Zoom lens with aspheric surface

Abstract: In a zoom lens having a lens unit (G₁) of negative refractive power most adjacent to the object side, distortion can be corrected better and an excellent imaging performance is maintained. In a zoom lens having a lens unit of negative refractive power most adjacent to the object side, a negative lens (L₁) whose image side surface in contact with air is formed by a concave surface is provided in the lens unit of negative refractive power, and the shape of the concave surface of the negative lens is made into an aspherical surface which satisfies the condition that -5<κ<0.75 with regard to a cone constant κ and of which the curvature monotonously decreases away from the optical axis.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a zoom lens for use, for example, in a single-lens reflex camera, an electronic still camera, a video camera or the like. Related Background Art Some of zoom lenses in which a lens unit having negative refractive power precedes have a relatively great angle of view. In order to obtain a great back focal length, this forward negative refractive power has the action of decrease the angle of view relative to a rearward lens unit and turning the correction of curvature of image field and Petzval sum to the negative, and is suitable for aberration correction of a wide angle of view. Therefore, a lens construction in which a lens unit of negative refractive power is disposed most adjacent to the object side is adopted as a lens for a wide angle of view. In this case, however, negative distortion occurs due to the negative refractive power most adjacent to the object side, and the angle of view increases and the bad influence thereof appears more remarkably. Therefore, use is made of a technique of disposing a positive lens near a negative lens to thereby mitigate the turning of distortion to the negative and correct distortion. This technique is considerably effective for aberration correction, but the addition of the positive lens increases the number of lenses and increases the volume of the lens, and this has led to the problem that the construction of the lens becomes complicated and the manufacturing cost is increased. Also, in a super-wide angle zoom lens, the correction of distortion is still insufficient even with the above-described correcting method, and the designing thereof has been extremely difficult and it has been difficult to maintain a good imaging performance. US-A-5 046 833 describes a zoom lens system consisting of, in order from the object side, a first lens unit of negative refractive power, a first lens unit of positive refractive power, a second lens unit of negative refractive power and a second lens unit of positive refractive power. The first negative lens unit consists of a negative lens component and positive and negative lens components on the image side thereof. The second positive lens unit consists solely of a radial GRIN (graded refractive index) lens. FR-A-2 466 785 describes a variable focus objective lens configuration for use with colour television cameras. The configuration comprises three movable groups between two stationary groups. The foremost stationary group consists of a stationary diverging meniscus lens. The rearmost group consists of a stationary combination of converging and diverging lenses. The three movable groups between the two stationary groups are constituted by a quasi plano-convex converging lens, a bi-convex converging lens and a bi-concave diverging lens. Various surfaces of the lens elements are aspherical, including the image side surface of the foremost stationary lens, the object side surface of the foremost movable lens, the object side surface of the second movable lens, the image side surface of the third movable lens and the object side surface of the rearmost stationary combination of lenses. SUMMARY OF THE INVENTION It would be beneficial to provide a zoom lens having a lens unit of negative refractive power most adjacent to the object side and which can correct distortion better and has an excellent imaging performance. The present invention is set out in the appended claims. The zoom lens according to an embodiment of the present invention is a zoom lens having a lens unit of negative refractive power most adjacent to the object side and a lens unit of positive refractive power disposed on the image side thereof, and when the focal length of the entire system is to be varied, the positive lens unit and the negative lens unit are removed relative to each other along the optical axis. The lens unit of negative refractive power has a negative lens having its image side surface in contact with air formed by a concave surface, and the shape of the concave surface of said negative lens is such that the curvature of the concave surface of said negative lens monotonically decreases away from the optical axis. Specifically, when the distance in the direction of the optical axis from the vertex of the curved surface at a distance y on the curved surface from the optical axis is x and the curvature of said curved surface at the vertex thereof is C and C4, C6, ... are constants and κ is a cone constant and x is expressed as x=Cy 2 /{1+(1-κC 2 y 2 ) 1/2 }+C4y 4 +C6y 6 + ..., the shape of the concave surface of said negatie lens is set within a range represented by -5<κ<0.75 By constructing the aspherical shape of the image side concave surface of the negative lens in the lens unit most adjacent to the object side as described above, it becomes easy to correct distortion well even for a wide angle of view. A sharp increase in distortion with an increase in the angle of view is attributable to the fact that the curvature of the spherical surface is constant. The aspherical surface heretofore introduced into the negative lens unit most adjacent to the object side with a view to correct distortion has been such that a spherical surface is adopted as the basic shape of the curved surface and yet the displacement from this spherical surface is small. Therefore, it has had only such a degree of meaning that auxiliary aberration correction is effected by the aspherical surface, for a system comprised of only a spherical surface. In contrast, in the present invention, with regard to the shape of the concave surface of the negative lens, the basic shape is made not spherical as expressed by expression (1) above. Although the action of the aspherical shape in the present invention will hereinafter be described, the lens is rotation-symmetrical with respect to the optical axis and therefore, it will suffice to prescribe only a two-dimensional curve in the meridional cross-section. According, in the following description, the shape of a rotation-symmetrical aspherical surface is evaluated by a consideration on the curve of the meridional cross-section. Generally, a quadratic curve is divided broadly into a circle (k=1), an ellipse (1<k, 0<k<1), a parabola (k=0) and a hyperbola (k<0). Of these, it is three except a circle that the curvature monotonically decreases as it becomes far from the optical axis, but in the case of an ellipse, the major axis and the optical axis must be coincident becomes possible. If the upper limit of this condition is exceeded, the reference surface will become approximate to a spherical surface and therefore, the effect of the present invention will become small and the good correction of distortion will become difficult. On the other hand, if the lower limit of this condition is departed from, the hyperbola will become approximate to a straight line close to an asymptote and therefore, it will become difficult to suitably bend an oblique light beam over a wide range of the angle of view and the moderate correction of distortion, curvature of image field and coma will become difficult. When the second and subsequent high-order terms in the above-mentioned aspherical surface expression enter, the curve becomes not a quadratic curve, but under the condition that it is contained between two curves determined from the upper and lower limits of the value of κ by conditional expression (1) and the curvature monotonically decreases relative to the distance y from the optical axis, the curve may be modified as required by adding a high-order term regarding y, and of course, better aberration correction is possible because the degree of freedom of aberration correction becomes higher. In this case, more practically, it is preferable that k be in the range of 0≦k<0.75. With regard to the other basic construction as a zoom lens and the specific lens construction of each lens unit, it is possible to use heretofore known various means. In the basic construction of the present invention as described above, a super-wide angle zoom lens of which the angle of view exceeds 90° requires stronger negative refractive power in the forward unit thereof, but at this time, it becomes more difficult to correct distortion sufficiently. However, by introducing an aspherical surface as shown by the above-mentioned condition, necessary negative refractive power can be provided and the occurrence of distortion can be suppressed. Also, the correction effect for distortion by the aspherical negative lens in the present invention becomes greater as the angle of view becomes greater and particularly, a zoom lens comprising four negative, positive, negative and positive units in succession from the object side is effective for providing a super-wide angle, and as shown in embodiments which will be described later, a zoom lens having an angle of view exceeding 90° becomes possible. In such a zoom lens of four-unit construction, it is suitable to make a three-lens construction in which a negative meniscus lens having its convex surface facing the object side is disposed most adjacent to the object side of the first lens unit G1 of negative refractive power, an aspherical surface as described above is provided on the image side concave surface thereof and another negative lens and a positive lens are disposed on the image side thereof. In this construction, it is desirable to satisfy the following conditions: 3.5<fp/fw<4.5 2.5<|fn|/fw<4.5, where fp: the focal length of the positive lens in the first lens unit, fw: the focal length of the entire system at the wide angle side; fn: the focal length of the image side negative lens in the first lens unit. If the upper limit of conditional expression (2) above is exceeded, the focal length of the positive lens in the first lens unit will become too great as compared with the focal length of the entire system and therefore, not only Petzval sum will increase in the negative direction, but also introversive coma will become apt to occur in the ray flux under the principal ray and good correction will become difficult. If the lower limit of this condition is departed from, the focal length of the positive lens will become too small relative to the focal length of the entire system and not only Petzval sum will become excessively great in the positive direction, but also extroversive coma will occur in the ray flux under the principal ray at the wide angle end and therefore, good correction will become difficult. If the upper limit of conditional expression (3) above is exceeded, the refractive power of the image side positive lens in the first lens unit will become small relative to the refractive power of the entire system and Petzval sum will become great in the positive direction and also, extroversive coma will occur in the ray flux under the principal ray at the wide angle end. If conversely, the lower limit of this condition is departed from, the refractive power of the image side negative lens will become too strong relative to the refractive power of the entire system and Petzval sum will become great in the negative direction and introversive coma will occur in the ray flux under the principal ray at the wide angle end. A zoom lens comprising two negative and positive units in succession from the object side, a four-unit zoom lens having a unit construction of negative, positive, negative, positive is practical as a zoom lens having a lens unit of negative refractive power most adjacent to the object side, and in these zoom lenses, it is desirable to further satisfy the following condition: 0.5<|fasp/fw|<2.0, where fasp: the inverse number of the on-axis refractive power of the aspherical surface. If the upper limit of this condition is exceeded, the curvature of the aspherical surface will become small and the difference thereof from a spherical surface will become small and therefore, the effect of the aspherical surface according to the present invention will become small and the good correction of distortion will become difficult. If the lower limit of this condition is exceeded, appropriate bending in aberration correction will become unobtainable. The first lens unit having negative refractive power is effective to obtain a long back focal length particularly for a single-lens reflex camera, and the greater becomes the angle of view, that is, the smaller becomes the focal length of the entire system relative to the back focal length, the greater negative refractive power is required forwardly. This tendency differs more or less depending on brightness and construction, but it is preferable that the negative lens having the aspherical surface as described above which is included in the negative lens unit most adjacent to the object side further satisfy the following condition: 0.5<|fA/fw|<2.5, where fA : the focal length of the negative lens having the aspherical surface. If the upper limit of this condition is exceeded, sufficient negative refractive power for obtaining a great back focal length cannot be obtained or the difference from the prior art will become small. If the lower limit of this condition is exceeded, various aberrations occurring from the lens having this aspherical surface will become great and the correction thereof will become difficult. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows the lens construction of a first embodiment of the present invention. Figure 2 shows various aberrations at the wide angle end and telephoto end of the first embodiment. Figure 3 shows the lens construction of a second embodiment of the present invention. Figure 4 shows various aberrations at the wide angle end and telephoto end of the second embodiment. DESCRIPTION OF THE PREFERRED EMBODIMENTS Some embodiments of the present invention will hereinafter be described. Figure 1 shows the lens construction of a first embodiment and the movement locus of each lens unit by zooming. Figure 2 shows various aberrations at the wide angle side and telephoto side of the first embodiment. As shown in Figure 1, there are disposed, in succession from the object side, a first lens unit G1 of negative refractive power, a second lens unit G2 of positive refractive power, a third lens unit G3 of negative refractive power including a stop S, and a fourth lens unit G4 of positive refractive power. This embodiment is a four-unit zoom lens in which the spacing d6 between the first lens unit G1 and the second lens unit G2, the spacing dll between the second lens unit G2 and the third lens unit G3 and the spacing d16 between the third lens unit G3 and the fourth lens unit G4 vary during zooming. Also, at this time, the second lens unit G2 and the fourth lens unit G4 are moved together. The first lens unit G1 comprises, in succession from the object side, a negative meniscus lens L1 having its convex surface facing the object side, a biconcave lens L2 and a positive meniscus lens L3 having its convex surface facing the object side. The second lens unit G2 comprises, in succession from the object side, a cemented lens L4 comprising a negative meniscus lens L41 having its convex surface facing the object side and a biconvex lens L42, the lenses L41 and L42 being cemented together, and a positive meniscus lens L5 having its convex surface facing the object side. The third lens unit G3 comprises a cemented lens L6 comprising a biconvex lens L61 and a biconvex lens L62 cemented together, and a positive meniscus lens L7 having its convex surface facing the object side. The fourth lens unit G4 comprises, in succession from the object side, a biconvex lens L8, a biconcave lens L9, a positive meniscus lens L10 having its concave surface facing the object side, and a cemented lens L11 comprising a biconvex lens L111 and a negative meniscus lens L112 having its concave surface facing the object side, the lenses L111 and L112 being cemented together. The focal length f of the entire system in the embodiment of Figure 1, F value indicative of the brightness thereof, the angle of view 2ω, the radius of curvature ri, the spacing di between adjacent surfaces, the refractive index ndi, the Abbe number udi and the aspherical surface coefficient are set as follows. The Abbe number νdi of air is represented as a blank. The second surface given a mark * is an aspherical surface, and the value of r2 which is the radius of curvature thereof means the radius of curvature on the optical axis. The values of d6, d11, d16 and back focal length d25 varied by zooming are shown in the table below in the relation thereof with the focal length of the entire system. (First Embodiment) Focal length f=20.6-34 F NO.=2.9 2ω=94.5°-64.6° i ri di ndi νdi 1 63.569 2.500 1.77279 49.45 *2 15.672 12.000 1.00000 3 528.969 2.200 1.84042 43.35 4 65.982 3.500 1.00000 5 43.826 6.000 1.80518 25.35 6 104.254 variable 1.00000 7 50.271 1.200 1.80518 25.41 8 23.800 5.500 1.61720 54.01 9 -70.110 1.913 1.00000 10 34.954 3.000 1.72300 53.93 11 83.652 variable 1.00000 12 -63.000 1.200 1.77279 49.45 13 33.400 4.000 1.80518 25.35 14 -467.820 0.200 1.00000 15 97.414 2.000 1.77279 49.45 16 43.068 variable 1.00000 17 46.500 3.500 1.67003 46.42 18 -66.364 0.500 1.00000 19 -530.788 2.000 1.80384 33.89 20 36.947 3.000 1.00000 21 -240.000 3.000 1.69350 53.72 22 -53.246 0.200 1.00000 23 62.000 9.000 1.59319 67.87 24 -19.800 1.700 1.80458 25.50 25 -49.454 κ=0.45 C4=0.000 C6=0.000 C8=0.000 C10=0.000 Focal length f of the entire system 20.6 28 34 d6 17.827 6.741 0.619 d11 1.379 6.339 10.198 d16 9.624 4.664 0.805 d25 38.246 44.445 49.269 fW=20.6, fA=-27.54 |fA/fW|=0.765 fasp=-20.280, |fasp/fw|=0.985 fp=89.922, fp/fw=4.365 fn=-89.894, |fn|/fw=4.364 Figure 3 shows the lens construction of a second embodiment and the movement locus of each lens unit by zooming. Figure 4 shows various aberrations at the wide angle side and telephoto side of the fifth embodiment. As shown in Figure 3, there are disposed, in succession from the object side, a first lens unit G1 of negative refractive power, a second lens unit G2 of positive refractive power, a third lens unit G3 of negative refractive power including a stop S, and a fourth lens unit G4 of positive refractive power. This embodiment is a four-unit zoom lens in which the spacing d6 between the first lens unit G1 and the second lens unit G2, the spacing dll between the second lens unit G2 and the third lens unit G3 and the spacing d14 between the third lens unit G3 and the fourth lens unit G4 vary during zooming. Also, at this time, the second lens unit G2 and the fourth lens unit G4 are moved together. The first lens unit G1 comprises, in succession from the object side, a negative meniscus lens L1 having its convex surface facing the object side, a biconcave lens L2 and a positive meniscus lens L3 having its convex surface facing the object side. The second lens unit G2 comprises, in succession from the object side, a cemented lens L4 comprising a negative meniscus lens L41 having its convex surface facing the object side and a biconvex lens L42, the lenses L41 and L42 being cemented together, and a biconvex lens L5. The third lens unit G3 comprises a cemented lens L6 comprising a biconcave lens L61 and a positive meniscus lens L62 having its convex surface facing the object side, the lenses L61 and L62 being cemented together. The fourth lens unit G4 comprises, in succession from the object side, a biconvex lens L7, a negative meniscus lens having its convex surface facing the object side, a positive meniscus lens L9 having its concave surface facing the object side, and a cemented lens L10 comprising a biconvex lens L101 and a negative meniscus lens L102 having its concave surface facing the object side, the lenses L101 and L102 being cemented together. The focal length f of the entire system in the embodiment of Figure 3, F value indicative of the brightness thereof, the angle of view 2ω, the radius of curvature ri, the spacing di between adjacent surfaces, the refractive index ndi, the Abbe number νdi and the aspherical surface coefficient are set as follows. The second surface given a mark * is an aspherical surface, and the value of r2 which is the radius of curvature thereof means the radius of curvature on the optical axis. Further, the values of d6, d11, d14 and back focal length d23 varied by zooming are shown in the table below in the relation thereof with the focal length of the entire system. (Second Embodiment) Focal length f=15.5-27.3 F NO.=2.9-3.6 2ω=110.06°-75.9° i ri di ndi νdi 1 44.913 2.500 1.78797 47.47 *2 15.754 14.500 1.00000 3 -328.018 2.000 1.80218 44.70 4 34.778 4.500 1.00000 5 36.438 5.000 1.74950 35.19 6 174.054 variable 1.00000 7 30.163 1.200 1.80384 33.89 8 18.063 5.200 1.46450 65.77 9 -62.648 1.407 1.00000 10 53.843 2.200 1.72000 50.19 11 -2680.597 variable 1.00000 12 -43.632 1.500 1.71300 53.93 13 13.830 3.000 1.72825 28.34 14 93.325 variable 1.00000 15 34.869 4.500 1.50137 56.41 16 -61.169 0.500 1.00000 17 226.288 2.000 1.79504 28.57 18 31.935 2.500 1.00000 19 -495.542 3.000 1.59319 67.87 20 -39.358 0.200 1.00000 21 122.675 8.300 1.60300 65.42 22 -17.202 1.800 1.86074 23.01 23 -33.645 κ=0.51 C4=-4.987x10-7 C6=7.015x10-9 C8=-2.808x10-11 C10=5.38x10-14 Focal length f of the entire system 15.5 24 27.3 d6 24.149 6.722 3.460 d11 3.125 8.179 10.087 d14 8.122 3.068 1.160 d23 37.773 46.9625 50.432 fW=15.54 fA=-32.00 |fa/fw|=0.889 fasp=-19.994, |fasp/fw|=1.294 fp=60.546, fp/fw=3.919 fn=-39.103, |fn|/fw=2.531 As shown in the above embodiments, there can be provided a zoom lens which requires a relatively great angle of view in the preceding negative lens unit and in which distortion is corrected well by the application of the present invention, irrespective of a four-unit construction, and irrespective of the specific lens construction of each lens unit. Also, thereby, it becomes possible to realize a super-wide angle zoom lens having a maximum angle of view exceeding 100°. Also, that concave surface of the negative lens in the first lens unit having negative refractive power which is in contact with air is formed by such an aspherical surface which monotonically decreases the curvature thereof away from the optical axis and therefore, it becomes possible to decrease the positive lenses for correcting distortion. This leads to the advantage that the construction of the lens system is simplified and the aperture of the negative meniscus lens disposed most adjacent to the object side becomes smaller and also the volume of the entire lens system becomes smaller.

Claims: A zoom lens arrangement comprising a first lens unit (G 1 ) of negative refracting power disposed at the object and a second lens unit (G 2 ) of positive refractive power disposed on the image side of said first negative lens unit (G 1 ), a third lens unit (G 3 ) of negative refractive power disposed on the image side of said second lens unit (G 2 ), and a fourth lens unit (G 4 ) of positive refractive power disposed on the image side of said third lens unit (G 3 ) of negative refractive power, said second positive lens unit (G 2 ) and said first negative lens unit (G 1 ) being moved relative to each other on the optical axis thereof when the focal length of the entire system is changed, wherein said first negative lens unit (G 1 ) has a first negative lens component (L 1 ) and, on the image side thereof, a second negative lens component (L 2 ) and a positive lens component (L 3 ), said first negative lens component having a concave surface (r 2 ) when viewed from the image side thereof which is in contact with air, characterised in that said concave surface has an aspherical surface shape of which the curvature decreases monotonically away from the optical axis, the curvature being defined by the equation: x = Cy 2 /{1 + (1-κC 2 y 2 ) ½ } plus, optionally, one or more higher-order terms of even powers of y, where κ is a constant within the range: - 5 < κ < 0.75, x is the distance measured along a direction parallel to the optic axis from the vertex of the surface to a point on the surface at a distance y from the optic axis and C is a constant, and in that the zoom lens satisfies the following conditions: 3.5<fp/fW<4.5 2.5<|fn|/fW<4.5, where fp is the focal length of the positive lens component (L 3 ) in said first negative lens unit (G 1 ), fW is the focal length of the entire system at the wide angle end, and fn is the focal length of said second negative lens component (L 2 ) in said first negative lens unit (G 1 ). A zoom lens according to claim 1, wherein κ is within the range: 0≤κ<0.75. A zoom lens according to claim 1 or 2, satisfying the following condition: 0.5<|fasp/fW|<2.0, where fasp is the inverse number of the refractive power of the aspherical surface (r 2 ) of said first negative lens component (L 1 ) on the optical axis. A zoom lens according to claim 3, satisfying the following condition: 0.5<|fA/fW|<2.5, where fA is the focal length of said first negative lens component (L 1 ). A zoom lens according to any of claims 1 to 4, wherein said first negative lens unit (G 1 ) and said fourth positive lens unit (G 4 ) are moved relative to each other for zooming.

Inventors: ISHIYAMA TOSHIRO, SUZUKI KENZABURO, ISHIYAMA, TOSHIRO, SUZUKI, KENZABURO
Applicants: NIPPON KOGAKU KK, NIKON CORPORATION
Application year: 1993
Main classification: G02B13
IPC codes: G02B15, G02B13