JP2006300972A - Retrofocusing type super-wide angle lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To propose a retrofocusing type super-wide angle lens whose f-number is about 4.5, the angle of view (2W) is about 102 degrees, the back focus can be secured to be twice or more, as long as the focal distance and which is easily manufactured. <P>SOLUTION: The retrofocusing type super-wide angle lens 1 has a first lens group GR1, having negative refractive power and a second lens group GR2, having positive refractive power. The first lens group GR1 has four negative meniscus lenses 2 to 5, and a cemented lens 6, obtained by bonding a positive lens 7 and a negative meniscus lens 8. The second lens group GR2 has a cemented lens 9, obtained by bonding positive/negative lenses 10 and 11 on the object side of a diaphragm 12, and has a positive lens 13, a cemented lens 14 obtained by bonding negative/positive lenses 15 and 16, a positive lens 17 and a doublet 18 on an image surface side. The convex lens surface 3a of the negative meniscus lens 3 in the first lens group GR1 is made aspherical. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a retrofocus ultra-wide-angle lens that is suitable for use as an interchangeable lens for a medium-format single-lens reflex camera and has a long back focus and excellent correction of various aberrations.

  With the recent trend of digital still cameras using an image sensor such as a CCD, digitalization is also progressing in medium format single-lens reflex cameras as well. Conventionally, most medium format single-lens reflex cameras are of the film back exchange type, and those capable of using both a silver salt film back and a digital back mounted with a CCD have been developed and marketed.

  However, the size of the imaging surface of the CCD mounted on the digital back is generally smaller than the format size of the silver salt film. For this reason, a CCD has a narrower field angle of view than a silver salt film, and in order to use a silver salt camera lens as a digital back lens, it is necessary to use a super wide angle lens in consideration of the CCD size.

  Here, since a wide-angle lens for a single-lens reflex camera needs to secure a sufficient back focus, a so-called retrofocus type having negative and positive refractive power arrangements from the object side is adopted. Therefore, a single-lens reflex camera combined with a digital back needs to ensure a long back focus and use a lens with a wide angle of view. However, a retrofocus type wide-angle lens having such characteristics has a stronger asymmetry, and it is difficult to correct various aberrations such as negative distortion and lateral chromatic aberration.

  Further, in the retrofocus type wide-angle lens, the entire lens is extended at the time of focusing to a short distance, and the so-called floating that changes the aperture interval is performed to correct the change in the positive direction of the image plane. However, if this is done, the negative distortion distortion and the upper coma will become excessive.

In order to solve such a problem, Patent Document 1 proposes a retrofocus super-wide-angle lens having an angle of view (2 W) of 100 degrees or more. The retrofocus super-wide-angle lens disclosed here employs a configuration in which the concave surface of the negative meniscus lens on the object side in the first lens group is aspherical.
JP 2004-102100 A

  The retrofocus super-wide-angle lens disclosed in the above-mentioned patent document is suitable for use as an interchangeable lens for a single-lens reflex camera combined with a digital back because distortion and the like are favorably corrected. However, it is necessary to use a lens having an aspherical shape on the concave surface, a large sag amount with respect to the radius of curvature of the reference spherical surface, and a large tangent angle. Such a lens has a problem that it is generally difficult to manufacture.

  In view of these points, the problem of the present invention is that the F number is about 4.5 and the angle of view (2 W) is about 102 deg. An object of the present invention is to propose a retrofocus super-wide-angle lens in which the focus is secured at least twice the focal length, various aberrations are corrected well, and the manufacture is easy.

In order to solve the above problems, the retrofocus super-wide-angle lens of the present invention is
The first lens group having negative refractive power arranged on the object side and the second lens group having positive refractive power arranged on the image plane side are fixed, and the first lens group is fixed for focusing. And the second lens group is moved by moving to the object side,
The first lens group includes four negative meniscus lenses arranged on the object side and at least one cemented lens having positive refractive power arranged on the image plane side. It has a negative refractive power cemented surface with a convex positive lens on the surface side and a concave negative meniscus lens on the object side,
Of the four negative meniscus lenses of the first lens group, the convex surface of any one negative meniscus lens is an aspherical surface,
The second lens group includes a single cemented lens having a configuration in which a positive lens and a negative lens are cemented together, arranged on the object side with a diaphragm interposed therebetween, and one positive lens arranged on the image plane side, One cemented lens having a configuration in which a negative lens and a positive lens are cemented, one positive lens, and one cemented lens;
Conditional expressions (1) and (2) are satisfied.
2.3 <| Fgr1 / F | <5.3 (1)
1.5 <Fgr2 / F <2.4 (2)
However,
F: Total system focal length Fgr1: First lens group focal length Fgr2: Second lens group focal length

  In the retrofocus super-wide-angle lens of the present invention, the first lens group having negative refractive power includes four negative meniscus lenses on the object side. Of these negative meniscus lenses, the negative meniscus lens located on the object side has a convex surface facing the object side so that the minimum deflection angle with respect to the incident light is generated. Aberration can be suppressed. In addition, since it is necessary to have a strong divergence in order to obtain a long back focus at an ultra-wide angle, the generation of aberrations such as distortion aberration that greatly affects the angle of view increases. In the present invention, four negative meniscus lenses are used. Since the refractive power is shared, the aberration is suppressed.

  Furthermore, since the convex surfaces of the four negative meniscus lenses are aspherical, the negative distortion aberration can be corrected by adopting a shape in which the radius of curvature decreases from the optical axis toward the periphery as an aspherical shape. Can do. In addition, since the radius of curvature of the aspherical reference sphere applied to the convex surface is larger than the radius of curvature of the aspherical reference sphere applied to the concave surface, an increase in the amount of sag and an increase in the tangential angle with respect to the radius of curvature can be reduced. And easy to manufacture.

  Of the four negative meniscus lenses, the aspherical lens surface is such that the luminous flux at each angle of view is thin and the height from the optical axis is not sufficiently separated and intersects, that is, the object side in the first lens group Selecting a lens surface located at is effective in correcting aberration angle variation. In practice, the convex surface of the second negative meniscus lens from the object side may be aspherical in consideration of the difficulty of aspherical processing.

  Next, in the second lens group, a cemented lens having a cemented surface having a negative refractive power and a concave surface on the object side is disposed at a position closer to the object side than the stop. Correction of lateral chromatic aberration affected by the angle of view is satisfactorily performed. A similar effect can be obtained by disposing one negative meniscus lens among the four negative meniscus lenses with the concave surface facing the object side.

  Focusing is performed by fixing the first lens group and moving the second lens group to the object side. In this case, aberration variation occurs. However, in the present invention, the aberration is corrected by the first lens group as described above, and the second lens group has a configuration in which a positive lens and a negative lens, which are arranged on the object side with a diaphragm interposed therebetween, are cemented together. One cemented lens, one positive lens arranged on the image plane side, one cemented lens having a construction in which a negative lens and a positive lens are cemented, one positive lens, and one sheet The lens has a cemented lens. Therefore, various aberrations can be corrected satisfactorily.

  Here, the conditional expression (1) defines the refractive power of the first lens group. If the lower limit value of the conditional expression is not reached, the divergent action is strengthened, so that a long back focus can be ensured, but the sensitivity of the second lens group to the focus position increases, and manual focusing operation becomes difficult. In addition, since the refractive action of each lens surface becomes strong, the occurrence of negative distortion becomes excessive, making it impossible to correct aberrations in the aspherical surface and the subsequent lens group. Furthermore, as the radius of curvature of the concave surface of the negative lens decreases, the amount of sag and tangential angle of the aspherical surface applied to the opposite convex surface increases, making it difficult to manufacture the lens. On the other hand, if the upper limit is exceeded, the divergence action is weakened, which is advantageous for aberration correction, but it is difficult to ensure a long back focus and the front lens diameter increases.

  Conditional expression (2) above defines the refractive power of the second lens group. If the lower limit value of this conditional expression is not reached, it becomes difficult not only to secure the back focus but also to correct aberrations, leading to an increase in the number of lenses, and ghosts and the like are likely to occur. On the other hand, when the upper limit value is exceeded, it is easy to secure the back focus, but there arises a problem that the entire lens length becomes long and the lens diameter increases.

  Next, the convex aspheric surface of the negative meniscus lens in the first lens group can be defined by the following aspheric function.

However,
R: radius of curvature of the reference sphere Y: height from the optical axis X: sag amount

In this case, it is desirable that the aspheric coefficient A0 satisfies the conditional expression (3).
A0> 1 (3)

  In order to correct distortion occurring in the negative meniscus lens on the object side with an aspherical surface provided on the convex surface, it is necessary to adopt an aspherical shape in which the radius of curvature decreases from the optical axis toward the periphery. An aspherical surface satisfying conditional expression (3) is an aspherical shape based on an elliptical surface with the optical axis as the short axis, and if such a shape is employed, distortion can be corrected well.

Next, in the retrofocus-type super-wide-angle lens of the present invention, the d-line Abbe number Vp of the glass material used for any one of the positive lenses located on the image plane side with respect to the stop has the following conditional expression (4). It is desirable to be satisfied.
Vp> 62 (4)

In the retrofocus super-wide-angle lens of the present invention, the d-line Abbe number Vn of a glass material used for at least one of the four negative meniscus lenses in the first lens group is expressed by the following conditional expression (5 ) Is desirable.
Vn> 62 (5)

  These conditional expressions (4) and (5) define the characteristics of the glass material used for the positive lens on the image plane side and the negative lens on the object side with the stop interposed therebetween. These conditional expressions are defined by these conditional expressions. If a lens made of a glass material having a high partial dispersion as described above is used, the downward declination of the short wavelength light beam (blue) increases, and the lateral chromatic aberration can be corrected efficiently.

  According to the present invention, the F-number is about 4.5, the angle of view (2W) is about 102 deg, the back focus is secured at least twice the focal length, and various aberrations are corrected well. A lens can be realized. In addition, since the aspherical surface is provided on the convex surface of the negative meniscus lens of the first lens group, the radius of curvature of the reference spherical surface is larger than when the aspherical surface is provided on the concave surface. Is small and the tangent angle is small. Therefore, it is easy to manufacture the lens.

  Therefore, according to the present invention, it is possible to realize a retrofocus super-wide-angle lens that is suitable for use as an interchangeable lens for a single-lens reflex camera combined with a digital back and has a wide angle of view, a long back focus, and is easy to manufacture. .

  Embodiments of a retrofocus type ultra-wide angle lens to which the present invention is applied will be described below with reference to the drawings.

  FIG. 1 is a configuration diagram illustrating the retrofocus-type ultra-wide-angle lens according to the first embodiment. In the retrofocus type super wide-angle lens 1 of this example, the first lens group GR1 having negative refractive power and the second lens group GR2 having positive refractive power are arranged in this order from the object side to the image plane side. Is arranged. During focusing, the first lens group GR1 is fixed, and the second lens group GR2 is moved to the object side.

  The first lens group GR1 includes four negative meniscus lenses 2, 3, 4, 5 arranged on the object side, and a cemented lens 6 having a positive refractive power arranged on the image plane side. Yes. Of the four negative meniscus lenses 2 to 5, the negative meniscus lenses 2 to 4 are arranged with the convex side facing the object side, and the remaining one negative meniscus lens 5 has the convex surface facing the image side Is arranged. The cemented lens 6 has a structure in which a positive lens 7 having a convex surface on the image side and a negative meniscus lens 8 having a concave surface on the object side are cemented, and the cemented surface has a negative refractive power. Yes.

  In the second negative meniscus lens 3 from the object side in the first lens group GR1, the convex lens surface 3a facing the object side is aspheric.

  The second lens group GR2 includes a cemented lens 9 having a configuration in which a positive lens 10 and a negative lens 11 are cemented, which are arranged on the object side with the diaphragm 12 interposed therebetween. In addition, a positive lens 13, a cemented lens 14 having a configuration in which a negative lens 15 and a positive lens 16 are cemented, a positive lens 17, and a cemented lens 18 are arranged on the image plane side of the diaphragm 12.

Table 1A shows parameters of the retrofocus super-wide-angle lens 1. The meaning of each code in this table is as follows.
F: Focal length FB: Back focus Fno: F number 2W: Full field angle I: Order of lens surfaces counted from the object side R (I): Curvature radius of I surface D (I): I surface and 1st surface (I + 1) spacing between surfaces N (I): Refractive index after the I-th surface (d-line)
V (I): Abbe number after surface I

(Table 1A)
F = 28.5 FB = 63.3 Fno = 4.6 2W = 102.4

[I] R (I) D (I) N (I) V (I)

[1] 65.7784 4.5000 1.83481 42.7
[2] 42.0164 6.3344
[3] 125.000 7.5000 1.48749 70.1
[4] 27.8109 7.7401
[5] 36.2097 2.5000 1.83481 42.7
[6] 20.3917 11.6534
[7] -30.2717 2.0000 1.805518 25.4
[8] -45.9494 3.2290
[9] 57.4333 12.1234 1.76182 26.5
[10] -20.1315 1.5079 1.805518 25.4
[11] -67.1960 7.2858
[12] 31.3605 7.409 1.56732 42.7
[13] 261.7203 1.5000 1.78800 47.4
[14] 24.1136 3.4263
[15] ∞ 1.5552
[16] 24.4200 4.8295 1.45600 87.0
[17] -21.0786 2.4469
[18] -155.5793 9.5000 1.83481 42.7
[19] 74.9796 8.1583 1.57099 50.7
[20] -19.151. 2000
[21] -1980.8662 4.0596 1.45600 87.0
[22] -44.6653. 2000
[23] 132.290 9.1722 1.48749 70.1
[24] -22.3984 2.0000 1.83481 42.7
[25] -106.2100

  The function defining the aspherical surface of the convex lens surface 3a of the negative meniscus lens 3 is given by the following equation, where Y is the height from the optical axis 1a, X is the sag amount in the optical axis direction, and R is the radius of curvature of the reference spherical surface. Can be represented by

  However, A0 to A10 are aspheric coefficients, and values are shown in Table 1B.

(Table 1B)
[Third surface aspherical surface]
A0 = .15565700E + 02
A2 = .00000000E + 00
A4 = .66436411E−05
A6 = −. 46755953E−08
A8 = .43412251E-11
A10 =-. 14078795E-14

The retrofocus super-wide-angle lens 1 of this example satisfies all the following conditional expressions (1) to (5).
(1) 2.3 <| Fgr1 / F | <5.3
(2) 1.5 <Fgr2 / F <2.4
(3) A0> 1
(4) Vp> 62
(5) Vn> 62
However,
F: Total system focal length Fgr1: Focal length of the first lens group Fgr2: Focal length of the second lens group Vp: d-line Abbe number of a glass material used for any positive lens located on the image plane side from the stop Vn: D-line Abbe number of glass material used for one of four negative meniscus lenses in the first lens group

  That is, in this example, | Fgr1 / F | = 4.45, Fgr2 / F = 2.02, and A0 = 15.6. Further, Vp = 87.0, and the Abbe number of the second negative meniscus lens from the object side in the first lens group is Vn = 70.1.

  2 and 3 are an aberration diagram of the retrofocus super-wide-angle lens 1 at an imaging distance ∞ and an aberration diagram of the imaging distance 0.35 m, respectively. In the figure, SA represents spherical aberration, DIST represents distortion (%), and AS represents astigmatism. Moreover, d in SA represents d line, g represents g line, S in AS represents sagittal, and M represents meridional. Furthermore, ΔY represents the image height ratio, and the numbers in parentheses represent 0%, 50%, 70%, and 10% lateral aberration, respectively.

  FIG. 3 is a configuration diagram illustrating the retrofocus-type ultra-wide-angle lens according to the second embodiment. Since the basic configuration of the retrofocus-type super-wide-angle lens 1A of this example is the same as that of the first embodiment, the corresponding parts are denoted by the same reference numerals and description thereof is omitted.

  Also in the retrofocus super-wide-angle lens 1A of the present example, the convex lens surface 3a of the second negative meniscus lens 3 counted from the object side is aspheric in the first lens group GR1.

  Table 2A shows parameters of the retrofocus super-wide-angle lens 1A. The meaning of each symbol is the same as in Table 1A. The aspheric surface 3a can also be defined by the function described in the first embodiment, and Table 2B shows each aspheric coefficient.

(Table 2A)
F = 28.4 FB = 63.4 Fno = 4.6 2W = 102.6

[I] R (I) D (I) N (I) V (I)

[1] 65.5084 4.55000 1.83481 42.7
[2] 42.4935 6.1560
[3] 125.000 6.9508 1.43875 90.9
[4] 27.9810 7.0304
[5] 38.2858 2.5000 1.83481 42.7
[6] 21.0220 11.7468
[7] -33.9296 2.8202 1.805518 25.4
[8] -53.9346 1.0437
[9] 92.3777 12.000 1.762182 26.5
[10] -21.901 4.4163 1.805518 25.4
[11] -59.9623 6.3328
[12] 20.1011 5.0000 1.56732 42.7
[13] 58.8429 1.5000 1.78800 47.4
[14] 14.8285 4.5233
[15] ∞ 2.5080
[16] 24.159 12.8830 1.48749 70.1
[17] -30.0170 3.4238
[18] -21.5658 2.0000 1.83481 42.7
[19] 51.5590 7.3202 1.57099 50.7
[20] -18.4844. 2000
[21] -141.5829 4.3330 1.43875 90.9
[22] -28.9422. 2000
[23] 890.6469 8.8708 1.48749 70.1
[24]-19.2171 2.0000 1.83481 42.7
[25] -71.5677

(Table 2B)
[Third surface aspherical surface]
A0 = .15445800E + 02
A2 = .00000000E + 00
A4 = .65164935E−05
A6 = −. 39338605E−08
A8 = .30526948E-11
A10 =-. 61829638E-15

  In this example, | Fgr1 / F | = 2.85, Fgr2 / F = 1.93, and A0 = 15.4. Further, Vp is 70.1 and 90.9, and Vn = 90.9. All satisfy the above conditional expressions (1) to (5).

  FIGS. 5 and 6 show an aberration diagram of the retrofocus super-wide-angle lens 1A of Example 2 with an infinite shooting distance and an aberration diagram with a shooting distance of 0.35 m, respectively. The meaning of each symbol in these figures is the same as in FIGS.

  FIG. 7 is a configuration diagram illustrating a retrofocus type ultra-wide-angle lens according to Example 3 of the present invention. Since the basic configuration of the retrofocus super-wide-angle lens 1B of this example is the same as that of the first embodiment, the corresponding parts are denoted by the same reference numerals, and the description thereof is omitted. In this example, the aspherical surface is given to the convex lens surface 3a of the second negative meniscus lens 3 counted from the object side in the first lens group GR1.

  Table 3A shows parameters of the retrofocus super-wide-angle lens 1B. The meaning of each symbol is the same as in Table 1A. The aspheric surface 3a can also be defined by the function described in the first embodiment, and Table 3B shows each aspheric coefficient.

(Table 3A)
F = 28.5 FB = 63.9 Fno = 4.6 2W = 102.4

[I] R (I) D (I) N (I) V (I)

[1] 70.1132 4.5000 1.83481 42.7
[2] 42.0674 6.2028
[3] 125.000 7.5000 1.43875 90.9
[4] 24.4604 5.9466
[5] 31.9807 2.5000 1.83481 42.7
[6] 20.7169 1.9233
[7] -33.2248 3.55837 1.80809 22.6
[8] -52.0263. 2000
[9] 61.2174 9.5000 1.76182 26.5
[10] -22.6518 3.0000 1.805518 25.4
[11] -68.9778 6.1340
[12] 21.3742 3.1411 1.56732 42.7
[13] 138.0333 1.5000 1.78800 47.4
[14] 16.3238 8.5441
[15] ∞ 2.2231
[16] 26.4457 8.6559 1.48749 70.1
[17] -32.6615 4.8124
[18] -23.4942 3.6149 1.83481 42.7
[19] 39.9581 7.76991 1.57099 50.7
[20] -20.7671. 2000
[21] -349.608608 5.0032 1.43875 90.9
[22] -31.5101. 2000
[23] ∞ 9.0264 1.48749 70.1
[24] -20.5898 2.0000 1.83481 42.7
[25] -67.0226

(Table 3B)
[Third surface aspherical surface]
A0 = .16235900E + 02
A2 = .00000000E + 00
A4 = .73817775E−05
A6 = −. 41918163E−08
A8 = .30834735E-11
A10 =-. 24494941E-15

  In this example, | Fgr1 / F | = 4.05, Fgr2 / F = 1.89, and A0 = 16.2. Vp is 70.1 and 90.9, and Vn is 90.9. Therefore, the above conditional expressions (1) to (5) are satisfied.

  FIGS. 8 and 9 show an aberration diagram of the retrofocus super-wide-angle lens 1B of Embodiment 3 with an infinite shooting distance and an aberration diagram of the shooting distance of 0.35 m, respectively. The meaning of each symbol in these figures is the same as in FIGS.

1 is a schematic configuration diagram of a retrofocus type ultra-wide-angle lens of Example 1. FIG. FIG. 6 is an aberration diagram when the photographing distance in the lens of Example 1 is infinite. FIG. 6 is an aberration diagram when the photographing distance in the lens of Example 1 is 0.35 m. FIG. 6 is a schematic configuration diagram of a retrofocus type ultra-wide angle lens according to Example 2. FIG. 6 is an aberration diagram when the photographing distance in the lens of Example 2 is infinite. FIG. 6 is an aberration diagram when the photographing distance in the lens of Example 2 is 0.35 m. 6 is a schematic configuration diagram of a retrofocus type ultra-wide-angle lens of Example 3. FIG. FIG. 10 is an aberration diagram when the photographing distance in the lens of Example 3 is infinite. FIG. 6 is an aberration diagram when the photographing distance in the lens of Example 3 is 0.35 m.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 1A, 1B Retro focus type super wide-angle lens 1a Optical axis GR1 1st lens group GR2 2nd lens groups 2-5, 8 Negative meniscus lens 6, 9, 14, 18 Joint lens 7, 10, 13, 16, 17 Positive lens 11, 15 Negative lens 12 Aperture

Claims (6)

The first lens group having negative refractive power arranged on the object side and the second lens group having positive refractive power arranged on the image plane side are fixed, and the first lens group is fixed for focusing. And the second lens group is moved by moving to the object side,
The first lens group includes four negative meniscus lenses arranged on the object side and at least one cemented lens having positive refractive power arranged on the image plane side. It has a negative refractive power cemented surface with a convex positive lens on the surface side and a concave negative meniscus lens on the object side,
Of the four negative meniscus lenses of the first lens group, the convex surface of any one negative meniscus lens is an aspherical surface,
The second lens group includes a single cemented lens having a configuration in which a positive lens and a negative lens are cemented together, arranged on the object side with a diaphragm interposed therebetween, and one positive lens arranged on the image plane side, One cemented lens having a configuration in which a negative lens and a positive lens are cemented, one positive lens, and one cemented lens;
A retrofocus super-wide-angle lens characterized by satisfying the following conditional expressions (1) and (2):
2.3 <| Fgr1 / F | <5.3 (1)
1.5 <Fgr2 / F <2.4 (2)
However,
F: Total system focal length Fgr1: Focal length Fgr2 of the first lens group: Focal length of the second lens group In claim 1,
The convex aspherical surface of the negative meniscus lens in the first lens group is defined by the following aspherical function, and the aspherical coefficient A0 satisfies the conditional expression (3). lens.

However,
R: radius of curvature of reference spherical surface Y: height from optical axis X: sag amount A0> 1 (3) In claim 1 or 2,
Vp :: A retrofocus super-wide angle characterized in that the d-line Abbe number Vp of a glass material used for at least one of the positive lenses positioned on the image plane side from the stop satisfies the conditional expression (4) lens.
Vp> 62 (4) In any one of Claims 1 thru | or 3,
The d-line Abbe number Vn of the glass material used for at least one of the four negative meniscus lenses in the first lens group satisfies the conditional expression (5). Wide angle lens.
Vn> 62 (5) In any of claims 1 to 4,
Of the four negative meniscus lenses, the convex surface of the second negative meniscus lens from the object side is an aspherical surface. In any of claims 1 to 5,
Of the four negative meniscus lenses, one negative meniscus lens is disposed with the concave surface facing the object side, and is a retrofocus type ultra-wide angle lens.

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