furthermore, a first condition υd 21 >63 and a second condition υd 22 >70 are satisfied, υd 21 being the Abbe number for a d-line in the first lens of the second lens group and υd 22 being the Abbe number for a d-line in the second lens of the second lens group.

The other objects, features, and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings.

FIG. 1 depicts a cross-sectional view (along an optical axis) of a zoom lens according to a first example;

FIG. 2 is a diagram of various types of aberration at a wide angle edge of the zoom lens according to the first example;

FIG. 3 is a diagram of various types of aberration at a telephoto edge of the zoom lens according to the first example;

FIG. 4 depicts a cross-sectional view (along the optical axis) of the zoom lens according to a second example;

FIG. 5 is a diagram of various types of aberration at the wide angle edge of the zoom lens according to the second example;

FIG. 6 is a diagram of various types of aberration at the telephoto edge of the zoom lens according to the second example;

FIG. 7 depicts a cross-sectional view (along the optical axis) of the zoom lens according to a third example;

FIG. 8 is a diagram of various types of aberration at the wide angle edge of the zoom lens according to the third example; and

FIG. 9 is a diagram of various types of aberration at the telephoto edge of the zoom lens according to the third example.

Referring to the accompanying drawings, exemplary embodiments according to the present invention are explained in detail below.

A zoom lens according to an embodiment includes sequentially from an object side, a first lens group having a negative refractive power, a diaphragm, and a second lens group having a positive refractive power. The zoom lens zooms from a wide angle edge to a telephoto edge by moving the second lens group along an optical axis, toward the object side; and corrects imaging plane (image location) variations accompanying zoom, by moving the first lens group along the optical axis.

An object of the present invention is to provide a zoom lens that achieves a higher focal ratio and a smaller size and that is further able to favorably correct, over the entire zoom range, various types of aberration occurring with respect to light in the visible range to the near-infrared range.

The second lens group includes from the object side, a positive first lens having at least 1 aspheric surface, and a positive second lens. By forming an aspheric surface on the first lens disposed farthest on the object side in the second lens group, spherical aberration occurring with a higher focal ratio can be favorably corrected.

In addition, the following conditional expressions are preferably satisfied, where υd 21 is the Abbe number for the d-line in the first lens of the second lens group, and υd 22 is the Abbe number for the d-line in the second lens of the second lens group.

υ d 21 >63  (1)

υ d 22 >70  (2)

Conditional expressions (1), (2) prescribe conditions to favorably correct, over the entire zoom range, chromatic aberration occurring with light in the visible range to the near-infrared range. Use of a low-dispersion material satisfying conditional expressions (1), (2) to form the first lens and the second lens of the second lens group, enables chromatic aberration occurring with light in the visible range to the near-infrared range to be favorably corrected over the entire zoom range. Below the lower limits of conditional expressions (1) and (2), axial chromatic aberration becomes difficult to correct, whereby chromatic aberration occurring with light in the visible range to the near-infrared range cannot be sufficiently corrected.

Further, the following conditional expression is satisfied, where D 2 is the displacement of the second lens group during zoom from a wide angle edge to a telephoto edge and Z is the zoom ratio.

2.25 <D 2 /Z< 2.55  (3)

Conditional expression (3) prescribes an appropriate stroke range for the second lens group during zoom. By satisfying conditional expression (3), high optical performance can be maintained while achieving a reduction in the size of the optical system. Above the upper limit of conditional expression (3), the displacement of the second lens group increases, making it difficult to reduce the size of the optical system. On the other hand, although advantageous in terms of size reduction, below the lower limit of conditional expression (3), the correction of spherical aberration and coma aberration, in particular at the wide angle edge, becomes difficult, arising in a problem of deteriorated optical performance.

Furthermore, in the zoom lens according to the embodiment, the second lens group includes on the image plane side of the second lens and sequentially from the object side, a cemented lens that includes a positive third lens and a negative fourth lens, and a positive fifth lens.

Further, in the zoom lens according to the embodiment, the following conditional expression is preferably satisfied, where υd 2 3 is the Abbe number for the d-line in the third lens of the second lens group.

υ d 23 >70  (4)

Similar to conditional expressions (1) and (2), conditional expression (4) also prescribes a condition to favorably correct, over the entire zoom range, chromatic aberration occurring with light in the visible range to the near-infrared range. Use of a low-dispersion material satisfying conditional expression (4) to form the third lens of the second lens group, enables chromatic aberration occurring with light in the visible range to the near-infrared range to be even more favorably corrected over the entire zoom range. Below the lower limit of conditional expression (4), the correction of axial chromatic aberration in the third lens becomes difficult.

Furthermore, in the zoom lens according to the embodiment, the first lens group includes sequentially from the object side, 3 lenses constituting 3 groups, including a first lens that is a negative meniscus lens having a convex surface facing toward the object side, a second lens that is a negative biconcave lens, and a positive third lens. Thus, farthest on the object side of the optical system, a negative meniscus lens having a convex surface facing toward the object side can be disposed which is advantageous in increasing the field of view.

In addition, the following conditional expression is preferably satisfied, where υd 13 is the Abbe number for the d-line in the third lens of the first lens group.

υ d 13 <20  (5)

Conditional expression (5) prescribes a condition that enables chromatic aberration occurring in the first lens group to be corrected by the first lens group. In other words, by satisfying conditional expression (5), the third lens, which is a positive lens, causes aberration of the same magnitude and in the opposite direction of the axial chromatic aberration and chromatic difference of magnification caused by the negative lens, whereby the first lens group is able to correct chromatic aberration that occurs. Above the upper limit of conditional expression (5), chromatic aberration of a magnitude necessary for correction cannot occur at the third lens, whereby chromatic aberration occurring in the first lens group increases.

As described, the zoom lens according to the embodiment, by satisfying the conditional expressions above, in addition to achieving a higher focal ratio a reduction in size, is able to correct extremely favorably, over the entire zoom range, various types of aberration occurring with light in the visible range to the near-infrared range. Consequently, the lens is ideal for video cameras, such as surveillance cameras, equipped with a megapixel imaging element. By simultaneously satisfying plural conditional expressions, even better optical performance can be achieved, i.e., surpassing that when only conditional expression is satisfied.

FIG. 1 depicts a cross-sectional view (along the optical axis) of the zoom lens according to a first example. The zoom lens includes sequentially from an object (non-depicted) side, a first lens group G 11 having a negative refractive power, a diaphragm STP, and a second lens group G 12 having a positive refractive power. Between the second lens group G 12 and an imaging plane IMG, a cover glass CG of an imaging element is disposed. The cover glass CG is disposed as needed and may be omitted when not necessary. Further, at the imaging plane IMG, the light receiving surface of the imaging element, e.g., CCD and CMOS, is disposed.

The first lens group G 11 includes sequentially from the object side, a first lens L 111 , a second lens L 112 , and a third lens L 113 . The first lens L 111 is a negative meniscus lens having a convex surface facing toward the object side. The second lens L 112 is a negative biconcave lens. The third lens L 113 is a positive lens.

The second lens group G 12 includes sequentially from the object side, a first lens L 121 , a second lens L 122 , a third lens L 123 , a fourth lens L 124 , and a fifth lens L 125 . The first lens L 121 is a positive lens, both surfaces of which are aspheric. The second lens L 122 is a positive lens. The third lens L 123 is a positive lens. The fourth lens L 124 is a negative lens. The third lens L 123 and the fourth lens L 124 are cemented. The fifth lens L 125 is a positive lens.

The zoom lens zooms from a wide angle edge to a telephoto edge by moving the second lens group G 12 along the optical axis, toward the object side; and corrects imaging plane (image location) variations accompanying zoom, by moving the first lens group G 11 along the optical axis.

Various values related to the zoom lens according to the first example are indicated below.

FIG. 2 is a diagram of various types of aberration at the wide angle edge of the zoom lens according to the first example; FIG. 3 is a diagram of various types of aberration at the telephoto edge of the zoom lens according to the first example. In the diagrams, d-line indicates aberration for a wavelength equivalent to 587.56 nm; and ΔS and ΔM in a portion depicting astigmatism, indicate aberration with respect to a sagittal image plane and a meridional image plane, respectively.

FIG. 4 depicts a cross-sectional view (along the optical axis) of the zoom lens according to a second example. The zoom lens includes sequentially from the object (non-depicted) side, a first lens group G 21 having a negative refractive power, a diaphragm STP, and a second lens group G 22 having a positive refractive power. Between the second lens group G 22 and the imaging plane IMG, a cover glass CG of an imaging element is disposed. The cover glass CG is disposed as needed and may be omitted when not necessary. Further, at the imaging plane IMG, the light receiving surface of the imaging element, e.g., CCD and CMOS, is disposed.

The first lens group G 21 includes sequentially from the object side, a first lens L 211 , a second lens L 212 , and a third lens L 213 . The first lens L 211 is a negative meniscus lens having a convex surface facing toward the object side. The second lens L 212 is a negative biconcave lens. The third lens L 213 is a positive lens.

The second lens group G 22 includes sequentially from the object side, a first lens L 221 , a second lens L 222 , a third lens L 223 , a fourth lens L 224 , and a fifth lens L 225 . The first lens L 221 is a positive lens, both surfaces of which are aspheric. The second lens L 222 is positive lens. The third lens L 223 is a positive lens. The fourth lens L 224 is a negative lens. The third lens L 223 and the fourth lens L 224 are cemented. Furthermore, the fifth lens L 225 is a positive lens.

The zoom lens zooms from a wide angle edge to a telephoto edge by moving the second lens group G 22 along the optical axis, toward the object side; and corrects imaging plane (image location) variations accompanying zoom, by moving the first lens group G 21 along the optical axis.

Various values related to the zoom lens according to the second example are indicated below.

FIG. 5 is a diagram of various types of aberration at the wide angle edge of the zoom lens according to the second example; FIG. 6 is a diagram of various types of aberration at the telephoto edge of the zoom lens according to the second example. In the diagrams, d-line indicates aberration for a wavelength equivalent to 587.56 nm; and ΔS and ΔM in a portion depicting astigmatism, indicate aberration with respect to a sagittal image plane and a meridional image plane, respectively.

FIG. 7 depicts a cross-sectional view (along the optical axis) of the zoom lens according to a third example. The zoom lens includes sequentially from the object (non-depicted) side, a first lens group G 31 having a negative refractive power, a diaphragm STP, and a second lens group G 32 having a positive refractive power. Between the second lens group G 32 and the imaging plane IMG, a cover glass CG of an imaging element is disposed. The cover glass CG is disposed as needed and may be omitted when not necessary. Further, at the imaging plane IMG, the light receiving surface of the imaging element, e.g., CCD and CMOS, is disposed.

The first lens group G 31 includes sequentially from the object side, a first lens L 311 , a second lens L 312 , and a third lens L 313 . The first lens L 311 is a negative meniscus lens having a convex surface facing toward the object side. The second lens L 312 is a negative biconcave lens. The third lens L 313 is a positive lens.

The second lens group G 32 includes sequentially from the object side, a first lens L 321 , a second lens L 322 , a third lens L 323 , a fourth lens L 324 , and a fifth lens L 325 . The first lens L 321 is a positive lens, both surfaces of which are aspheric. The second lens L 322 is a positive lens. The third lens L 323 is a positive lens. The fourth lens L 324 is a negative lens. The third lens L 323 and the fourth lens L 324 are cemented. The fifth lens L 325 is a positive lens.

The zoom lens zooms from a wide angle edge to a telephoto edge by moving the second lens group G 32 along the optical axis, toward the object side; and corrects imaging plane (image location) variations accompanying zoom, by moving the first lens group G 31 along the optical axis.

Various values related to the zoom lens according to the third example are indicated below.

FIG. 8 is a diagram of various types of aberration at the wide angle edge of the zoom lens according to the third example; FIG. 9 is a diagram of various types of aberration at the telephoto edge of the zoom lens according to the third example. In the diagrams, d-line indicates aberration for a wavelength equivalent to 587.56 nm; and ΔS and ΔM in a portion depicting astigmatism, indicate aberration with respect to a sagittal image plane and a meridional image plane, respectively.

Among the values for each of the examples above, r 2 , . . . indicate radii of curvature for each lens, aperture stop surface, etc.; d 1 , d 2 , . . . indicate the thickness of the lenses, aperture stop, etc. or the distance between surfaces thereof; nd 1 , nd 2 , . . . indicates the refraction index of each lens with respect to the d-line (λ=587.56 nm); and υd 1 , υd 2 , . . . indicates the Abbe number with respect to the d-line (λ=587.56 nm) of each lens.

Each of the aspheric surfaces above can be expressed by equation [1], where Z is the distance along a direction of the optical axis from the apex of the lens surface, y is the height in a direction normal to the optical axis, and the travel direction of light is positive.

Where, R is paraxial radii of curvature; K is constant of the cone; and A, B, C, D are the fourth, sixth, eighth, and tenth aspheric coefficients, respectively.

As described above, the zoom lens according to each of the examples above satisfies each of the conditional expressions, whereby the zoom lens achieves a high focal ratio and a smaller size while being able to favorably correct, over the entire zoom range, various types of aberration occurring with light in the visible range to the near-infrared range. Furthermore, the zoom lens according to each example employs a lens having an appropriately shape aspheric surface, whereby favorable optical performance can be maintained with fewer lenses.

Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.