JP2008180887A - Wide angle lens system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wide angle lens system which has a short entire length, a wide angle and a large diameter, by an extremely simple configuration. <P>SOLUTION: The wide angle lens system 1 with a total field angle (2ω) of 90 degrees or more comprises a first single lens and a second single lens in order from an object side and a third single lens on an image side, across a diaphragm surface 4. The first lens 5 is a negative meniscus lens whose convex surface faces the object side, the second lens 6 is a positive or negative meniscus lens whose concave surface faces the object side, and the third lens 7 is a biconvex lens. When the radius of curvature of the object side surface of the second lens is defined as R1 and the radius of curvature of the image side surface thereof is defined as R2, expression of 0.5<R1/R2<2 is satisfied. When the focal length of the entire system is defined as f, the entire length L and an image height H' satisfy L/(H'×f)≤5. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a small-sized and inexpensive wide-angle lens system having a short overall length used for a surveillance camera, an in-vehicle camera, and the like.

  Conventionally, in industrial fields such as surveillance cameras and in-vehicle cameras, it has been necessary to realize a wide angle of view under a special projection method while miniaturizing an optical system. In addition, since a CCD or CMOS is used as the image pickup device, it is also required that the back focus be increased and that the principal ray be parallel to the optical axis (telecentricity).

  In a wide-angle lens different from the normal projection method, it is necessary to refract the chief ray incident with a very large angle of view and to enter the image plane at an appropriate angle. Therefore, a divergent group that reduces the chief ray angle with a large angle of view is disposed in the group located on the object side from the stop, i.e., the front group, and the group located on the image sensor side from the stop, i.e., the rear group. A positive lens having a role of causing the principal ray whose angle is relaxed in the front group to be incident on the image plane in a substantially telecentric manner and forming an image is disposed.

  As a result, the lens configuration employs a so-called retrofocus type (asymmetric type) configuration in which negative power is disposed on the object side and positive power is disposed on the image side.

  Under these roles, it is desirable that the front group is composed of only negative lenses and the rear group is composed of only positive lenses.

  In particular, regarding the configuration of the front group, the negative lens group has an active role of controlling astigmatism and distortion when considering only the chief ray, but spherical aberration mainly related to the increase in aperture. Has the negative side of increasing. Also, increasing the negative component of the front group has the effect of increasing the retrofocus effect and lengthening the total length.

  In a conventional wide-angle lens system, a configuration has been adopted in which the power that the front group should have is shared by a plurality of lenses, and the burden of aberration per lens is reduced, but in such a configuration, The negative power per lens increased on the contrary, and the above problem was amplified. In addition, with the conventional lens configuration, it is generally difficult to increase the aperture, and at the same time, there is a problem that the entire length of the wide-angle lens system becomes long.

  By the way, in recent years, the practical use of a resin that can be used as an optical material has facilitated the production of an aspherical surface, and various types of lens systems in which a resin lens having an aspherical surface is incorporated have been put into practical use. The use of an aspheric lens is useful for widening and widening the wide-angle lens system. However, the demands for a large aperture and a wide angle are contradictory, and at the same time, sufficiently shortening the entire length of the wide angle lens system is a major problem.

JP 2001-750006 A

JP 2001-83409 A

JP 2005-309210 A

  In view of such a situation, the present invention provides a wide-angle lens system having a short overall length and a wide angle and a large aperture with an extremely simple configuration in order to solve the above problems.

  The present invention is a wide-angle lens system having a total angle of view (2ω) of 90 ° or more, and includes a first single lens from the object side and a second single lens from the object side across the diaphragm surface, The first lens is a negative meniscus lens having a convex surface facing the object side, the second lens is a positive or negative meniscus lens having a concave surface facing the object side, and the third lens is a biconvex lens, When the radius of curvature of the object side surface of the second lens is R1 and the radius of curvature of the image side surface is R2, when 0.5 <R1 / R2 <2 is satisfied and the focal length of the entire system is f, the total length L The image height H ′ relates to a wide-angle lens system that satisfies L / (H ′ × f) ≦ 5.

  According to the present invention, in a wide-angle lens system in which the total angle of view (2ω) is 90 ° or more, each of the first and second images from the object side and the third lens from the image side with the stop surface interposed therebetween. The first lens is a negative meniscus lens having a convex surface facing the object side, the second lens is a positive or negative meniscus lens having a concave surface facing the object side, and the third lens is a biconvex lens. Yes, when the radius of curvature of the object side surface of the second lens is R1 and the radius of curvature of the image side surface is R2, when 0.5 <R1 / R2 <2 is satisfied and the focal length of the entire system is f, Since the total length L and the image height H ′ satisfy L / (H ′ × f) ≦ 5, the overall length of the wide-angle lens system can be shortened with a simple configuration, and a small and inexpensive wide-angle lens system. The excellent effect of being able to be demonstrated.

  The best mode for carrying out the present invention will be described below with reference to the drawings.

  The wide-angle lens system according to the present invention has a configuration in which positive power is incorporated into the front group in an appropriate form.

  FIG. 1 shows a configuration of a wide-angle lens system 1 according to the first embodiment, in which 2 denotes an optical axis, and 3 denotes an image plane on which an image sensor and the like are arranged.

  A diaphragm 4 is disposed on the optical axis 2, and a first lens 5 and a second lens 6 are disposed on the object side of the diaphragm 4, and a front group is formed by the first lens 5 and the second lens 6. Is configured. A third lens 7 is disposed on the image plane 3 side of the diaphragm 4, and the third lens 7 forms a rear group.

  As the first lens 5 located closest to the object side in the front group, a meniscus lens having a negative power with the convex surface facing the object side is adopted, and the second lens 6 that should bear the negative power is used. A thick meniscus lens having a concave surface facing the object side and a convex surface facing the diaphragm 4 side is employed.

  The third lens 7 in the rear group is a convex lens having convex surfaces on the diaphragm 4 side and the image plane 3 side.

  By sharing negative power on the object side surface of the second lens 6 to reduce the chief ray tilt angle, and giving positive power to the surface of the second lens 6 on the diaphragm 4 side, Shortening and large diameter are realized. The power of the second lens 6 itself is the power required by the entire wide-angle lens system 1 and may be positive or negative.

  The second lens 6 does not have a bonding surface and does not have an element that positively corrects chromatic aberration.

  By the way, in the assumed use, since the image plane size is generally small and the angle of view is large, the image is manufactured under a design with a small focal length and is put into practical use.

  Here, various aberrations including chromatic aberration are proportional to the focal length. In addition, it is known that the allowable amount of aberration depends only on the size of the emulsion grains on the photosensitive surface when the image is acquired electronically, when the size of the image sensor is acquired, and when the image is acquired with a film. Therefore, by selecting an appropriate glass material and resin material, axial and lateral chromatic aberration can be reduced to an appropriate size suitable for the element size.

  In FIG. 1, 11 indicates an on-axis light beam, and 12 indicates a most off-axis light beam (view angle ω = 80 °).

  In the first embodiment, the object side surface of the first lens 5 is the first surface, the diaphragm 4 side surface is the second surface, and the object side surface of the second lens 6 is the third surface (curvature radius R1). When the diaphragm 4 side surface is the fourth surface (curvature radius R2), the diaphragm 4 is the fifth surface, the diaphragm 4 side surface of the third lens 7 is the sixth surface, and the image surface 3 side surface is the seventh surface. The third and fourth surfaces of the second lens 6 are aspheric surfaces, and the sixth and seventh surfaces of the third lens 7 are aspheric surfaces.

  In the first embodiment, the first lens 5 is made of glass, and the second lens 6 and the third lens 7 are made of resin. The specifications of each lens are shown in FIG. 2, in which N is the refractive index and ν is the Abbe number. Incidentally, the resin 1 is used for the second lens 6, the resin 2 is used for the third lens 7, and the refractive indexes of the resin 1 and the resin 2 are shown in FIG. 17.

  When the curvature of the third surface of the second lens 6 is R1, the curvature of the fourth surface is R2, R1 / R2 = 1.018135, and the focal length of the wide-angle lens system 1 is f. f = 1.625 mm, total length L = 12.5 mm, image height H ′ = 2.3 mm, and L / (H ′ × f) = 12.5 / (2.3 × 1.625) = 3.3445 It is.

  In the wide-angle lens system 1, the F number of the lens is F / no, and the angle of view, focal length, and back focus are shown in FIG. Further, it is assumed that the aspherical shape of the lens is expressed by (Expression 1) as x in the optical axis direction and a separation distance H from the optical axis, and the aspherical coefficients R, K, A, B, C of each surface of the lens Is represented in FIG.

x = H ² / R (1 + √ [1− (1 + K) (H / R) ² ]) + AH ⁴ + BH ⁶ + CH ⁸ (1 formula)

  FIG. 5 shows spherical aberration and sinusoidal aberration in the wide-angle lens system 1 having the above-described configuration.

  As shown in FIG. 5, the spherical aberration and the coma aberration are corrected in a desirably small state.

  Further, astigmatism is shown in FIG. In FIG. 6, ΔS is a sagittal image plane, and ΔM is a meridional image plane. As shown in FIG. 6, astigmatism and field curvature are corrected in a desirably small state.

  Further, the distortion aberration is shown in FIG. 7, and as shown in FIG. 7, the distortion is corrected in a desirably small state under an equidistant projection.

  FIG. 8 shows the MTF characteristics when the angle of view F1 of the incident light F1 (light on the optical axis), the angle of view ω of the incident light F2 = 70 °, and the angle of view ω = 80 ° of the incident light F3 ( (Spatial frequency response characteristics). RAD indicates a characteristic in the radial direction, and TAN indicates a characteristic in the tangential direction. The MTF characteristics indicated by F1, F2, and F3 are the MTF characteristics for the C line (wavelength 656.27 nm), the d line (wavelength 587.56 nm), and the F line (wavelength 486.13 nm), respectively. Line: F line = 1: 2: 1.

  According to this graph, the wide-angle lens system 1 is corrected to a state in which various aberrations including chromatic aberration are desirably small, and a spatial frequency (50 lp / mm: a pair of black and white lines per 1 mm with respect to the general size of the image sensor. Means 50. Note that lp is a line pair).

  FIG. 9 shows a configuration of a wide-angle lens system 15 according to the second embodiment. The wide-angle lens system 15 has a configuration equivalent to that of the wide-angle lens system 1 according to the first embodiment. 9, the same symbols are given to the same components as those shown in FIG.

  In FIG. 9, 11 indicates an on-axis light beam, and 12 indicates a most off-axis light beam (view angle ω = 55 °).

  The object side surface of the first lens 5 is the first surface, the diaphragm 4 side surface is the second surface, the object side surface of the second lens 6 is the third surface (curvature radius R1), and the aperture 4 side surface is the fourth surface (curvature radius R2). The diaphragm 4 is the fifth surface, the diaphragm 4 side surface of the third lens 7 is the sixth surface, and the image surface 3 side surface is the seventh surface, and the third and fourth surfaces of the second lens 6 are aspherical surfaces, The sixth surface and the seventh surface of the third lens 7 are aspherical surfaces.

  In the second embodiment, glass is used as the material of the first lens 5, and resin is used as the material of the second lens 6 and the third lens 7. The specifications of each lens are shown in FIG. 10, in which N is the refractive index and ν is the Abbe number. The resin 1 is used for the second lens 6, the resin 3 is used for the third lens 7, and the refractive indexes of the resin 1 and the resin 3 are shown in FIG.

  When the curvature of the third surface of the second lens 6 is R1 and the curvature of the fourth surface is R2, R1 / R2 = 1.4380, and when the focal length of the wide-angle lens system 15 is f, f = 2.365 mm, total length L = 6.385 mm, image height H ′ = 2.3 mm, and L / (H ′ × f) = 6.385 / (2.3 × 2.365) = 1.1738. .

  In the wide angle lens system 1, the F number of the lens is F / no, and the angle of view, focal length, and back focus are shown in FIG. Further, it is assumed that the aspherical shape of the lens is expressed by (Expression 1) as x in the optical axis direction and a separation distance H from the optical axis, and the aspherical coefficients R, K, A, B, and C of each lens surface Is represented in FIG.

  As shown in FIG. 13, the spherical aberration and the coma aberration are corrected in a desirably small state.

  Further, astigmatism is shown in FIG. In FIG. 14, ΔS is a sagittal image plane and ΔM is a meridional image plane. As shown in FIG. 14, astigmatism and curvature of field are corrected in a desirably small state.

  Further, the distortion aberration is shown in FIG. 15, and as shown in FIG. 15, the distortion is corrected in a desirably small state under the equidistant projection.

  FIG. 16 shows the MTF characteristics when the angle of view ω = 0 of incident light F1 (ray on the optical axis), the angle of view ω = 40 ° of incident light F2, and the angle of view ω = 55 ° of incident light F3 ( (Spatial frequency response characteristics). The MTF characteristics indicated by F1, F2, and F3 are the MTF characteristics for the C line (wavelength 656.27 nm), the d line (wavelength 587.56 nm), and the F line (wavelength 486.13 nm), respectively. Line: F line = 1: 2: 1. In FIG. 16, RAD indicates the MTF characteristic in the radial direction, and TAN indicates the MTF characteristic in the tangential direction.

  According to this graph, the wide-angle lens system 15 is corrected to a state in which various aberrations including chromatic aberration are desirably small, and a spatial frequency (50 lp / mm: a pair of black and white lines per 1 mm) with respect to the general size of the image sensor. It can be seen that there is sufficient resolving power at 50).

  As described above, glass is used for the first lens 5, and resin is used for the second lens 6 and the third lens 7. Examples of the characteristics of the resin glass materials 1, 2, and 3 used for the second lens 6 and the third lens 7 include those shown in FIG. 17, and FIG. 17 shows 656.27 nm (C line) and 587.56 nm. (D line), The refractive index of 486.13 nm (F line) is shown.

  The specifications of the wide-angle lens system in the first and second embodiments are summarized as shown in FIG.

  The curvature R1 of the third surface and the curvature R2 of the fourth surface of the second lens 6 satisfy 0.5 <R1 / R2 <2, and the wide-angle lens system 1 and the wide-angle lens are not limited to the above embodiments. The focal length f, the total length L, and the image height H ′ of the entire lens system 15 may satisfy L / (H ′ × f) ≦ 5.

  Conventionally, as shown in Patent Documents 1 to 3, f = 1 mm, L = 14 mm, and image height 2.3 (1/4 ″ CCD), but the limit is 0.5 < By satisfying R1 / R2 <2 and L / (H ′ × f) ≦ 5, the overall length of the wide-angle lens system can be shortened.

  Furthermore, the present invention can be applied not only to a surveillance camera and an in-vehicle camera but also to a camera-equipped mobile phone camera, a digital camera, a video camera, and the like. Therefore, the device body can be made compact.

It is a schematic block diagram which shows the 1st Embodiment of this invention. It is a figure which shows the item of each lens in 1st Embodiment same as the above. It is a figure which shows the item of the wide angle lens system in 1st Embodiment before the same. It is a figure which shows the coefficient regarding the aspherical shape of each lens in 1st Embodiment same as the above. It is a figure which shows the spherical aberration and sine condition in 1st Embodiment same as the above. It is a figure which shows the astigmatism in 1st Embodiment before the same. It is a figure which shows the distortion aberration in 1st Embodiment before the same. It is a figure which shows the MTF characteristic in 1st Embodiment same as the above. It is a schematic block diagram which shows the 2nd Embodiment of this invention. It is a figure which shows the item of each lens in 2nd Embodiment same as the above. It is a figure which shows the item of the wide angle lens system in 2nd Embodiment same as the above. It is a figure which shows the coefficient regarding the aspherical shape of each lens in 2nd Embodiment same as the above. It is a figure which shows the spherical aberration and sine condition in 2nd Embodiment same as the above. It is a figure which shows the astigmatism in 2nd Embodiment same as the above. It is a figure which shows the distortion aberration in 2nd Embodiment same as the above. It is a figure which shows the MTF characteristic in 2nd Embodiment same as the above. It is a figure which shows the refractive index of the resin material used for a lens. It is a figure which shows the summary of the item of 1st Embodiment and 2nd Embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Wide-angle lens system 2 Optical axis 3 Image surface 4 Aperture 5 1st lens 6 2nd lens 7 3rd lens 11 On-axis light beam 12 Most off-axis light beam 15 Wide-angle lens system

Claims (1)

  In a wide-angle lens system in which the total angle of view (2ω) is 90 ° or more, the first lens from the object side and the second lens from the object side, and the third lens from the image side across the diaphragm surface, the first lens is , A negative meniscus lens having a convex surface directed toward the object side, the second lens is a positive or negative meniscus lens having a concave surface directed toward the object side, and the third lens is a biconvex lens, When the radius of curvature of the object side surface of the lens is R1 and the radius of curvature of the image side surface is R2, when 0.5 <R1 / R2 <2 is satisfied and the focal length of the entire system is f, the total length L and the image height H ′ Is a wide-angle lens system characterized by satisfying L / (H ′ × f) ≦ 5.

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