JP2006072188A - Imaging lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an imaging lens capable of demonstrating good aberration performance, even though the lens is simply constituted. <P>SOLUTION: The imaging lens comprises, in order from an object side, a front group G1 having negative refractive power, a diaphragm, and a rear group G2 having positive refractive power. The front group G1 comprises, in order from the object side, a 1st positive lens L1 having a convex face on the object side, a 2nd negative meniscus lens L2 having a convex face on the object side, a 3rd biconcave lens L3 and a 4th biconvex lens L4. The rear group G2 is provided with, in order from the object side, a 5th positive lens L5, a 1st doublet L67 constituted of a 6th positive lens L6 and a 7th negative lens L7, and a 2nd doublet L89 constituted of an 8th positive lens L8 and a 9th negative lens L9. Further, focusing is performed on an imaging surface Simg by shifting only the rear group G2 in the optical axis direction. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a fixed-focus imaging lens suitable for an image processing camera, a surveillance camera, and the like.

  Conventionally, television cameras equipped with an image sensor such as a CCD (Charge Coupled Device) have become widespread. As such an imaging lens system in a television camera, a retrofocus imaging lens (hereinafter referred to as a retrofocus lens) that generally has a wide angle of view and easily ensures a sufficiently long back focus with respect to the focal length is adopted. Has been. This is because a space for arranging other optical components such as a low-pass filter and an infrared cut filter is required between the imaging lens system and the imaging element. In this retrofocus lens, a negative group (front group) and a positive group (rear group) are arranged in order from the object side so as to sandwich the stop, and have a configuration that is asymmetrical with respect to the stop.

As such a retrofocus type lens, there is a lens in which a positive lens is arranged on the most object side so as to suppress a negative distortion due to a wide angle of view (for example, see Patent Document 1). .
JP-A-7-181376

The retrofocus lens of Patent Document 1 performs focusing by extending the entire lens. For this reason, the total length of the lens system at the time of extension is increased, and the burden on the lens driving unit is increased. Therefore, as a solution to such a problem, a retrofocus lens having a two-group seven-element configuration that performs focusing by extending only the rear group of the lens system is disclosed (for example, see Patent Document 2). According to this, it is possible to perform focusing while keeping the entire length of the lens system constant.
JP-A-8-166537

  By the way, in recent years, the image quality of television cameras has been improved due to the development of an image sensor having a larger number of pixels. In parallel with this, downsizing of the entire camera is also progressing. Due to such a background, there is an increasing demand for improvement in optical performance and miniaturization of an imaging lens mounted on a television camera. Especially when mounted on an image processing camera used in applications such as FA (Factory Automation), extremely high imaging performance is required, so various aberrations such as spherical aberration, astigmatism, and distortion are extremely good. It is necessary to correct it.

  However, the retrofocus lens described in Patent Document 2 has large aberrations, particularly distortion, and it is expected that it will be difficult to cope with further higher pixels in the future.

  The present invention has been made in view of such a problem, and an object thereof is to provide an imaging lens capable of exhibiting good aberration performance while having a compact configuration.

  The imaging lens according to the present invention includes, in order from the object side, a front group having a negative refractive power, a stop, and a rear group having a positive refractive power. Here, in order from the object side, the front group includes a positive first lens having a convex surface facing the object side, a negative second lens having a meniscus shape having a convex surface facing the object side, and a biconcave shape. 3 lenses and a biconvex fourth lens. One rear group, in order from the object side, includes a positive fifth lens, a first cemented lens including a positive sixth lens and a negative seventh lens, a positive eighth lens, and a negative ninth lens. And a second cemented lens. Further, this imaging lens performs focusing by moving only the rear group in the optical axis direction with respect to the imaging plane.

  In the imaging lens according to the present invention, a sufficient back focus can be obtained by including, in order from the object side, a front group having a negative refractive power, a stop, and a rear group having a positive refractive power. In addition, by adopting a configuration in which only the rear group is extended with respect to the imaging surface, the total length is shortened, and a two-group nine-lens configuration is used, so that aberration is better than in the conventional seven-sheet configuration. It is corrected. In the front group, positive distortion is generated by the first lens having positive refractive power, and this and the negative distortion generated in the subsequent second lens and third lens cancel each other. Become. Furthermore, the negative refractive power is distributed between the image-side surface of the second lens and both surfaces of the third lens, so that the occurrence of negative distortion itself is suppressed. On the other hand, in the rear group, the positive refractive power is distributed by the fifth, sixth, and eighth lenses to suppress the occurrence of spherical aberration, and the presence of the first and second cemented lenses causes chromatic aberration ( In particular, the occurrence of axial chromatic aberration) is suppressed.

It is desirable that the imaging lens of the present invention is further configured to satisfy the following conditional expressions (1) to (3).
4.0 <f1 / f <6.6 (1)
−7.3 <fF / f <−4.4 (2)
1.23 <fR / f <1.36 (3)
Here, f1 is the focal length of the first lens, fF is the focal length of the front group, fR is the focal length of the rear group, and f is the focal length of the entire system.

  According to the imaging lens of the present invention, in order from the object side, the front group having a negative refractive power, a stop, and a rear group having a positive refractive power, the front group is arranged in order from the object side to the object side. A positive first lens with a convex surface facing the surface, a negative second lens with a meniscus shape with a convex surface facing the object side, a third lens with a biconcave shape, and a fourth lens with a biconvex shape, The rear group includes, in order from the object side, a first cemented lens including a positive fifth lens, a positive sixth lens, and a negative seventh lens, a second lens including a positive seventh lens, and a negative eighth lens. Focusing is performed by moving only the rear group in the direction of the optical axis with respect to the image plane, so that focusing can be performed with the entire length being constant, and compact. Suitable for mounting on image processing cameras, etc. while ensuring the configuration It can be exhibited very good aberration performance.

  Specifically, in the front group, positive distortion is generated by arranging a positive first lens, and this positive distortion and negative distortion generated in the subsequent second lens and third lens are generated. Can be canceled each other. Furthermore, since the second lens and the third lens distribute negative refractive power, the occurrence of negative distortion itself can be suppressed. On the other hand, in the rear group, since the fifth, sixth and eighth lenses distribute the positive refractive power, it is possible to suppress the occurrence of spherical aberration and to provide chromatic aberration by providing the first and second cemented lenses. The occurrence of (especially longitudinal chromatic aberration) can also be suppressed.

  Furthermore, by satisfying conditional expressions (1) to (3), it is possible to maintain a good balance between correction of various aberrations and securing of the back focus.

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

  FIG. 1 shows a configuration example of an imaging lens according to an embodiment of the present invention. This configuration example corresponds to the lens configuration of a first numerical example (FIG. 3) described later. FIG. 2 shows another configuration example of the imaging lens according to the present embodiment. The configuration example of FIG. 2 corresponds to the lens configuration of a second numerical example (FIG. 4) described later. Since the basic configuration is the same in each configuration example, the following description is based on the configuration of the imaging lens shown in FIG. The imaging lens of the present embodiment is particularly suitable for an image processing camera provided with a high pixel imaging device. In FIG. 1, the side indicated by the symbol Zobj is the object side (subject side), and the side indicated by the symbol Zimg is the image plane side (imaging side). The reference symbol Ri (R1 to R19) denotes the i-th component that sequentially increases toward the image plane side, with the surface of the most object-side component including the aperture stop (hereinafter simply referred to as the stop) St as the first. The curvature radius of the surface Si (i = 1 to 19) is shown. Here, the symbol S9 corresponds to the aperture stop St. Reference symbol Di (D1 to D19) indicates a surface interval on the optical axis Z1 between the i-th surface Si and the i + 1-th surface Si + 1.

  This imaging lens includes, in order from the object side along the optical axis Z1, a front group G1 having a negative refractive power as a whole, a stop St, and a rear group G2 having a positive refractive power as a whole. . An imaging element (not shown) such as a CCD is disposed on the imaging surface Simg of the imaging lens, and a parallel flat plate GC such as a cover glass is disposed between the imaging element and the rear group G2. Yes. In this imaging lens, focusing is performed by moving only the rear group G2 in the direction of the optical axis Z1 with respect to the imaging plane Simg.

  The front group G1 includes, in order from the object side, a positive first lens L1 having a convex surface facing the object side, a negative second lens L2 having a meniscus shape having a convex surface facing the object side, and a biconcave first shape. It has three lenses L3 and a fourth lens L4 having a biconvex shape.

  On the other hand, the rear group G2, in order from the object side, includes a positive fifth lens L5, a first cemented lens L67 including a positive sixth lens L6 and a negative seventh lens L7, a positive eighth lens L8, and a negative lens. And a second cemented lens L89 including the ninth lens L9.

  Further, this imaging lens is configured to satisfy the following conditional expressions (1) to (3). Here, f1 is the focal length of the first lens L1, fF is the focal length of the front group G1, fR is the focal length of the rear group G2, and f is the focal length of the entire system.

4.0 <f1 / f <6.6 (1)
−7.3 <fF / f <−4.4 (2)
1.23 <fR / f <1.36 (3)

  Next, operations and effects of the imaging lens configured as described above will be described.

  In this imaging lens, a sufficient back focus is obtained by including, in order from the object side, a front group G1 having a negative refractive power as a whole, a stop St, and a rear group G2 having a positive refractive power as a whole. I am doing so. In addition, by adopting a configuration in which only the rear group is extended with respect to the image formation surface, the aberration is corrected more satisfactorily by adopting a two-group nine-lens configuration while shortening the overall length. In the front group G1, the first lens L1 having a positive refractive power generates positive distortion with respect to the transmitted light, while the subsequent second lens L2 and the third lens L3 are negative with respect to the transmitted light. It acts to generate the distortion aberration. Therefore, although incident light from the object side causes positive distortion when transmitted through the first lens L1, it acts as negative distortion by sequentially transmitting through the second lens L2 and the third lens L3. Therefore, the distortion is canceled when the entire front group G1 is transmitted. Further, the negative refractive power is distributed by the surface S4 of the second lens L2 and the surfaces S5 and S6 of the third lens L3, thereby suppressing the occurrence of negative distortion itself. On the other hand, in the rear group G2, the positive refractive power is distributed by the fifth, sixth, and eighth lenses L5, L6, and L8 to suppress the occurrence of spherical aberration, and in addition, the first cemented lens L67 and The second cemented lens L89 acts to suppress the occurrence of chromatic aberration (particularly axial chromatic aberration).

  Here, the significance of the conditional expressions (1) to (3) will be described.

  Conditional expression (1) represents an appropriate range of an amount (f1 / f) representing the magnitude of the refractive power (1 / f1) of the first lens L1 with respect to the refractive power (1 / f) of the entire system. . By optimizing the refractive power distribution of the first lens group G1, it is possible to ensure a good balance between correction of various aberrations and size reduction. Here, if the refractive power of the first lens L1 becomes too strong below the lower limit of the conditional expression (1), the negative refractive power of the entire front group G1 becomes weak, so that sufficient back focus is ensured. It becomes difficult. On the other hand, if the refractive power of the first lens L1 becomes too weak beyond the upper limit of the conditional expression (1), the negative distortion generated in the second lens L2 and the third lens L3 cannot be corrected.

  Conditional expression (2) is an expression representing an appropriate range of an amount (fF / f) representing the magnitude of the refractive power (1 / fF) of the front group G1 with respect to the refractive power (1 / f) of the entire system. By optimizing the refractive power distribution of the front group G1, it is possible to ensure a good balance between correction of various aberrations and miniaturization. Here, if the negative refractive power of the front group G1 becomes too weak below the lower limit of the conditional expression (2), it is difficult to ensure sufficient back focus. On the other hand, if the negative refractive power of the front group G1 exceeds the upper limit of the conditional expression (2), the Petzval sum is excessively corrected, the image surface is tilted in the positive direction (image side), and distortion is increased. Resulting in.

  Conditional expression (3) is an expression that represents an appropriate range of an amount (fR / f) that represents the magnitude of the refractive power (1 / fR) of the rear group G2 with respect to the refractive power (1 / f) of the entire system. By optimizing the refractive power distribution of the rear group G2, it is possible to ensure a good balance between correction of various aberrations and size reduction. Here, if the positive refractive power of the rear group G2 becomes too strong below the lower limit of the conditional expression (3), it is difficult to ensure sufficient back focus. On the other hand, if the upper limit of conditional expression (3) is exceeded and the positive refractive power of the rear group G2 becomes too weak, the amount of feeding at the time of focusing increases and aberration fluctuations due to focusing increase.

  As described above, according to the imaging lens according to the present embodiment, the front group G1 and the rear group G2 are configured as described above, and the conditional expressions (1) to (3) are satisfied. In addition, it is possible to achieve a compact configuration while ensuring a good back focus, and it is possible to obtain extremely good aberration performance suitable for mounting on an image processing camera or the like.

  Next, specific numerical examples of the imaging lens according to the present embodiment will be described.

  Hereinafter, the first and second numerical examples (Examples 1 and 2) will be described together. Here, FIGS. 3 and 5 show basic lens data corresponding to the configurations of the imaging lenses shown in FIGS. 1 and 2, respectively.

  In the column of Si (surface number) in FIG. 3 and FIG. 5, regarding the imaging lens of each example, the surface of the component closest to the object side is the first, and the image plane side including the diaphragm St and the plane parallel plate GC is included. The number of the surface of the i-th (i = 1 to 19) component that sequentially increases toward is shown. In the column of Ri (curvature radius), the value of the curvature radius of the i-th (i = 1 to 19) surface Si from the object side corresponding to the symbol Ri attached in FIGS. 1 and 2 is shown. The portion where the value of the curvature radius Ri is ∞ indicates that it is a plane. Similarly, in the column of Di (surface interval), the optical axis between the i-th surface Si and the i + 1-th surface Si + 1 from the object side is associated with the reference symbol Di attached in FIG. 1 and FIG. Indicates the interval. Here, the unit of curvature radius Ri and surface interval Di is millimeter (mm). Further, in the columns of Ndj (refractive index) and νdj (Abbe number), the d-line (wavelength; 587) of the jth lens element (j = 1 to 10) from the object side, including the plane parallel plate GC, respectively. The refractive index and Abbe number values for.

  Further, the focal length f (mm) and F number (FNO.) Of the entire system in Examples 1 and 2 are shown in the margins of FIGS. 3 and 5.

  Furthermore, the numerical values of the conditional expressions in the imaging lenses according to Examples 1 and 2 are collectively shown in FIG.

  As is clear from the data shown in FIG. 4, the imaging lenses of Examples 1 and 2 both satisfy the conditional expressions (1) and (2).

  Next, FIGS. 6A to 6D show spherical aberration, astigmatism, distortion (distortion aberration), and lateral chromatic aberration in the imaging lens of Example 1. FIG. In FIG. 6A showing spherical aberration, values for g-line (wavelength 435.8 nm), d-line (wavelength 587.6 nm) and C-line (656.3 nm) are shown. In FIG. 6B showing astigmatism, the solid line shows the aberration with respect to the sagittal image surface, and the broken line shows the aberration with respect to the tangential (meridional) image surface. In FIG. 6 (D) showing lateral chromatic aberration, the values for the g-line and C-line are shown. In FIGS. 6 (A) to (D), those for which the wavelength is not specified indicate aberration with respect to the d-line. FNO. Represents the F number, and ω represents the half angle of view.

  Similarly, various aberrations with respect to the imaging lens of Example 2 are shown in FIGS.

  As described above, as shown in each numerical data and each aberration diagram, in each example, the aberration is corrected extremely well while having a sufficiently long back focus and securing an angle of view exceeding 50 °. .

  The present invention has been described with reference to the embodiment and examples. However, the present invention is not limited to the above embodiment and example, and various modifications can be made. For example, the values of the radius of curvature, the surface interval, and the refractive index of each lens component are not limited to the values shown in the above numerical examples, but can take other values.

1 is a cross-sectional view illustrating a first configuration example of an imaging lens according to an embodiment of the present invention and corresponding to Example 1. FIG. 2 is a cross-sectional view illustrating a second configuration example of an imaging lens according to an embodiment of the present invention and corresponding to Example 2. FIG. It is a figure which shows the basic lens data of Example 1 in the imaging lens which concerns on one embodiment of this invention. It is a figure which shows the numerical value of each conditional expression in the imaging lens which concerns on the 1st and 2nd Example. It is a figure which shows the basic lens data of Example 2 in the imaging lens which concerns on one embodiment of this invention. It is a figure which shows the spherical aberration, astigmatism, distortion, and lateral chromatic aberration of the imaging lens which concerns on a 1st Example. It is a figure which shows the spherical aberration, astigmatism, distortion, and lateral chromatic aberration of the imaging lens which concerns on a 2nd Example.

Explanation of symbols

G1: front group, G2: rear group, GC: plane parallel plate, L1 to L9: first to ninth lenses, L67, L89: cemented lens, Si: i-th lens surface from the object side, Ri: object side To i-th lens surface radius of curvature, Di: distance between the i-th and (i + 1) -th lens surfaces from the object side, Simg: imaging surface (imaging surface), St: stop, Z1, light axis.

Claims (2)

In order from the object side, a front group having a negative refractive power, a stop, and a rear group having a positive refractive power,
The front group includes, in order from the object side, a positive first lens having a convex surface facing the object side, a negative second lens having a meniscus shape having a convex surface facing the object side, and a third lens having a biconcave shape. And a fourth lens having a biconvex shape,
The rear group includes, in order from the object side, a positive fifth lens, a first cemented lens including a positive sixth lens and a negative seventh lens, a positive eighth lens, and a negative ninth lens. A second cemented lens,
An imaging lens, wherein focusing is performed by moving only the rear group in the optical axis direction with respect to the imaging plane. Furthermore, it is comprised so that the following conditional expressions (1) to (3) may be satisfy | filled. The imaging lens of Claim 1 characterized by the above-mentioned.
4.0 <f1 / f <6.6 (1)
−7.3 <fF / f <−4.4 (2)
1.23 <fR / f <1.36 (3)
However,
f1: focal length of the first lens fF: focal length of the front group fR: focal length of the rear group f: focal length of the entire system

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