JP3337666B2 - Objective lens - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an objective lens particularly suitable for an endoscope.

[0002]

2. Description of the Related Art A lens system disclosed in Japanese Patent Application Laid-Open No. 61-162221 is known as an endoscope optical system having a simple configuration in which distortion is well corrected and the number of components is relatively good. . However, since this lens system has a narrow angle of view and does not include a concave lens, the field curvature is large and the aberration is not satisfactorily corrected.

Also, Japanese Patent Application Laid-Open No. 2-208617 is unsuitable for endoscopes because the total length is long and the distance from the third lens unit to the image is long.

In Japanese Patent Application No. 63-222884, the third lens unit does not use an aspherical surface, and does not eliminate distortion.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide an objective lens which is small in size, has a wide angle of view, and is excellent in correcting astigmatism.

[0006]

According to the present invention, there is provided an objective lens comprising:
A first group having a negative power for correcting the Petzval sum and widening the angle, a second group having a positive power, a third group having a positive power and including an aspheric surface, and a third group. This is an objective lens for an image pickup device including an image pickup device placed in close proximity.

The objective lens according to the present invention has, for example, the configuration shown in FIG. 1 and includes a first unit L1, a second unit L2, and a third unit L3, respectively.
The stop S is located between the first unit L1 and the second unit L2, and is positioned near the front focal position of the second and third units to make the principal ray incident on the image guide IG in parallel. ing. Note that an image sensor such as a solid-state image sensor may be used instead of the image guide IG.

The reason that the third lens unit L3 is disposed close to the image pickup device is to simplify the frame structure for supporting the third lens unit by integrating the third lens unit with the image pickup device. .

As described above, the aspherical surface provided in the third lens group is for correcting distortion, and the aspherical shape has a weaker effect of converging a light beam as the distance from the optical axis increases. It is a shape including a part.

The shape of this aspherical surface is such that the optical axis is the x-axis,
Assuming that the direction of the image is positive and the values of the coordinates taken in the direction orthogonal to the x-axis with the intersection of the y-axis as the origin and the intersection point of the surface and the optical axis are x and y, the following expression is used.

Where r is the radius of curvature of a circle in contact with the aspheric surface near the optical axis, P is a parameter representing the shape of the aspheric surface, and B, E, F, G,. 8
The following are the coefficients of the aspheric surface. Also, if P = 1, E, F, G,
If all of H ... are 0, the above equation represents a spherical surface.

In the following description, it is assumed that B = 0 and P = 1.

In the present invention, it is preferable to use an aspherical surface which satisfies the following expression in order to correct astigmatism (astigmatic difference). (1) A2sp + A3As ≒ 0 where A2sp is a value obtained by dividing a third-order coefficient of astigmatism by the F-number of the objective lens among Seidel aberration coefficients of the spherical surface of the second group, and A3As is a third group. Is the value obtained by dividing the third-order coefficient of astigmatism due to the aspherical surface by the F number of the objective lens.

The condition (1) may practically satisfy the following conditions (2) and (3). (2) 0.05 <| A2sp / A3As | <20 (3) A2sp <0 and A3As> 0 Further, Seidel's aberration coefficient is defined by the following equation. This is the same as that used in the general-purpose lens design program ACCOS-V. However, ACCOS-V
Let the object distance be OB and the marginal ray numerical aperture be N
A, when the refractive index of the medium on the object side from the first surface is n0,
The ray height H0 of the paraxial ray on the first surface is determined by H0 = OB × tan {sin-1 (NA / n0)}, whereas in the present application, it is determined by H0 = OB × (NA / n0). . Therefore, in the present application, H determined by the latter
Each aberration coefficient is obtained by performing paraxial tracking based on 0.

ΔY = (SA3) for a meridional ray (X = 0) H^Three+ (CMA3) Y¹H^Two + ｛3 (AST3) + (PTZ3)｝ Y^TwoH + (DIS3) Y^Three + (SA5) H^Five+ (CMA5) YH^Four+ (TOBSA) Y^TwoH^Three + (ELCMA) Y^ThreeH^Two+ ｛5 (AST5) + (PTZ5)｝ Y^FourH + (DIS5) Y^Five+ (SA7) H⁷………… ΔZ = (SA3) H with respect to the sagittal ray (Y = 0)^Three+ ｛(AST3) + (PTZ3)｝ Z^TwoH + (SA5) H^Five+ (SOBSA) Z^TwoH^Three + ｛(AST5) + (PTZ5)｝ Z^FourH + (SA7) H⁷……… The above equation gives the paraxial image point (aberration
The difference between the actual image point and the image point when there is no
Therefore, Y is the paraxial principal light at the image plane standardized by the maximum image height.
The incident position of the line, H is the marker normalized by the pupil diameter in the pupil plane.
This is the incident position of the signal beam. SA3, SA5, S
A7 is the third-order, fifth-order, and seventh-order spherical aberration, respectively, CMA3, C
MA5 is the third and fifth order tangential coma, AS respectively
T3 and AST5 are third and fifth order astigmatism, PTZ
3, PTZ5 is 3rd and 5th order Petzval sum, DI
S3 and DIS5 are third-order and fifth-order distortions, respectively, TOBS
A is the fifth-order oblique tangential spherical aberration, ELC
MA is the fifth-order elliptical frame, SOBSA is the fifth-order oblique support.
It is digital spherical aberration.

In the above equation, the third order coefficient AST of astigmatism is expressed as follows when the objective lens includes a spherical surface and an aspherical surface. AST3 = (A1sp + A2sp + A3sp + A1As + A2As
+ A3As) × F where Ajsp is a cubic coefficient of astigmatism of the spherical portion of the j group,
AjAs is a third-order coefficient of astigmatism due to the aspheric surface of the J group. Here, the aberration coefficient of the spherical portion is the aberration coefficient of the surface itself when the surface is a spherical surface. However, when the surface is an aspheric surface, the so-called reference spherical surface (aspherical surface on the optical axis) is used. (Spherical surface in contact). Further, the aberration coefficient of the aspherical surface portion is obtained by subtracting the aberration coefficient of the reference spherical surface from the aberration coefficient of the aspherical surface itself. In the above equation showing AST3, the value of AAs of the group that includes all the spherical and aspherical components for each group but does not include the aspherical surface is 0.
It is.

Since the objective lens of the present invention has large values of A2sp and A3As, astigmatism can be corrected by canceling each other. The conditions necessary for that are the above-mentioned conditions (2) and (3).

If the upper limit of the condition (2) is exceeded among these conditions, the sphere lacking image plane becomes large to the plus side with respect to the meridional image plane (Δm), and the astigmatic difference becomes large. When the lower limit is exceeded, the opposite tendency occurs, and the astigmatic difference similarly increases.

Further, A2sp and A3As are expressed by the following components of the lens, that is, paraxial ray height, paraxial incident angle, paraxial exit angle, refractive index, and fourth-order aspherical coefficient of each surface. A3As = Σ8 (hai) ² (hbi) ² Ei (N _i -N _{i + 1} ) where hai is the paraxial marginal ray height on the i plane, hbi is the principal ray height on the i plane, and Ei is the aspherical surface on the i plane. The fourth-order coefficient, _Ni is the refractive index of the i-side on the object side, Ni _{+ 1} is the refractive index of the i-side on the image side, and Σ is the sum of aspherical surfaces in the three lens units. The A2sp = ΣA2spi = ΣSi (I'l) 2 Si = N i (Ki -1) hai (Ii + ui) The _{Ki = N i / N i +} 1 where A2spi = Si (Ii) 2 and defined.

Where Ii is the angle of incidence of the paraxial marginal ray on the i-plane, ui is the angle of inclination of the paraxial marginal ray emitted from the i-plane with respect to the optical axis, and I 'is the angle of incidence of the paraxial principal ray on the i-plane. is there. Σ means that the sum of the two groups of spherical surfaces is taken. Note that Ii, ui, Ii ', and ui' are all positive clockwise with respect to the optical axis.

Next, in order to better correct the distortion,
It is desirable to use an aspheric surface also for the first lens unit. Used here
Aspheric surface is N_i <N_{i + 1} , As y increases
Is a surface that includes the part where the convergence of light rays gradually increases.
And N_i > N_{i + 1} In the case of, as y increases
Where the divergence of light gradually weakens or the convergence of light
It is desirable that the surface includes a portion that gradually becomes stronger. This
If the aspheric surface of 含 む includes a fourth-order aspherical term,
If the surface number of the spherical surface is l, A1As and A3As are as follows, respectively.
Can be expressed as A1As = $ 8 (hall)^Two  (Hbl)^Two El (N_l-N_{l + 1}A3As = $ 8 (hai)^Two(Hbi)^Two Ei (N_i -N_{i + 1} A1As <0 A3As> 0 In this case, the following condition (4) is satisfied instead of the condition (2).
There is a need to. (4) 0.005 <| A1As / A3As | <5 If this condition (4) is not satisfied, | A1As + A3A
s + A2sp | increases, it is difficult to remove astigmatism
It will be strange.

From the above, when the first and third units include aspheric surfaces in the present invention, it is preferable that the following condition (5) is satisfied for correcting astigmatism. (5) 0.055 <| (A2sp + A1As) / A3As | <
25

[0023]

Next, embodiments of the objective lens for an endoscope imaging apparatus according to the present invention will be described. Example 1 f = 1.000, F / 2.649, IH = 1.0535, object distance = 13.8169, distance from the last lens surface to the image = 0.016 r1 = 86.3558 (aspherical surface) d1 = 0.5181 n1 = 1.51633 v1 = 64.15 r2 = 0.8152 d2 = 0.7761 r3 = ∞ (aperture) d3 = 0.0010 r4 = 3.9092 d4 = 2.0862 n2 = 1.72916 ν2 = 54.68 r5 = -1.7392 d5 = 0.2591 r6 = 1.6792 (aspherical surface) d6 = 2.7813 = 1.53 n3 = 1.53 Aspheric coefficient (first surface) P = 1.0000, E = 0.39580 × 10 ⁻¹ , F = −0.29530 × 10 ⁻² , G = −0.19633 × 10 ⁻² (Sixth surface) P = 1.0000, E = −0.60922 × 10 ⁻¹ , F = −0.91382 × 10 ⁻² , G = 0.47341 × 10 ⁻² , H = 0.42571 × 10 ⁻² , I = −0.49067 × 10 ⁻² f2 = 1.955, f12 = 2.223, | A2sp / A3As | = 0.55072, | A1As / A3As | = 0.15580, | (A2sp + A1As) /A3As|=0.70652 Example 2 f = 1.000, F / 2.646, IH = 1.0517, object distance = -13.7931, from the last lens surface to the image Distance = 0.001 r1 = 86.2069 (aspherical surface) d1 = 0.5172 n1 = 1.51633 ν1 = 64.15 r2 = 0.8655 d2 = 0.8655 r3 = ∞ (aperture) d3 = 0.0050 r4 = 4.2005 d4 = 2.0518 n2 = 1.75416 7802 d5 = 0.2586 r6 = 1.6724 (aspherical surface) d6 = 2.7953 n3 = 1.51633 ν3 = 64.15 r7 = ∞ aspherical surface coefficient (first surface) P = 1.0000, E = 0.39785 × 10 ⁻¹ , F = −0.29785 × 10 ⁻² , G = -0.19872 × 10 ⁻² (Sixth surface) P = 1.0000, E = −0.56047 × 10 ⁻¹ , F = −0.57663 × 10 ⁻² , G = −0.88319 × 10 ⁻³ f2 = 2.007, f12 = 2.224, | A2sp / A3As | = 0.5806, | A1As / A3As | = 0.2092, | (A2sp + A1As) /A3As|=0.7815 Example 3 f = 1.000, F / 2.424, IH = 1.0535, object distance = -17.2712, Distance from the last lens surface to the image = 0.016 r1 = 86.3558 (aspheric surface) d1 = 0.5181 n1 = 1.51633 v1 = 64.15 r2 = 0.8152 d2 = 0.8619 r3 = ∞ (aperture) d3 = 0.0010 r4 = 3.9655 d 4 = 1.9313 n2 = 1.72916 v2 = 54.68 r5 = -1.7392 d5 = 0.4318 r6 = 1.6538 (aspheric surface) d6 = 2.6598 n3 = 1.51633 v3 = 64.15 r7 = ∞ aspheric surface coefficient (first surface) P = 1.0000, E = 0.39580 × 10 ⁻¹ , F = −0.29530 × 10 ⁻² , G = −0.19633 × 10 ⁻² (Sixth surface) P = 1.0000, E = −0.26822 × 10 ⁻¹ , F = −0.57910 × 10 ⁻¹ , G = 0.12977 × 10 ^-1 , H = 0.28091 × 10 ^-1 , I = -0.16645 × 10 ^-1 f2 = 1.934, f12 = 2.163, | A2sp / A3As | = 1.3296, | A1As / A3As | = 0.4527, | (A2sp + A1As) /A3As|=0.7824 Example 4 f = 1.000, F / 2.726, IH = 0.9472, object distance = -15.5280, distance from the last lens surface to the image = 0.225 r1 = 77.6398 (aspherical surface) d1 = 0.46559 n1 = 1.51633 v1 = 64.15 r2 = 0.7329 d2 = 0.7749 r3 = ∞ (aperture) d3 = 0.0009 r4 = 3.5652 d4 = 1.7364 n2 = 1.72916 v2 = 54.68 r5 = -1.5637 d5 = 0.3882 r6 = 1.4869 (non-spherical) = 1. 51633 ν3 = 64.15 r7 = −1.7081 d7 = 0.7764 n4 = 1.84666 ν4 = 23.78 r8 = ∞ Aspherical coefficient (first surface) P = 1.0000, E = 0.54462 × 10 ⁻¹ , F = −0.50268 × 10 ⁻² , G = −0.41347 × 10 ⁻² (Sixth surface) P = 1.0000, E = −0.36908 × 10 ⁻¹ , F = −0.998580 × 10 ⁻¹ , G = 0.27330 × 10 ⁻¹ , H = 0.73187 × 10 ⁻¹ , I = −0.53649 × 10 ⁻¹ f2 = 1.739, f12 = 1.944, | A2sp / A3As | = 1.328, | A1As / A3As | = 0.525, | (A2sp + A1As) /A3As|=1.789 Example 5 f = 1.000, F / 2.483, IH = 1.0167, object distance = -16.6667, distance from the last lens surface to the image = 0.000 r1 = 8.3333 d1 = 0.5000 n1 = 1.51633 v1 = 64.15 r2 = 1.0267 d2 = 1.1018 r3 = ∞ (aperture) d3 = 0.0000 r4 = 2.6261 d4 = 1.8908 n2 = 1.72916 v2 = 54.68 r5 = -1.8039 d5 = 0.2770 r6 = 1.6010 (aspherical surface) d6 = 2.0458 n3 = 1.51633 v3 = 64.15 r7 = ∞ aspherical surface coefficient P = 1.095, E = 0.300 × 10 ^-1 , F = −0.51667 × 10 ⁻¹ , G = 0.26459 × 10 ⁻¹ , H = 0.39158 × 10 ⁻¹ , I = −0.34342 × 10 ⁻¹ f2 = 1.788, f12 = 1.746, | A2sp / A3As | = 1.607, | A1As / A3As | = 0, | (A2sp + A1As) /A3As|=1.3607 Example 6 f = 1.000, F / 2.417, IH = 1.0535, object distance = -17.2712, distance from lens final surface to image = 0.018 r1 = 17.17212 d1 = 0.5181 n1 = 1.51633 v1 = 64.15 r2 = 0.8047 (aspherical surface) d2 = 0.9121 r3 = ∞ (aperture) d3 = 0.0010 r4 = 4.4820 d4 = 1.9981 n2 = 1.72916 = 54.61 = 54.75 r6 = 1.6446 (aspherical surface) d6 = 2.7040 n3 = 1.51633 v3 = 64.15 r7 = ∞ aspherical surface coefficient (second surface) P = 1.0000, E = -0.11598, F = -0.40346, G = -0.38855 (sixth surface) P = 1.0000, E = −0.31119 × 10 ⁻¹ , F = −0.58652 × 10 ⁻¹ , G = 0.12859 × 10 ⁻¹ , H = 0.27636 × 10 ⁻¹ , I = −0.16595 × 10 ⁻¹ f2 = 2.011, f12 = 2.223, | A2sp / A3As | = 1.003, | A1As / A3As | = 0.4540, | (A2sp + A1As) /A3As|=1.457 where r1, r2,... Are the radii of curvature of the respective lens surfaces, and d1, d2,. .. Are the refractive indices of the lenses, .nu.1, .nu.2,... Are the Abbe numbers of the lenses.

Note that hai, hbi, u ^′ i, I
The values of i and I'i are as follows. Example 1 Paraxial marginal ray k hai ui Ii 0 0.000000 -0.012906 1 0.178322 -0.007808 0.014971 2 0.182368 -0.127348 0.231518 3 0.281205 -0.127348 0.127348 4 0.281336 -0.043300 0.199315 5 0.371670 0.080950 -0.170401 6 0.3506980.124500 0.003105 0.188783 -0.120717 Paraxial chief ray khbi u'i I'i 0 -15.410584 -1.049522 1 -0.909416 -0.695732 1.038991 2 -0.548933 -0.707277 0.022359 3 0.000000 -0.707277 0.707277 4 0.000726 -0.408951 0.707463 5 0.853892 -0.349148 0.944344 -0.038765 0.911517 7 1.052163 -0.037587 0.038765 8 1.052574 -0.058781 0.037587 Example 2 Paraxial marginal ray k hai ui Ii 0 0.000000 -0.012906 1 0.178015 -0.007808 0.014971 2 0.182054 -0.120445 0.218150 3 0.286301 -0.1204450.12044520 0.370728 0.080974 -0.167079 6 0.349787 0.124622 0.128183 7 0.001431 0.120836 -0.124622 8 0.000114 0.18 8968 -0.120836 Paraxial chief ray khbi u'i I'i 0 -15.399097 -1.046844 1 -0.959871 -0.694171 1.035709 2 -0.600817 -0.694171 -0.000001 3 0.000000 -0.694171 0.694171 4 0.003471 -0.401101 0.694997 5 0.826462 -0.355571 -0.0624426 0.918420 -0.047493 0.904745 7 1.051179 -0.046051 0.047493 8 1.051681 -0.072016 0.046051 Example 3 Paraxial marginal ray k hai ui Ii 0 0.000000 -0.011395 1 0.196813 -0.006739 0.013675 2 0.200304 -0.137087 0.252452 3 0.318462 -0.137017 0.1 0.406285 0.091831 -0.188204 6 0.366634 0.136049 0.129857 7 0.004776 0.131916 -0.136049 8 0.003336 0.206295 -0.131916 Paraxial chief ray khbi u'i I'i 0 -19.072572 -1.049370 1 -0.948742 -0.695787 1.038383 2 -0.588230 -0.682469 -0.07920000 -0.682469 0.682469 4 0.000701 -0.394608 0.682646 5 0.762822 -0.362528 -0.043995 6 0.919354 -0.049794 0.918423 7 1.051792 -0.048281 0.049794 8 1.052320 -0.075503 0 .048281 Example 4 Paraxial Marginal Ray k hai ui Ii 0 0.000000 -0.011395 1 0.176948 -0.006739 0.013675 2 0.180087 -0.137087 0.252452 3 0.286319 -0.137087 0.137087 4 0.286446 -0.045400 0.217432 5 0.365278 0.091831 -0.188204 469.3031 0.205351 8 0.041264 0.117277 -0.099316 Paraxial chief ray khbi u'i I'i 0 -15.245533 -0.932973 1 -0.758370 -0.618610 0.923205 2 -0.470198 -0.606770 -0.022932 3 0.000000 -0.606770 0.606770 4 0.000560 -0.350838 0.606927 5 0.609756 -0.3 -0.039115 6 0.734879 -0.044270 0.816551 7 0.803622 -0.120511 -0.426214 8 0.897187 -0.142306 0.120511 Example 5 Paraxial marginal ray k hai ui Ii 0 0.000000 -0.011395 1 0.189924 0.000245 0.034186 2 0.189802 -0.095078 0.184617 395045 0.007687 0.207243 5 0.309088 0.111647 -0.163658 6 0.278156 0.132791 0.062097 7 0.001357 0.128757 -0.132791 8 0.00 0001 0.201356 -0.128757 Paraxial chief ray k hbi u'i I'i 0 -17.965483 -1.010614 1 -1.121921 -0.712330 0.875983 2 -0.765756 -0.695034 -0.033499 3 0.000000 -0.695034 0.695034 4 0.000000 -0.401949 0.695034 5 0.759994 -0.3878320.0 6 0.867441 -0.071271 0.929659 7 1.016004 -0.069106 0.071271 8 1.016732 -0.108071 0.069106 Example 6 Paraxial marginal ray k hai ui Ii 0 0.000000 -0.011395 1 0.196813 -0.003635 0.022791 2 0.198696 -0.132996 0.250540 0.30.320006 -0.396 5 0.413640 0.089478 -0.186888 6 0.374047 0.136455 0.137957 7 0.005069 0.132309 -0.136455 8 0.003624 0.206910 -0.132309 Paraxial chief ray k hbi u'i I'i 0 -19.130935 -1.050336 1 -0.990415 -0.712210 0.992991 2 -0.621395 -0.681 -0.0521 0.000000 -0.681255 0.681255 4 0.000700 -0.393915 0.681411 5 0.787783 -0.356629 -0.051135 6 0.945590 -0.039412 0.931587 7 1.052160 -0.038214 0.039412 8 1.052578 -0 .059761 0.038214 Embodiments 1, 2, and 3 are shown in FIGS. 1, 2, and 3, respectively, and are each examples in which each group is constituted by one lens. The first and third lens units have aspherical surfaces to correct distortion.

In these embodiments, focus adjustment is performed by changing the distance between the second lens unit and the third lens unit. If the marginal rays emitted from the second lens group are parallel to the optical axis, the focus adjustment described above cannot be performed. Therefore, the combined focal length f12 of the first lens group and the second lens group must satisfy the following condition. | F12 | <10f where f is the focal length of the entire system.

In order to satisfy the above conditions (2) and (3), Si of the surface closest to the object in the second lens unit must be negative. Therefore, the second group surface R2 must satisfy the following condition. If 15f>R2> 0 R2 exceeds the upper limit, it becomes difficult to correct spherical aberration, and the overall length of the lens system becomes large because the convex power of the second unit is rearward.

The distance S from the last surface of the lens system to the image plane
k is a small value because, for example, the final surface of the objective lens is bonded to the image guide. It is desirable that the following condition (6) is satisfied. (6) If Sk is larger than 0 ≦ Sk ≦ ff, the configuration of the third lens unit becomes complicated due to the frame structure. When Sk is within the above range, the third lens unit can be integrated with the image sensor, which is convenient.

In order to satisfy the above condition, the thickness D3 of the final lens of the third group (the total thickness when this lens is a cemented lens) must satisfy the following condition (7). is there. (7) D3> 0.5f Further, in order to correct coma and shorten the overall length of the lens system, the second group should be a biconvex lens in which the convex power on the image side is stronger than the convex power on the object side. , The focal length f2 preferably satisfies the following condition. 0 <f2 <7f The focal length f3 of the third lens group preferably satisfies the following condition in order to prevent the total length from being extended by the strong concave power of the first lens group. 0 <f3 <12f In the shape of the third lens group, it is preferable that the convex power of the object-side surface is stronger than the convex power of the image-side surface in order to correct astigmatism and coma.

In Example 4, as shown in FIG. 4, the lenses in the third group were cemented in order to correct chromatic aberration of magnification. Incidentally, the chromatic aberration of magnification may be corrected by using the lens in the second group as a cemented lens.

The fifth embodiment is an example in which the first lens unit is not provided with an aspherical surface but the third lens unit is provided with an aspherical surface in the structure shown in FIG. 5, which is characterized in that the cost is reduced.

The sixth embodiment shown in FIG. 6 is an example in which the image-side surface of the first lens unit is formed into an aspherical surface including a portion where the divergence of a light beam gradually decreases as y increases. If the concave surface is made aspheric as described above, when the aspheric lens is made of glass press, plastic, or the like, there is an advantage that processing is easy because the mold is convex.

[0032]

According to the present invention, it is possible to obtain an objective lens for an image pickup device having a short overall length and free of distortion.

[Brief description of the drawings]

FIG. 1 is a sectional view of a first embodiment of the present invention.

FIG. 2 is a sectional view of a second embodiment of the present invention.

FIG. 3 is a sectional view of a third embodiment of the present invention.

FIG. 4 is a sectional view of a fourth embodiment of the present invention.

FIG. 5 is a sectional view of a fifth embodiment of the present invention.

FIG. 6 is a sectional view of a sixth embodiment of the present invention.

FIG. 7 is an aberration curve diagram according to the first embodiment of the present invention.

FIG. 8 is an aberration curve diagram according to the second embodiment of the present invention.

FIG. 9 is an aberration curve diagram according to the third embodiment of the present invention.

FIG. 10 is an aberration curve diagram according to the fourth embodiment of the present invention.

FIG. 11 is an aberration curve diagram according to the fifth embodiment of the present invention.

FIG. 12 is an aberration curve diagram according to the first embodiment of the present invention.

Continuation of the front page (56) References JP-A 1-319009 (JP, A) JP-A 2-208617 (JP, A) JP-A 2-293709 (JP, A) JP-A 1-1113714 (JP) JP-A-1-314216 (JP, A) JP-A-4-23812 (JP, A) JP-A-4-238313 (JP, A) JP-A-4-246606 (JP, A) JP-A-4-261510 (JP, A) JP-A-4-261511 (JP, A) JP-A-5-173067 (JP, A) JP-A-4-275514 (JP, A) JP-A-61-162021 (JP, A A) JP-A-63-163317 (JP, A) JP-A-50-137729 (JP, A) JP-A-58-58515 (JP, A) JP-A-60-37514 (JP, A) JP-A-116315 (JP, A) JP-A-61-144616 (JP, A) JP-A-62-173415 (JP, A) JP-A-63-61213 (JP, A) JP-A-63-61214 (JP, A) JP-A-63-149618 (JP, A) JP-A-63-161421 (JP, A) JP-B-49-20215 (JP, A) B1) (58) Field surveyed (Int. Cl. ⁷ , DB name) G02B 13/04

Claims (2)

(57) [Claims] A first group having a negative refractive power, a second group having a positive refractive power, and a third group having a positive refractive power and including an aspherical surface, in order from the object side, Each group has one lens and satisfies the following conditions (2), (3) and (6). (2) 0.05 <| A2sp / A3As | <20 (3) A2sp <0 and A3As> 0 (6) 0 ≦ Sk ≦ f where A2sp is a third-order coefficient of astigmatism due to the spherical surface of the second lens unit. A3As is a value obtained by dividing the third-order coefficient of astigmatism by the aspheric surface of the third group by the F-number.
Sk is the distance from the last surface of the lens system to the image plane, and f is the focal length of the entire system. 2. In order from the object side, there are a first group having a negative refractive power, a second group having a positive refractive power, and a third group having a positive refractive power and including an aspheric surface, Each group has one lens and satisfies the following conditions (2), (3) and (7). (2) 0.05 <| A2sp / A3As | <20 (3) A2sp <0 and A3As> 0 (7) D3> 0.5f where A2sp is a third-order coefficient of astigmatism due to the spherical surface of the second group. A3As is a value obtained by dividing the third-order coefficient of astigmatism by the aspheric surface of the third group by the F-number.
D3 is the thickness of the third lens, and f is the focal length of the entire system.