JP2008134540A - Fisheye lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fisheye lens which achieves a super wide angle by a small number of component lenses. <P>SOLUTION: In the fisheye lens comprising a lens system constituted of three lenses, a first concave lens, a second concave lens, a diaphragm and a third convex lens are arranged in order. The first lens positioned nearest to an object side is a glass lens having a high refractive index and its both surfaces are spherical. The second lens is a resin lens or a molded glass lens and has aspherical shape. The fisheye lens has such a super wide angle that a horizontal viewing angle is 160° or more and has such brightness as F/3 or more. The entire length of the fisheye lens is shorter than that of an imaging element, and the aberration thereof is corrected relative to a certain single wavelength in a near infrared region. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a super-wide angle fisheye lens.

  In recent years, in addition to the fields of weather observation and photography, the demand for super-wide-angle lenses has also increased in the fields of surveillance cameras and in-vehicle cameras.

  In such applications, it is desirable that the lens be as small and bright as possible.

  Conventionally, super wide-angle lenses including fisheye lenses have been widely used for scientific applications such as weather photography due to their characteristic projection methods. The fisheye lens with a simple structure has a long history, and goes back to the “hill sky lens” manufactured by Beck in 1924 (see Non-Patent Document 1).

  This type of lens is composed of three lenses, and is used when recording the whole sky on a single photograph for weather recording purposes.

  Patent Document 1 discloses a super wide-angle lens having three lenses.

Patent Document 2 shows that the arrangement of the three lenses is concave | convex / convex (that is, concave lens, stop, convex lens, convex lens).
JP 2003-195161 A JP-A-57-173810 "A HISTORY OF PHOTOGRAPHIC LENS" (R. Kingslake, Academic Press, 1989)

  Conventional small fisheye lenses are very dark.

  Bright fisheye lenses have also been developed but are limited to large ones. In the past, in order to achieve a bright and ultra-wide angle, many lens elements had to be used, and an increase in size was inevitable.

  In Patent Document 1, since the refractive power of the first lens located closest to the object side is extremely weak and the principal ray cannot be effectively bent, correction of off-axis aberrations (particularly high-order astigmatism) is performed. It was difficult.

  In the conventional fisheye lens, on the other hand, there is a drawback that it becomes dark with a simple configuration, and on the other hand, there is a disadvantage that the configuration becomes complicated to make it bright and an increase in size is inevitable. The main reason for this is that there was no practically precise aspheric processing technology. Therefore, in order to make it bright, it is necessary to construct a super-wide-angle lens only with a spherical surface, but in that case, a large number of lenses are required.

  An object of the present invention is to provide a fish-eye lens that can be bright and have a super wide angle with a small number of lenses.

  Examples of the solving means of the present invention are as follows.

  (1) The lens system is composed of three lenses, and the first lens located closest to the object side is a glass lens with a high refractive index, both surfaces are spherical, and the second lens is made of a resin lens or molded glass. A fish-eye lens having an aspherical shape and a super wide angle of 160 ° or more in horizontal angle of view.

  (2) The fisheye lens described above, wherein a concave first lens, a concave second lens, a stop, and a convex third lens are arranged in this order.

  (3) The aforementioned fisheye lens, wherein the third lens is a resin lens or a molded glass lens.

  (4) The overall length is shorter than that of the image sensor, and various aberrations are corrected for a single wavelength in the near infrared region, and | t ′ |> 10f (f is the focal length) with respect to the exit pupil position t ′. The fisheye lens described above, wherein the image side represented by the formula is substantially telecentric.

  (5) The first lens and the second lens constitute a first group (G1) whose refractive power is negative as a whole, and the third lens is a spherical surface or an aspherical surface on the image side across the stop. The fisheye lens described above, which constitutes the second group (G2) consisting of a positive single lens.

  (6) The fisheye lens described above, wherein the first lens is a negative meniscus lens.

  (7) The above-described fisheye lens, which is based on equidistant projection.

Best Mode for Carrying Out the Invention

  By combining a glass lens with a high refractive index whose both surfaces are spherical and a resin lens or a molded glass lens, the degree of freedom of creating an aspheric surface for the resin lens or molded glass lens is dramatically improved. Can do. It is possible to achieve a super-wide-angle lens with a bright and very simple structure by making the best use of the aspherical surface obtained in this way and the fact that the focal length is short, so that there is no need for color correction. it can.

  The function of the lens system group in the fisheye lens according to the best embodiment of the present invention will be described. The first group has a function of relaxing the tilt angle of the off-axis principal ray having a large tilt angle. At the same time, ∞ light is formed into a virtual image at a finite distance from the lens. The second group forms the virtual image as a real image on the imaging surface.

  Therefore, the shape of the first group is mainly related to off-axis aberrations, particularly distortion, and the shape of the second group is related to spherical aberration and coma aberration.

  For ultra-wide-angle lenses, the projection method is an important factor in the specification.

  The first lens located closest to the object side belongs to the first group and has both spherical surfaces. The first lens is a glass lens in response to weather resistance requirements.

  In addition, the first lens has a strong refractive power, and uses a glass material with a high refractive index as much as possible to reduce the curvature while effectively bending the principal ray and preventing the deterioration of the axial aberration.

  Although it is conceivable to apply aspherical processing to the first lens, resin and mold glass that are easy to process generally have a low refractive index. Therefore, a glass material having a high refractive index is used as much as possible. However, when a glass material having a high refractive index is used, aspherical surface creation is expensive. Therefore, it is desirable that the first lens is composed of spherical surfaces that are easy to process.

  However, since the first lens made only of a spherical surface cannot sufficiently control the off-axis principal ray, the first lens made of high refractive index glass and made only of a spherical surface is relatively difficult to create an aspherical surface. A first lens is formed by combining a second lens made of easy resin or molded glass to achieve a desired purpose.

  The second lens has almost no refractive power, but preferably has a very large conic coefficient.

  In addition, it is possible to design a so-called rear stop lens in which the first lens and the second lens constitute one group and there is no lens in the rear group, but each image point is formed in an image sensor such as a CCD. It is desirable that the incident angles of the principal rays be as parallel as possible. In other words, the optical system is desirably close to image side telecentric, but in order to achieve this, it is desirable to use a positive lens in which the object side principal point is substantially at the stop position. Therefore, the arrangement of the three lenses constituting the first group and the second group is desirable, and the arrangement configuration of concave / convex (that is, concave lens, concave lens, stop, convex lens) is desirable.

  Here, a preferable “aspherical shape” will be described. The positive single lens (third lens) constituting the second group has a rotational hyperboloid shape on both surfaces, and the weak surface faces the diaphragm. For example, the action of this type of lens has a role of forming a virtual image formed at a finite distance by the front group as a real image on the imaging surface as described above.

  Further, when focusing on pupil imaging, this has a role of forming an image of a diaphragm as an exit pupil. In particular, at this time, it is desirable that the exit pupil is at ∞. In terms of pupil imaging, this shape is the configuration of an aplanatic single lens (that is, a lens in which aplanatic = spherical aberration and coma aberration are corrected), similar to the pickup lens of an optical disk.

  Speaking of the base curvature with respect to the aspherical surface, in order to reduce pupil spherical aberration, a strong surface is directed to the far side of the pupil conjugate distance. In general, it is known that spherical aberration is generated by becoming stronger than the curvature necessary for collecting parallel light at one point as it goes to the peripheral part of the surface. Therefore, by setting the conic constant k of each surface to be negative, keeping the central curvature as it is and reducing the peripheral curvature is effective in reducing pupil spherical aberration.

  In addition, regarding the fact that “various aberrations are corrected for a single wavelength”, in applications such as surveillance cameras and in-vehicle cameras, it has a very large angle of view (2ω> 160 °) and records moving images. From the need, lenses that are as bright as possible are needed.

  In such an application, in order to enable night photography, an image sensor having high sensitivity in the near infrared region is often used. As a result, the lens specification, unlike the scientific observation field and the art field such as photography, does not require much color correction in the entire visible range.

  In addition, the refractive index of the lens material generally increases rapidly as it approaches the ultraviolet region, but is characterized by a moderate decrease in the red to near infrared region. Therefore, if the spherical aberration is corrected for a single wavelength in the near infrared region corresponding to the characteristics of the image sensor, the axial aberration does not collapse so rapidly in the red to near infrared region.

  For the same reason, off-axis aberrations including chromatic aberration of magnification do not show so much collapse (the image is colored) in the red to near infrared region.

  In view of the above, a lens that “corrects various aberrations for a single wavelength in the near infrared region” is preferable.

The definition of the aspheric coefficient is as follows.

  Here, x is a sag amount in the optical axis direction of the surface. R is a radius of curvature. H = y ^ 2 + z ^ 2, which indicates that the surface shape is a rotationally symmetric shape. K is a conic coefficient and − (eccentricity) ^ 2. A, B, and C are fourth-order, sixth-order, and eighth-order aspherical coefficients, respectively, and represent the distance from the conical curve of the surface shape.

  In the present invention, the lens shape is not necessarily rotationally symmetric. In the field of use such as in-vehicle cameras and surveillance cameras, a so-called anamorphic surface is introduced, and the imaging magnification of each of the y-axis and z-axis perpendicular to the optical axis (x-axis) is changed, so that the subject can have a desired image circle Because it fits inside. Even if comprised in this way, imaging performance does not deteriorate and desired imaging performance can be achieved.

  The fisheye lens according to a preferred embodiment of the present invention is a fisheye lens based on equidistant projection.

  In a preferred embodiment of the present invention, the fisheye lens is composed of three lenses, has a super wide angle (2ω> 160 °), and is very bright (F / 3 or less).

  In a preferred embodiment of the present invention, the overall length is shorter than the image sensor (total length ≦ 10 f), various aberrations are corrected for a single wavelength in the near infrared region, and the image side is substantially telecentric (exit pupil). | T ′ |> 10f with respect to the position t ′, where f is a focal length).

  In a preferred embodiment of the present invention, a negative meniscus lens (first lens) made of glass and a lens (second lens) including an aspheric shape are provided on the object side across the stop. The first group G1 having a negative refractive power as a whole group is composed of a first lens and a second lens. The second group G2 is composed of a spherical or aspherical positive single lens (third lens) made of resin or moldable glass on the image side with the stop interposed therebetween.

  Here, the aspherical shape of the positive lens (third lens) is conic constant K <0, and conditions for power distribution and surface separation are given.

  If configured as described above, since there is no need for color correction, a super-wide-angle lens having a bright and very simple configuration can be obtained.

  In addition, the arrangement of the three lenses is concave / convex (that is, concave lens, concave lens, aperture, convex lens), and concave | convex / convex (concave lens, aperture, convex lens, convex lens) or concave / convex | (that is, concave lens, concave lens). , A convex lens, a diaphragm) can be made superior.

  First to third embodiments of the present invention will be described.

Table 1 shows the half angle of view of the fisheye lens, image circle, total length, F / no, focal length, exit pupil, fB, f1 to f3, f (G1), and conic coefficient, that is, K (S5) in the first to third embodiments. ) And K (S6), radius of curvature, ie R (S5) and R (S6), diameter / total length.

<First embodiment>
FIG. 1 schematically shows a fisheye lens according to a first embodiment of the present invention.

  FIG. 2 shows the spherical aberration of the fisheye lens shown in FIG.

  FIG. 3 shows the aspherical aberration of the fisheye lens shown in FIG.

  FIG. 4 shows the distortion of the fisheye lens shown in FIG.

Table 2 shows the surface data of the fisheye lens shown in FIG.

<Second embodiment>
FIG. 5 schematically shows a fisheye lens according to a second embodiment of the present invention.

  FIG. 6 shows the spherical aberration of the fisheye lens shown in FIG.

  FIG. 7 shows the aspherical aberration of the fisheye lens shown in FIG.

  FIG. 8 shows the distortion of the fisheye lens shown in FIG.

Table 3 shows surface data of the fisheye lens shown in FIG.

<Third embodiment>
FIG. 9 schematically shows a fisheye lens according to a third embodiment of the present invention.

  FIG. 10 shows the spherical aberration of the fisheye lens shown in FIG.

  FIG. 11 shows the aspherical aberration of the fisheye lens shown in FIG.

  FIG. 12 shows the distortion of the fisheye lens shown in FIG.

Table 4 shows the surface data of the fisheye lens shown in FIG.

  In Tables 2-4, R is a curvature radius. K is a conic coefficient and − (eccentricity) ^ 2. A, B, and C are fourth-order, sixth-order, and eighth-order aspheric coefficients, respectively, and represent the distance from the conical curve of the surface shape.

  The lenses shown in Examples 1 to 3 are all fisheye lenses based on equidistant projection.

<Lens surface molding method>
It is preferable that the spherical surfaces and aspheric surfaces of the first to third lenses (L1, L2, L3) are precisely measured by the following method and formed into desired shapes.

  FIG. 13 shows an example of an apparatus for measuring the shape of the convex surface of a lens according to the present invention.

  In FIG. 13, the rotating member 23 has a first contact 22 for contacting the aspherical second surface 10b of the lens 10 and measuring the aspherical lens 10 precisely, preferably a spherical contact member. The first contact 22 is brought into contact with the second surface 10 b of the aspherical lens 10, and the second contact 24 a on the linear length measuring device 26 side is in contact with the contact surface 23 b of the quadrangular columnar rotation member 23, The reaction force from the lens surface is measured by the linear length measuring device 26.

  A rotating member 23 is provided on a leaf spring 23c attached to the tip of the base portion 26a of the linear length measuring device 24.

  The contact point (spherical contact) of the first contactor 22 on the locus (dotted line in FIG. 13) of the second contactor 24a (spherical contact member) that swivels in response to the reaction force received from the second surface 10b of the aspherical lens 10. And the lens 10 is accurately measured.

  In this case, the first contact 22 (spherical contact member) makes point contact when contacting the second contact surface 10b of the aspherical lens 10, and is rectangular from the fulcrum 23a so as to draw an arc around the fulcrum 23a. The second contactor 24a (spherical contact member) swivels around the radius of the distance to the position where the second contactor 22 (spherical contact member) provided on the rotating member 23 has a shape, and the aspherical lens 10 is moved. Since the second contact 24a is disposed on the trajectory of the swivel movement of the first contact 22 about the fulcrum 23a while contacting the second surface 10b from the normal direction, the first spherical contact member 22 is transmitted to the linear movement of the second spherical contact member 24a, accurately transmits the pressing force from the second surface 10b of the aspherical lens 10, and accurately measures the aspherical lens 10. it can. The first contactor 22 and the second contactor 24a accurately follow the second surface 10b of the aspherical lens 10 by a plate-like member 23c such as a plate spring that supports the quadrangular columnar rotation member 23. And aspherical shape can be measured accurately.

  FIG. 14 shows an example of an apparatus for measuring the shape of the concave surface of a lens. In the case of the lens 50 having the concave surface 51, the contact itself does not enter the concave surface 51 of the lens 50 well in the conventional shape measuring apparatus, and the shape of the lens 50 and the inclination of the concave and convex portions cannot be measured accurately. However, the measuring device of FIG. 14 uses a needle-like contact as the first contact 53, and the support member that supports the first contact 53 can be rotated about the fulcrum 55 with respect to the base 58. The rotating member 54 is configured. The first contact 53 is brought into contact with the concave surface 51 of the lens 50. A spherical second contact 56 is arranged at a position corresponding to the back side of the needle-shaped first contact 53 with a rotating member 54 interposed. Then, by transmitting the movement (arc-shaped movement) of the needle-like first contactor 53 to the spherical second contactor 56, the concave surface 51 of the lens 50 is accurately measured. Reference numeral 57 denotes a part of the rod-like member of the linear side length device.

  In FIG. 15, the needle-like first contactor 53 is brought into contact with the convex surface 61 of the lens 60, and the movement (arc-shaped movement) of the needle-like first contactor 53 is transmitted to the spherical second contactor 56. Thus, the shape of the convex surface 61 of the lens 60, the inclination of the unevenness, and the like are precisely measured. In this case, since the needle-like first contactor 53 can be accurately brought into contact with the vicinity of the boundary between the convex surface 61 and the flat surface 63, the boundary between the convex surface 61 and the flat surface 63 and the inclination of the concave and convex portions are, for example, 60 degrees or more. An aspherical shape with a high contact angle can be measured with extremely high accuracy.

  Preferably, the lens grinding apparatus includes the above-described measuring device. While grinding the lens with a grinding wheel, measure the deflection and shape of the lens, and grind and correct the lens again.

  The lens polishing apparatus also includes the above-described measuring apparatus. While polishing the lens, measure the deflection and shape of the lens, and polish and correct the lens again.

  The measuring device described above can also be applied to measure a concave surface or a convex surface of a mold for manufacturing a lens with a mold.

  Details of such a measuring apparatus are described in Japanese Patent Application No. 2006-270572 and drawings. The description is made a part of the description in the present specification and drawings.

  Various conventional grinding and polishing methods can be employed. As an example, an aspheric lens polishing method described in Japanese Patent No. 3756130 can be given.

  In addition, this invention is not limited to the above-mentioned Example. For example, a lens with a more limited angle of view can be easily designed based on the aforementioned conditions.

1 shows a fisheye lens according to a first embodiment of the present invention. The spherical aberration and sine condition of the fisheye lens shown in FIG. 1 are shown. The astigmatism of the fisheye lens shown in FIG. 1 is shown. The distortion aberration of the fisheye lens shown in FIG. 1 is shown. 3 shows a fisheye lens according to a second embodiment of the present invention. The spherical aberration and sine condition of the fisheye lens shown in FIG. 5 are shown. 6 shows astigmatism of the fisheye lens shown in FIG. 6 shows distortion of the fisheye lens shown in FIG. 7 shows a fisheye lens according to a third embodiment of the present invention. The spherical aberration and sine condition of the fisheye lens shown in FIG. 9 are shown. 10 shows astigmatism of the fisheye lens shown in FIG. 10 shows distortion of the fisheye lens shown in FIG. An example of the lens surface shape measuring apparatus used when manufacturing the fisheye lens of the present invention is shown. The other example of the lens surface shape measuring apparatus used in manufacturing the fisheye lens of the present invention will be described. Still another example of a lens surface shape measuring apparatus used in manufacturing the fisheye lens of the present invention will be described.

Explanation of symbols

L1 1st lens L2 2nd lens L3 3rd lens

Claims (7)

  The first lens located on the most object side is composed of a three-lens lens system. The first lens is a glass lens with a high refractive index, both surfaces are spherical, and the second lens is a resin lens or a molded glass lens. A fisheye lens having an aspherical shape and having a super-wide angle of 160 ° or more in total angle of view.   2. The fisheye lens according to claim 1, wherein a concave first lens, a concave second lens, a stop, and a convex third lens are arranged in this order.   The fisheye lens according to claim 1 or 2, wherein the third lens is a resin lens or a molded glass lens.   The total length is shorter than that of the image sensor, and various aberrations are corrected for a single wavelength in the near-infrared region, and expressed by | t ′ |> 10f (f is a focal length) with respect to the exit pupil position t ′. The fisheye lens according to any one of claims 1 to 3, wherein the image side of the image is substantially telecentric.   The first lens and the second lens constitute a first group (G1) having a negative refractive power as a whole group, and the third lens is a positive spherical or aspherical surface on the image side across the stop. The fisheye lens according to any one of claims 1 to 4, comprising a second group (G2) made of a single lens.   The fisheye lens according to claim 1, wherein the first lens is a negative meniscus lens.   The fisheye lens according to claim 1, wherein the fisheye lens is based on equidistant projection.

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