JP2007139985A - Super-wide-angle lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a compact and inexpensive super-wide-angle lens few in the number of lenses. <P>SOLUTION: The super-wide-angle lens is composed of a combination of positive lenses and negative lenses. At least one face of the lens surfaces is a diffractive optical face. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an ultra-wide angle lens.

Ultra-wide-angle lenses are widely used for in-vehicle cameras and security cameras. These cameras are required not only to have performance but also to be small and low cost.
Conventionally, as such an ultra-wide-angle lens, two groups and six super-wide-angle lenses have been proposed (Patent Document 1). This super wide-angle lens realizes a wide angle of view exceeding 166 degrees with a small number of lenses of 6 in 2 groups, but uses a bonded lens in the rear group, and the number of bonding steps in lens manufacturing, etc. Therefore, it is not easy to reduce the cost.

JP 2002-72085 A

  In view of the circumstances described above, an object of the present invention is to realize a compact, inexpensive and high-performance ultra-wide-angle lens with a small number of lenses.

The super wide-angle lens of the present invention has a plurality of positive lenses and a plurality of negative lenses, and at least one of the lens surfaces is a diffractive optical surface.
The super wide-angle lens according to claim 1 is configured by six lenses of three front groups and three rear groups with a diaphragm interposed therebetween, and two lenses on the object side in the front group (one lens counted from the object side) Eye and second lens) have a negative focal length, and in the rear group, the two lenses on the image plane side (the first and second lenses counted from the image plane side) have positive focal points, respectively. A configuration having a distance is preferable (claim 2). In this case, it is preferable to provide a diffractive optical surface in the rear group.

The super wide-angle lens according to claim 2 or 3, wherein the focal length of the first and second negative lenses counted from the object side in the front group is f1, and the first lens is counted from the image plane side in the rear group. The combined focal length of the second positive lens: f2 and the focal length of the entire lens: f are the conditions:
(1) -4 ≦ f1 / f ≦ -2
(2) 2.5 ≦ f2 / f ≦ 4.5
Is preferably satisfied.

  The inventors have conducted research aimed at further widening the angle of view in an ultra-wide-angle lens, but it is important to correct chromatic aberration when expanding the angle of view and achieving good performance. I found it. Although there is no mention of chromatic aberration in Patent Document 1, it is effective as a means for correcting chromatic aberration to use a cemented lens in the lens system as in the super wide-angle lens described in the document. However, using a bonded lens requires a troublesome bonding process as a manufacturing process of the lens as described above, and it is difficult to realize a low-cost super-wide-angle lens.

  The inventors noticed that the Abbe number of the diffractive optical surface is negative, and came to recognize that the diffractive optical surface can be used for correcting chromatic aberration.

  Since the diffractive optical surface has an Abbe number, chromatic aberration can be corrected by a combination of the diffractive optical surface and the lens. In general, positive and negative lenses with different Abbe numbers are combined to correct chromatic aberration. However, since the Abbe number is uniquely determined by the lens material, a lens material having an appropriate Abbe number is selected for correcting chromatic aberration. The degree of freedom of material selection is not so great.

  On the other hand, the negative Abbe number of the diffractive optical surface can be set by design according to the form of the diffractive optical surface itself, so that the optimum Abbe number for chromatic aberration correction can be easily realized. Further, the diffractive optical surface can be effectively used not only for correcting chromatic aberration but also for correcting other aberrations such as spherical aberration.

  In the super wide-angle lens according to claim 1, as described above, by using at least one of the lens surfaces as a diffractive optical surface, the aberration correction function of the diffractive optical surface is used, so that the number of lenses is more than necessary. Without increasing, optical aberrations, particularly chromatic aberrations, are effectively corrected to achieve super wide angle and good performance.

  As in the super wide-angle lens according to the second aspect, “comprising six lenses of three front groups and three rear groups with a diaphragm interposed therebetween” can further improve the optical performance. Further, the 6-group 6-lens configuration is the same number as the super wide-angle lens described in Patent Document 1, and can be realized in a compact manner similar to that described in Patent Document 1, and does not include a bonded lens in the lens system. Therefore, it can be manufactured at a lower cost.

  The role of the front group is mainly to “guide the off-axis light to the rear group after the stop while suppressing the occurrence of optical aberration as much as possible”. Therefore, in the front group, it is necessary to “refractively refract light” at each lens surface. In view of this point, the super wide-angle lens according to claim 2 includes three lenses as the front group, At least two lenses located on the object side are lenses having negative power.

  On the other hand, since the rear group plays a role of correcting optical aberrations such as astigmatism, coma aberration, and chromatic aberration, the super wide-angle lens according to claim 2 has a rear group as a rear group so as to effectively correct the optical aberration. Three lenses are arranged, and at least two lenses located on the image plane side are lenses having a positive power in order to further enhance the image forming effect of light on the image plane.

  By providing the diffractive optical surface as the lens surface of the lens in the rear group as in the super wide-angle lens according to claim 3, the aberration correction function that the rear group should have can be effectively used. Can be increased.

  When the parameter of condition (1): f1 / f becomes smaller than the lower limit value, the negative composite power of the “two negative lenses on the object side” in the front group becomes smaller. In order to ensure a super wide angle, The effective diameter of the lens closest to the object side in the front group (the first lens counted from the object side) must be increased, making it difficult to achieve compactness and low cost.

  When the parameter: f1 / f increases beyond the upper limit of the condition (1), the negative composite power of the “two negative lenses on the object side” in the front group increases, which is advantageous for super-wide angle. It becomes difficult to “reflect light gently” on each lens surface of the front group, and it becomes difficult to correct deterioration of optical aberrations such as astigmatism and coma aberration by the rear group.

  When the parameter of condition (2): f2 / f becomes smaller than the lower limit of condition (2), the positive combined power of the “two positive lenses on the image plane side” in the rear group becomes smaller, and astigmatism, Optical aberrations such as coma tend to deteriorate. When the parameter f2 / f exceeds the upper limit of the condition (2), the positive combined power of the “two lenses on the image plane side” in the rear group increases, and the total length of the super wide-angle lens increases. And lacks compactness.

  The super wide-angle lens of the present invention can be effectively implemented with a field angle of 140 degrees or more, but can be effectively implemented particularly with a large field angle exceeding 170 degrees. In an example described later, an ultra-wide-angle lens having an angle of view of 190 degrees and good performance is realized.

  As described above, according to the present invention, a compact and excellent performance ultra-wide-angle lens can be realized by using a diffractive optical surface as a lens surface and contributing to aberration correction, particularly correction of chromatic aberration. Since the super wide-angle lens of the present invention does not use a bonded lens, a troublesome bonding process is not required during the manufacturing process, so that the manufacturing is easy and can be realized at low cost.

FIG. 1 shows an embodiment of an ultra-wide-angle lens.
The super wide-angle lens shown in FIG. 1 includes a first lens L1, a second lens L2, a third lens L3, a fourth lens L4, a fifth lens L5, and a sixth lens L6 in order from the object side (left side in the figure). The diaphragm I is provided between the third lens L3 and the fourth lens L4.

  The first lens L1 to the third lens L3 constitute a “front group”, and the first lens L1 and the second lens L2 are both “negative lenses”. The third lens L3 can be a “negative lens” or a “positive lens”. The fourth lens L4 to the sixth lens L6 constitute a “rear group”, and the fifth lens L5 and the sixth lens L6 are both “positive lenses”. The fourth lens can be a “positive lens” or a “negative lens”.

  Examples 1 to 4 of the super wide-angle lens of the present invention will be described below. In addition, as an example for comparison, an example “an example that does not use a diffractive optical surface” is given. These Examples 1 to 4 and Comparative Example are all used for in-vehicle cameras and security cameras, and an object image is formed on a light receiving surface of an image sensor (CCD area sensor or the like). The “cover glass” in FIG. 1 represents a cover glass and a filter that protect the light receiving surface of the light receiving element, and the “image surface” matches the light receiving surface.

  The object distance is infinite, the design F number is 3.2, and the design wavelengths are 450 nm, 530 nm, and 630 nm. In each of Examples 1 to 4 and the comparative example, the maximum field angle is 190 degrees (half field angle: 95 degrees).

  The aspheric surfaces in Examples and Comparative Examples are represented by the following well-known expressions.

Z = (1 / R) h ² / [1+ {1- (1 + K) (h / R) ² } ^1/2 ] + Ah ² + Bh ⁴ + Ch ⁶
Here, Z is an aspherical amount, h is a distance from the optical axis, R is a paraxial radius of curvature, K is a conic constant, and A, B, and C are aspherical coefficients.
Further, the phase difference Φ generated on the diffractive optical surface (DOE surface) is expressed by the following equation.

φ = M {E (r / Rn) ² + F (r / Rn) ⁴ + G (r / Rn) ⁶ }
Here, M is the diffraction order, r is the radial coordinate on the surface, and E, F, and G are coefficients. These M, E, F, and G are hereinafter referred to as “phase coefficients”.
FIG. 2 shows a “conceptual diagram in which a diffractive optical surface is formed on a lens surface”. In the figure, the “reference surface” indicated by a continuous smooth line is the shape of the lens surface determined by the aspheric data and the radius of curvature, and the “sawtooth portion (shape)” along the reference surface is the phase difference. : The shape of the diffractive optical surface specified by Φ.

f = 0.8 mm, f1 / f = −2.83, f2 / f = 3.11
The specifications of Example 1 are shown in Table 1.

  Surfaces: 1 to 15 are surface numbers counted from the object side in FIG. The radius of curvature is indicated by R1 to R13 in FIG. 1, and the surface spacing is indicated by D1 to D15 in FIG. The same applies to the following.

  Table 2 shows the conic constant and the aspheric coefficient on the aspheric surface (the surface marked with * in Table 1).

In the above notation, for example, “5.7081E-3” means “5.7081 × 10 ⁻³ ”. The same applies to the following.

  Table 3 shows the phase coefficient of the diffractive optical surface (the surface marked with * in Table 1).

  Table 4 shows the positive and negative powers of the first to sixth lenses.

f = 0.8 mm, f1 / f = −2.96, f2 / f = 3.14
The specifications of Example 2 are shown in Table 5.

  Table 6 shows the conic constant and aspheric coefficient of the aspheric surface (the surface marked with * in Table 5).

  Table 7 shows the phase coefficient of the diffractive optical surface (the surface marked with * in Table 5).

  Table 8 shows the positive and negative powers of the first to sixth lenses.

f = 0.8 mm, f1 / f = −3.34, f2 / f = 4.11
Table 9 shows the data of Example 3.

  Table 10 shows the conic constant and the aspheric coefficient on the aspheric surface (the surface marked with * in Table 9).

  Table 11 shows the phase coefficient of the diffractive optical surface (the surface marked with * in Table 9).

  Table 12 shows the positive and negative powers of the first to sixth lenses.

f = 0.8 mm, f1 / f = −2.48, f2 / f = 4.13
Table 13 shows the data of Example 4.

  Table 14 shows the conic constant and the aspheric coefficient on the aspheric surface (the surface marked with * in Table 13).

  Table 15 shows the phase coefficient of the diffractive optical surface (the surface marked with * in Table 13).

  Table 16 shows the positive and negative powers of the first to sixth lenses.

"Comparative example (no optical diffractive surface)"
f = 0.8mm
Table 17 shows the specifications of the comparative example.

  Table 18 shows the conic constant and the aspheric coefficient on the aspheric surface (the surface marked with * in Table 17).

  Table 19 shows the positive and negative powers of the first to sixth lenses.

  Aberration diagrams of Example 1 are shown in FIGS. 3 is a diagram of astigmatism and distortion (distortion aberration), FIG. 4 is a diagram of coma aberration and lateral chromatic aberration (the left diagram at each angle of view is the tangential direction, and the right diagram is the sagittal direction), diagram. 5 is an MTF diagram. In these aberration diagrams, T means tangential and S means sagittal. The same applies to other aberration diagrams.

Aberration diagrams of Example 2 are shown in FIGS. 6 is a diagram of astigmatism and distortion (distortion aberration), FIG. 7 is a diagram of coma aberration, and FIG. 8 is an MTF diagram.
Aberration diagrams of Example 3 are shown in FIGS. 9 is a diagram of astigmatism and distortion (distortion aberration), FIG. 10 is a diagram of coma aberration and lateral chromatic aberration, and FIG. 11 is an MTF diagram.
Aberration diagrams of Example 4 are shown in FIGS. 12 is a diagram of astigmatism and distortion (distortion aberration), FIG. 13 is a diagram of coma aberration and lateral chromatic aberration, and FIG. 14 is an MTF diagram.
Aberration diagrams of the comparative examples are shown in FIGS. 15 is a diagram of astigmatism and distortion (distortion aberration), FIG. 16 is a diagram of coma aberration and lateral chromatic aberration, and FIG. 17 is an MTF diagram.
The comparative example has the same lens configuration as in Example 1 (“front group” negative / negative / positive, “back group” negative / positive / positive), and satisfies the conditions (1) and (2), while satisfying the diffractive optical surface. This is an example in which each aberration is optimized without using the lens, but as is clear when each aberration in the comparative example is compared with that in Example 1, any one of curvature of field, coma aberration, lateral chromatic aberration, and MTF is observed. Also, it does not reach the performance of the first embodiment. From this, the effect of using the diffractive optical surface is clear.

It is a figure which shows one Embodiment of the super-wide-angle lens of this invention. It is a conceptual diagram of a diffractive optical surface. FIG. 4 is an astigmatism diagram and a distortion diagram of Example 1. FIG. 6 is a coma aberration diagram and a chromatic aberration diagram of magnification in Example 1. 1 is an MTF diagram of Example 1. FIG. FIG. 4 is an astigmatism diagram and a distortion diagram of Example 2. FIG. 4 is a coma aberration diagram and a lateral chromatic aberration diagram of Example 2. 6 is an MTF diagram of Example 2. FIG. FIG. 6 is an astigmatism diagram and a distortion diagram of Example 3. FIG. 6 is a coma aberration diagram and a lateral chromatic aberration diagram of Example 3. 6 is an MTF diagram of Example 3. FIG. FIG. 6 is an astigmatism diagram and a distortion diagram of Example 4. FIG. 6 is a coma aberration diagram and a lateral chromatic aberration diagram of Example 4. FIG. 6 is an MTF diagram of Example 4. It is an astigmatism figure of a comparative example, and a distortion figure. It is a coma aberration figure of a comparative example, and a magnification chromatic aberration figure. It is a MTF figure of a comparative example.

Explanation of symbols

L1 1st lens L2 2nd lens L3 3rd lens L4 4th lens L5 5th lens L6 6th lens I Aperture

Claims (4)

  An ultra-wide-angle lens having a plurality of positive lenses and a plurality of negative lenses, wherein at least one of the lens surfaces is a diffractive optical surface. The super wide-angle lens according to claim 1,
It consists of 6 lenses, 3 in the front group and 3 in the rear group, across the aperture.
An ultra-wide-angle lens, wherein two lenses on the object side in the front group each have a negative focal length, and two lenses on the image plane side in the rear group each have a positive focal length. The super wide-angle lens according to claim 2,
An ultra-wide-angle lens having a diffractive optical surface in the rear group. The super wide-angle lens according to claim 2 or 3,
The combined focal length of the first and second negative lenses counted from the object side in the front group: f1, and the combined focal length of the first and second positive lenses counted from the image plane side in the rear group: f2. The focal length of the entire lens: f is the condition:
(1) -4 ≦ f1 / f ≦ -2
(2) 2.5 ≦ f2 / f ≦ 4.5
An ultra-wide-angle lens characterized by satisfying

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