Zoom lens

Abstract: A zoom lens is provided, in which a fixed first lens group (1) having a positive refracting power, a second lens group (2) having a negative refracting power and which realizes a variable magnification, a fixed third lens group (3) having a positive refracting power and a fourth lens group (4) having a positive refracting power which compensates fluctuation of an image surface caused by the variable magnification and which focuses an image are sequentially provided from an object side. In this zoom lens, the third lens group (3) is composed of two separate lenses comprising a positive lens (31) and a negative lens (32). At least one surface of the positive lens (31) is formed as an aspherical surface and a surface of the negative lens (32) opposing the image surface side is concaved to the image surface side. Thus, the zoom lens of this invention can be miniaturized and reduced in weight without deteriorating the characteristics which comprise a large variable magnification and that a small F number.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention generally relates to zoom lenses and, more particularly, is directed to a zoom lens for use with electronic still cameras, video cameras or the like, for example. Description of the Related Art At present, consumer video cameras employ a lens having a zooming ratio of 6 times or more and whose F number is smaller than 2.0 as a zoom lens. Recently, it has been requested that the video camera be still further miniaturized and reduced in weight while maintaining the features such that the zoom lens has a large variable magnification and has also a large brightness. To meet the requirements such that the video camera is miniaturized and reduced in weight, the imager size of a charge-coupled device (CCD) image pickup device utilized as the image pickup element should be more miniaturized because it is to be noted from a scaling law standpoint that a camera lens itself can be miniaturized in accordance with the reduction of the imager size. The conventional zoom lens, however, comprises 10 spherical lenses or more. There is then the disadvantage that a conventional lens cannot be miniaturized and reduced in weight sufficiently. Further, when the camera lens is miniaturized from a scaling law standpoint, then precision prescribed of each of the lenses constructing the zoom lens also is increased so that, if the number of the lenses constructing the zoom lens remains the same, then the zoom lens cannot be processed without difficulty. Particularly, when a lens element is finished as a spherical surface by the polishing-treatment, if the radius of curvature of the spherical surface is reduced, there is then the risk that a manufacturing cost for achieving the precision prescribed is increased considerably. In order to remove the above-mentioned disadvantage and risk, a method of manufacturing a lens by molding some suitable materials such as plastics, glass or the like has been established recently. According to this method of manufacturing the lens by the molding-process, there is then the possibility that upon mass-production the manufacturing cost of the lens can be reduced as compared with the above-mentioned method of manufacturing the lens by the polishing-process. Further, in accordance with the molding-process, the lens surface can be formed as an aspherical surface with ease and the number of lens elements constructing the camera lens can be reduced, which can make a great contribution to the camera lens when the camera lens is miniaturized and reduced in weight. For an example of a prior art zoom lens composed of four lens groups, reference may be made to US-A-4 859 042. All the lens groups except the second have a positive refractive power. The first and third lens groups are stationary with respect to the optical axis, while the second and fourth lens groups are axially displaceable to effect zooming. The fourth lens group is also axially displaceable on its own to effect focusing. WO-A-8706717 discloses an infrared afocal zoom telescope. OBJECTS AND SUMMARY OF THE INVENTION Therefore, it is an object of the present invention to provide an improved zoom lens in which the aforesaid shortcomings and disadvantages encountered with the prior art can be eliminated. More specifically, it is an object of the present invention to provide a zoom lens which can be miniaturized while maintaining the characteristics such that the variable magnification thereof is large and that the F number thereof is small. Another object of the present invention is to provide a zoom lens which can be reduced in weight while maintaining the characteristics such that the variable magnification thereof is large and that the F number thereof is small. A further object of the present invention is to provide a zoom lens whose full length can be reduced. Still another object of the present invention is to provide a zoom lens in which bad influence such as ghost or the like can be reduced. In the zoom lens of the present invention, there are sequentially provided from an object side a fixed first lens group having a positive refracting power, a second lens group having a negative refracting power and which realizes a variable magnification, a fixed third lens group having a positive refracting power and a fourth lens group having a positive refracting power which compensates fluctuation of an image surface caused by the variable magnification and which focuses an image. In this zoom lens, the third lens group comprises a positive lens of which at least one surface is formed as an aspherical surface and a negative lens having a surface, opposing the image surface side, formed to be concave to the image surface side. Thus, the zoom lens of this invention can be miniaturized and reduced in weight without deteriorating the characteristics such that the variable magnification is large and the F number is small. BRIEF DESCRIPTION OF THE DRAWINGS The following detailed description, given by way of example and not intended to limit the present invention solely thereto, will best be understood in conjunction with the accompanying drawings in which: FIG. 1 is a schematic representation of an embodiment of a zoom lens according to the present invention; FIG. 2 is a schematic diagram showing an allocation of shape parameters in the arrangement of FIG. 1; FIGS. 3A through 3C are respectively schematic diagrams showing spherical aberration, astigmatism and distortion presented when the zoom lens of this embodiment is placed in the wide angle lens position; FIGS. 4A through 4C are respectively schematic diagrams showing spherical aberration, astigmatism and distortion presented when the zoom lens of this embodiment is placed in the intermediate position; and FIGS. 5A through 5C are schematic diagrams showing spherical aberration, astigmatism and distortion presented when the zoom lens of this embodiment is placed in the telephoto lens position, respectively. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Prior to the description of the preferred embodiment, a fundamental principle of the present invention will be described below. As shown in FIG. 1, the present invention is directed to a zoom lens in which a fixed first lens group 1 having a positive refracting power, a second lens group 2 having a negative refracting power and which realizes a variable magnification, a fixed third lens group 3 having a positive refracting power and a fourth lens group 4 having a positive refracting power which compensates fluctuation of an image surface caused by the variable magnification and which focuses an image are sequentially provided from an object side. In this zoom lens, the third lens group 3 is composed of two separate lenses comprising a positive lens 31 and a negative lens 32. Further, according to the present invention,the shape of the surface 31a of the positive lens 31 opposing the object side is expressed by the following equation (4): Z = Y²/R31 1 + 1-(K31+1)Y²/R31² + (AD31)Y⁴ + (AE31)Y⁶ + (AF31)Y⁸ + (AG31)Y¹⁰ where Y represents the height from the optical axis, Z represents the distance of lens apex from the tangential surface in a point on the aspherical surface in the height Y from the optical axis where the object side is assumed to be negative, R31 represents the paraxial radius of curvature, K31, AD31, AE31, AF31 and AG31 respectively represent coefficients . The shape of the surface 31b of the positive lens 31 opposing the image surface side is expressed by the following equation (5): Z = Y²/R32 1 + 1-(K32+1)Y²/R32² + (AD32)Y⁴ + (AE32)Y⁶ + (AF32)Y⁸ + (AG32)Y¹⁰ where Y represents the height from the optical axis, Z represents the distance of lens apex from the tangential surface in a point on the aspherical surface in the height Y from the optical axis where the object side is assumed to be negative, R32 represents the paraxial radius of curvature, K32, AD32, AE32, AF32 and AG32 respectively represent coefficients.The shape of the positive lens 31 satisfies the following relation expressed by the equation (6) below: -K31/(8R31³) - AD31 + K32/(8R32³) + AD32 > 0 Further, according to the present invention, when R34 (corresponding to R13) represents the radius of curvature of the surface of the negative lens 32, the positive lens 31 being convexed to the object side, fw represents the focal length of the wide angle lens end of the zoom lens on the whole and ft represents the vocal length of the telephoto lens end of the zoom lens on the whole, the following relation is established : 0 < R34 < 6 fw·ft As described above, the present invention employs as a fundamental arrangement a zoom lens of a rear-focus system formed of four lens groups, that is of positive, negative, positive and positive lens groups composed of a fixed first lens group 1 having a positive refracting power, a second lens group 2 having a negative refracting power as a variator, a fixed third lens group 3 having a positive refracting power and a fourth lens group 4 having a positive refracting power which includes both a compensator function for correcting a movement of an image surface by the variable magnification of the second lens group so as to keep the image surface position constant and a focusing function. Further, according to the present invention, since the zoom lens of the present invention is of the telephoto lens type in which the third lens group 3 is formed of separate positive lens 31 and negative lens 32 provided from the object side, a principal point position of the third lens group 3 is provided near the object side. Thus, the movable range of the second lens group 2 can be extended effectively and the full length of the zoom lens can be reduced. Also, since the zoom lens of the present invention is of the telephoto lens type, a bundle of light incident on the fourth lens group 4 can be made smaller than that on the third lens group 3 and therefore the fourth lens group 4 can be reduced in weight. As a result,the burden on the fourth lens group driving system can be reduced, the responsiveness to the magnification and focusing can be improved and a lens barrel structure including the lenses and the driving system can be reduced in diameter. Further, since the third lens group 3 is comprised of two lenses, the number of the lenses in the zoom lens can be reduced on the whole and the entirety of the zoom lens can be made more compact. Further, the shapes of one surface 31a and the other surface 31b of the positive lens 31 are respectively expressed by the above equations (4) and (5) and the relationship expressed by the equation (6) is also established, which means that at least one surface of the positive lens 31 is formed as the aspherical surface.In this case, such an aspherical surface acts as spherical aberration so that, by the combination of the positive lens 31 and the negative lens 32 within the third lens group 3, the correction of chromatic aberration on the axis which is the same aberration on the axis can be carried out independently of the correction of the spherical aberration. Accordingly, there is more freedom in the aberration correction of the third lens group 3. As a result, the burden on other lens groups, particularly on the fourth lens group 4 which focuses an image from an aberration correction standpoint can be reduced and therefore the focal length of the fourth lens group 4 can be reduced, the full length of the zoom lens being thus reduced. Furthermore, in the zoom lens of a so-called rear focus system formed of four lens groups of positive, negative, positive and positive lens groups, the front three lens groups of the first to third lens groups 1 to 3 constitute an afocal system. That is, a bundle of light from the third lens group 3 becomes a parallel bundle of light. In this case, if the surface of the negative lens 32 in the third lens group 3 opposing the image side is shaped as substantially plane, then a re-focusing point of a reflected light from a target screen is formed near the target screen, causing a deterioration of image quality such as ghost or the like. If the surface of the negative lens 32 opposing the image side is convexed to the image side, then the re-focusing point is formed at the rear side of the target screen as viewed from the object side. However, if the fourth lens group 4 is moved toward the object side in the wide angle lens mode, the above re-focusing point is moved near the focal point or a bundle of light converged is passed through the target screen so that problems such as ghost or the like occur. On the other hand, if the radius of curvature of the surface of the negative lens 32 opposing the image side is selected so as to satisfy the condition expressed by the above equation (7) and this surface is concaved to the image side, then a re-focusing point of a reflected light from the target screen is formed on the object side rather than the focal point. Accordingly, since only a weak diverging light from the re-focusing point is irradiated on the target screen, an offending ghost or the like can be prevented from being produced. An embodiment of the present invention will now be described with reference to the drawings. FIG. 1 shows a zoom lens according to the embodiment of the present invention. As shown in FIG. 1, a fixed first lens group 1 having a positive refracting power, a second lens group 2 having a negative refracting power as a variator, a fixed third lens group 3 having a positive refracting power and a fourth lens group 4 having a positive refracting power acting as a compensator are provided in that order from the object side to an image surface 5. Assuming that components of arrows A1 and A2 perpendicular to the optical axis of the zoom lens correspond to magnifications and that components of arrows A1 and A2 parallel to the optical axis of the zoom lens correspond to the movements of the lens groups, then upon variable magnification the second lens group 2 is moved in response to the arrow A1 and the fourth lens group 4 is moved in response to the arrow A2 so as to compensate a fluctuation of the image surface 5 upon variable magnification. The fourth lens group 4 is moved solely in response to an arrow A3 so that it may have a focusing function. That is, the zoom lens of this embodiment is composed of four lens groups of positive, negative, positive and positive groups and is formed as a rear-focus system which belongs, for example, to a so-called inner-focus system. Also, as shown in FIG. 1, an iris 6 is provided between the second lens group 2 and the third lens group 3, and a glass block 7 is provided between the fourth lens group 4 and the image surface 5 as an optical low-pass filter. Further, the first lens group 1 is comprised of a negative meniscus lens 11 whose convex surface is opposed to the object side and a positive lens 12 bonded to the negative meniscus lens 11. The second lens group 2 is comprised of a negative meniscus lens 21 whose convex surface is opposed to the object side and a positive lens 23 bonded to the negative meniscus lens 21. The third lens group 3 is comprised of two separated lenses of a positive lens 31 and a negative lens 32 from the object side. The fourth lens group 4 is comprised of a negative meniscus lens 41 whose convex surface is opposed to the object side and a positive lens 42 bonded to the meniscus lens 41. Both surfaces of the positive lens 31 of the third lens group 3 are formed as aspherical surfaces, and the iris 6 is located just in front of the positive lens 31 in an opposing relation to the object side. There is then the advantage that spherical aberration can be satisfactorily corrected while aberration outside the optical axis is not deteriorated by the aspherical surface. FIG. 2 shows the allocation of shape parameters of respective elements in the zoom lens according to this embodiment.As shown in FIG. 2, Ri represents the radius of curvature of an i'th surface (i = 1, 2, ..., 18) and di represents the space between the i'th surface and (i + 1)'th surface, respectively. Further, ni and νi represent the refractive index of d line of medium between the i'th surface and (i + 1)'th surface and Abbe number, respectively. In this case, third, tenth, eleventh and sixteenth surfaces are formed of aspherical surfaces, respectively and the shape of the aspherical surface can be expressed by the following equation (8): Z = Y²/Ri 1 + 1-(Ki+1)Y²/ Ri² + (ADi)Y⁴ + (AEi)Y⁶ + (AFi)Y⁸ + (AGi)Y¹⁰ In the above equation (8), Y represents the height from the optical axis, Z represents the distance of lens apex from the tangential surface in a point on the aspherical surface in the height Y from the optical axis, Ri (i = 3, 10, 11, 16) represents the paraxial radius of curvature, Ki represents the constant of the cone of the i'th surface and ADi, AEi, AFi and AGi represent aspherical surface constants of the i'th surface, respectively. These aspherical surface coefficients are set as follows: In that case, the 10'th surface and the 11'th surface are respectively the surfaces of the positive lens 31 of the third lens group 3 in an opposing relation to the object side and the image surface side and a condition expressed by the following equation (9) is imposed between the above two surfaces: -K10/(8R10³) - AD10 + K11/(8R11³) + AD11 > 0 Then, the values of the respective elements shown in FIG. 2 are set as follows: In the above table, values of Ri corresponding to i (i = 3, 10, 11, 16) as shown by symbols * respectively represent radiuses of reference spherical surfaces of aspherical surfaces, respectively and the fact that R9 is ∞ corresponds to the fact that the iris is planar. Also, blanks in ni and νi correspond to areas in which a medium is air. The whole focal length f of the zoom lens according to this embodiment lies in a range of from 10 to 60 mm, F number lies in a range of from 2.0 to 2.8 and a semi field angle w lies in a range of from 23.20 to 4.09°. Further, values of the above-mentioned variable spacings di (i = 3, 8, 13, 16) in the wide angle lens position, the intermediate position and in the telephoto lens position are set as follows: If the above-mentioned values are utilized, then the focal length ft on the telephoto lens side becomes 60 mm, and the focal length fw on the wide angle lens side becomes 10 mm. Further, since a spacing ZL from the first surface of the lens to the focusing position is 88.5 mm,the zooming magnification ft/fw is 6 and the following equation is established: ZL/ft = 1.48 As described above, according to this embodiment, while maintaining the characteristics such that the zooming magnification is large and that the F number is small, the full length of the zoom lens can be reduced considerably and the zoom lens can be miniaturized. The radius of curvature R13 of the surface of the negative lens 32 of the third lens group 3 opposing the image surface 5 side according to this embodiment will be described with reference to FIG. 1. In the zoom lens according to this embodiment, the first to third lens groups constitute an afocal system so that, when a light bundle 1 incident on the first lens group 1 is a parallel light bundle (particularly a light bundle emitted from one point of the object located in the distant position can be regarded as a parallel light bundle in front of the lens), a light bundle 2 emitted from the third lens group 3 to the fourth lens group 4 becomes close to a parallel light bundle. This parallel light bundle 2 is converged at one point on the image surface 5 through the fourth lens group 4. In that case, a reflected light from the image surface 5 is returned through an opposite optical path to the negative lens 32 as a parallel light bundle 3 so that, if the surface of the negative lens opposing the image surface 5 side is formed with a shape substantially equal to the plane, then a ghost or the like is produced on the image surface 5 by the reflected light of the negative lens 32. In order to avoid the occurrence of ghost or the like, the radius of curvature R13 of the surface of the negative lens 32 opposing the image surface 5 side must satisfy the condition expressed by the following equation (10): 0 < R13 < 6 fw·ft where the sign of the radius of curvature is determined so as to become a positive value when the lens surface becomes convexed to the object side. The above equation (10) means that LF < LI is established when the surface of the negative lens 32 opposing the image surface side becomes convexed relative to the image surface 5 side, LF represents the distance to a point at which a light bundle 4 is converged from the negative lens 32 and the distance LI represents the distance between the negative lens 32 and the image surface 5. Accordingly, a weak reflected light, which is gradually diffused, is irradiated on the image surface 5 and the influence of ghost or the like can be reduced. FIGS. 3A to 3C, FIGS. 4A to 4C and FIGS. 5A to 5C respectively show various aberrations of the zoom lens according to this embodiment. FIGS. 3A, 3B and 3C show spherical aberration, astigmatism and distortion in the wide angle lens position (f = 10.0 mm), respectively. FIGS. 4A, 4B and 4C show those in the intermediate position (f = 30.3 mm), and FIGS. 5A, 5B and 5C show those in the telephoto lens position (f = 60.0 mm), respectively. According to the zoom lens of the present invention, in the zoom lens formed of four lens groups of positive, negative, positive and positive lenses, since the third lens component is composed of two lenses of positive and negative lenses having aspherical surfaces, there is then the advantage that the zoom lens can be miniaturized and reduced in weight while maintaining the characteristics such that the variable magnification ratio is large and that the F number is small. Further, when the surface of the negative lens opposing the image surface side is convexed to the image surface side, then bad influence such as ghost or the like due to the reflected light from the negative lens can be reduced. Having described the preferred embodiment of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to that precise embodiment and that various changes and modifications thereof could be effected by one skilled in the art without departing from the scope of the invention as defined in the appended claims.

Claims: A zoom lens in which are sequentially provided, from an object side, a fixed first lens group (1) having a positive refracting power, a second lens group (2) having a negative refracting power and which realizes a variable magnification, a fixed third lens group (3) having a positive refracting power and a fourth lens group (4) having a positive refracting power which compensates fluctuation of an image surface caused by the variable magnification and which focuses an image, characterized in that said third lens group (3) is composed of two separate lenses comprising a positive lens (31) and a negative lens (32), said positive lens (31) having at least one surface (31a, 31b) formed as an aspherical surface, and said negative lens (32) having a surface opposing the image side formed to be concave to the image side. A zoom lens according to claim 1, wherein the shape of the surface (31a) of said positive lens (31) of said third lens group (3) opposing the object side is expressed by the following equation (1) : Z = Y²/R31 1 + 1-(K31+1)Y²/R31² + (AD31)Y⁴ + (AE31)Y⁶ + (AF31)Y⁸ + (AG31)Y¹⁰ where Y represents the height from the optical axis, Z represents the cistance of lens apex from the tangential surface in a point on the aspherical surface in the height Y from the optical axis where the object side is assumed to be negative, R31 represents the paraxial radius of curvature, K31, AD31, AE31, AF31 and AG31 respectively represent coefficients, wherein the shape of the surface (31b) of said positive lens (31) opposing the image surface side is expressed by the following equation (2) : Z = Y²/R32 1 + 1-(K32+1)Y²/R32² + (AD32)Y⁴ + (AE32)Y⁶ + (AF32)Y⁸ + (AG32)Y¹⁰ where Y represents the height from the optical axis, Z represents the distance of lens apex from the tangential surface in a point on the aspherical surface in the height Y from the optical axis where the object side is assumed to be negative, R32 represents the paraxial radius of curvature, K32, AD32, AE32, AF32 and AG32 respectively represent coefficients, and wherein the shape of said positive lens (31) satisfies the following relation: -K31/(8R31³) - AD31 + K32/(8R32³) + AD32 > 0. A zoom lens according to claim 1, wherein R13 represents the radius of curvature of the surface of said negative lens (32) opposing the image side, wherein the positive lens (31) is convex to the object side, wherein fw represents the focal length of the zoom lens on the whole in the wide angle position and ft represents the focal length of the zoom lens on the whole in the telephoto position, the following relation is satisfied: 0 < R13 < 6 fw·ft .

Inventors: NAKAMURA AKIRA, NAKANO YUICHI, NAKAMURA, AKIRA, NAKANO, YUICHI
Applicants: SONY CORP, SONY CORPORATION
Application year: 1991
Main classification: G02B15
IPC codes: G02B15, G02B13