JP2572237B2 - Wide angle lens with long back focus - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a wide-angle lens for a single-lens reflex electronic still camera having a long back focus.

[Conventional technology]

The electronic still camera has a split mirror to guide light rays to the viewfinder and a long back focus to secure the space required for low-pass filters, even though the imaging size is extremely small compared to the conventional 35mm silver halide camera. Required. For example, a wide-angle lens having an angle of view (2ω) of about 64 ° requires a back focus of about 2 to 2.5 times the focal length. In order to realize this, it is necessary to provide a so-called retrofocus power arrangement in which a lens having a strong negative power is arranged in the first lens unit of the lens system. In such a retrofocus type lens system, a four-group type of negative, positive, negative, and positive or a five-group type of negative, positive, negative, positive, and positive can be considered. Among these types of conventional examples, the four-group type includes:
Japanese Unexamined Patent Publication No. Sho 60-37514 discloses a type, and examples of the five-group type include those described in Japanese Utility Model Unexamined Publication No. Sho 60-54111. However, in these lenses, the back focus is about 1 times the focal length in the former case and about 1.3 times the focal length in the latter case, and the back focus is sufficiently secured as a lens for a single-lens reflex electronic still camera. I can't say that.

[Problems to be solved by the invention]

The present invention provides a wide-angle lens for a single-lens reflex electronic still camera having an angle of view of about 64 °, an aperture ratio of about 1.8 to 2.8, a back focus of about 2 to 2.5 times the focal length of the entire system, and about 4 to 5 lenses. A lens is provided.

[Means for solving the problem]

The wide-angle lens according to the present invention includes, in order from the object side, a first lens group including one negative lens, four lenses including a positive lens, a negative lens, a positive lens, and a positive lens, or three lenses including a positive lens, a negative lens, and a positive lens. A negative lens of the first lens group, wherein at least one surface of the negative lens of the first lens group has a shape in which negative refractive power decreases as going to the periphery, and The lens system satisfies the conditions (1) to (4).

(1) −4 <f _I /f<−1.5 (2) 1.5 <f _II / f <4 (3) 2 <D / f <8 (4) 1 × 10 ⁻⁵ <| Δ | / f _I < 1 × 10 ⁻¹ (y = y _EC ) where f is the focal length of the entire lens system, f _I is the focal length of the first lens group, f _II is the focal length of the second lens group, and D is the first lens group. Is the distance between the rear principal point and the front principal point of the second lens group, Δ is the amount of deviation of the aspheric surface from the reference spherical surface, y is the height from the optical axis, and y _EC is the principal ray height at the maximum angle of view. .

In order to obtain a long back focus of about 2 to 2.5 times the focal length of the entire system, it is necessary to provide a strong negative power to the first lens unit. Condition (1) defines the focal length of the first lens group. f _I / f is the condition (1)
If the refractive power of the first lens unit becomes weaker than the lower limit value of −4, it becomes impossible to secure a long back focus. In addition, since the lens system of the present invention has an asymmetric lens configuration with the stop interposed therebetween, off-axis aberrations such as astigmatism and distortion are likely to occur in the first lens group located at a position away from the stop. Therefore, the upper limit of condition (1) -1.5
When the negative refractive power of the first lens unit exceeds the above, astigmatism and distortion are deteriorated, and correction becomes difficult.

Condition (2) defines the combined focal length of the second lens group. In the lens system of the present invention, the light beam diverged by the first lens group is converged by the second lens group. For this reason, spherical aberration and coma are likely to occur in the second lens group. In consideration of this point, the second lens group is configured with three or four lenses to disperse the refractive power. However, if f _II / f becomes smaller than the lower limit of 1.5 of the condition (2) and the refractive power of the second lens group becomes too strong, it becomes difficult to correct these aberrations. When the value exceeds the upper limit value 4 of the condition (2), the second
The convergence of the lens group is so weak that a long back focus cannot be obtained.

The condition (3) defines the distance between the principal points of the first lens group and the second lens group. In order to obtain a long back focus, it is conceivable to increase the negative refractive power of the first lens unit and the positive refractive power of the second lens unit, or to increase the distance between the first lens unit and the second lens unit. . However, as described above, it becomes difficult to satisfactorily correct aberrations when the refractive power is increased. Accordingly, in the present invention, the principal point interval between the first lens unit and the second lens unit is increased in order to obtain a long back focus of 2f to 2.5f.

If D / f becomes smaller than the lower limit 2 of the condition (3) as the principal point interval becomes shorter, a long back focus cannot be obtained. If the value is larger than the condition value 8 of the condition (3), the total length of the lens system becomes too long, which is not preferable.

The lens system according to the present invention is asymmetric with respect to the stop, and since off-axis rays pass away from the optical axis in the first lens group, off-axis aberrations are likely to occur in the first lens group. In order to correct this, it is desirable that at least one surface of the negative lens of the first lens group is formed into an aspheric surface having a shape in which the negative refractive power becomes weaker toward the periphery. Condition (4)
Defines the amount of deviation of the aspheric surface from the reference plane.

Exceeding the lower limit (1 × 10 ⁻⁵ ) of the condition (4) results in insufficient correction of negative distortion, and exceeding the upper limit (1 × 10 ⁻¹ ) results in excessive correction of distortion and increases astigmatism. I do.

In the wide-angle lens according to the present invention described above, it is preferable that the following condition (5) is further satisfied.

(5) 0.2 <d ₆ / f <2 where d ₆ is the thickness of the negative lens in the second lens group.

Condition (5) defines the thickness of the negative lens in the second lens group. If d ₆ exceeds the upper limit of 2 of this condition, spherical aberration increases, and outward coma generated on the image-side lens surface increases. When the value is smaller than the lower limit of 0.2, negative distortion and astigmatism are insufficiently corrected.

Furthermore, when the lens system is composed of five lenses, it is desirable that the following condition (6) is satisfied.

However d ₂ is the air gap between the first lens group and the second lens group, d ₄₅ is the air space between the positive lens and the negative lens on the most object side in the second lens group.

Condition (6) defines the relationship between the air gap between the positive and negative lenses closest to the object side in the second lens group and the air gap between the first lens group and the second lens group. . When the value exceeds the upper limit (0.5) of the condition (6), it becomes impossible to secure a sufficient back focus, so that the refractive power of the negative lens of the first lens unit and the positive lens closest to the object side in the second lens unit is reduced. It is necessary to increase the intensity, and if it is increased, each aberration increases, which is not preferable. On the other hand, if the lower limit (0.05) is exceeded, it is advantageous to secure the back focus, but the spherical aberration increases because the height of the marginal rays passing through the most object-side positive lens in the second lens group becomes high.

In the lens system according to the present invention, not only the entire system can be extended and focusing can be performed, but also one negative lens in the first lens group or one positive lens closest to the image in the second lens group can be extended. Can focus.

The shape of the aspherical surface used in the present invention is expressed by the following equation when the optical axis direction is set to the x-axis and the direction perpendicular thereto is set to the y-axis.

Here, r is the paraxial radius of curvature of the aspherical surface, and E, F, and G are aspherical surface coefficients.

〔Example〕

 Next, an embodiment of the wide-angle lens of the present invention will be described.

Example 1 f = 6.5, F / 2.0, 2ω = 63.2 ゜ r ₁ = 71.7280 (aspherical surface) d ₁ = 1.0000 n ₁ = 1.51728 ν ₁ = 69.56 r ₂ = 5.9643 d ₂ = 13.4817 r ₃ = 14.0756 d ₃ = 2.0000 n ₂ = 1.64769 ν ₂ = 33.80 r ₄ = -15.3124 d ₄ = 2.5361 r ₅ = ∞ (aperture) d ₅ = 1.7773 r ₆ = -6.6090 d ₆ = 5.2994 n ₃ = 1.84666 ν ₃ = 23.88 r ₇ = 18.5920 _{d 7 = 0.8707 r 8 = -246.8487} d 8 = 1.9353 n 4 = 1.58900 ν 4 = 48.61 r 9 = -7.6475 d 9 = 0.4000 r 10 = 16.2176 d 10 = 2.0000 n 5 = 1.61700 ν 5 = 62.79 r 11 = - 16.4176 d ₁₁ = 2.0000 r ₁₂ = ∞ d ₁₂ = 14.5000 n ₆ = 1.5163 ν ₆ = 64.15 r ₁₃ = ∞ Aspherical coefficient E = 0.23207 × 10 ^-3 , F = −0.30600 × 10 ^-6 G = −0.80407 × 10 ^-8 f _I / f = -1.945, f _II / f = 2.212 D / f = 4.571, | Δ | / f _I = 1.017 × 10 ^-2 d ₆ /f=0.815 Example 2 f = 6.5, F / 2.0, 2ω = 63.2 ゜ r ₁ = 168.1264 d ₁ = 1.0000 n ₁ = 1.49216 ν ₁ = 57.50 r ₂ = 5.5858 (aspherical surface) d ₂ = 11.5452 r _{3 = 10.9818 d 3 = 2.0000 n} 2 = 1.67270 ν 2 = 32.10 r 4 = -22.8751 d 4 = 2.6166 r 5 = ∞ ( _{stop) d 5 = 1.9769 r 6 =} -5.8544 d 6 = 4.6399 n 3 = 1.84666 ν 3 = 23.88 r ₇ = 18.7945 d ₇ = 0.9176 r ₈ = -53.4197 d ₈ = 1.8000 n ₄ = 1.61484 ν ₄ = 51.17 r ₉ = -6.7 128 d ₉ = 0.7000 r ₁₀ = 16.2182 d ₁₀ = 2.1000 n ₄ = 1.60311 ν ₅ = 60.70 r ₁₁ = -14.5495 d ₁₁ = 2.0000 r ₁₂ = ∞ d ₁₂ = 14.5000 n ₆ = 1.51633 ν ₆ = 64.15 r ₁₃ = ∞ Aspherical coefficient E = −0.63786 × 10 ^-3 , F = 0.35216 × 10 ^-5 G = −0.80999 × 10 ⁻⁶ f _I /f=−1.810, f _II /f=2.260 D / f = 4.541, | Δ | / f _I = 1.475 × 10 ⁻² d ₆ /f=0.714 Example 3 f = 6.5, F / 2.8, 2ω = 63.2 ゜ r ₁ = 104.1484 d ₁ = 1.0000 n ₁ = 1.56873 ν ₁ = 63.16 r ₂ = 6.5241 (aspherical surface) d ₂ = 20.8732 r _{3 = 10.2632 d 3 = 1.7000 n} 2 = 1.85026 ν 2 = 32.28 r 4 = -22.1157 d 4 = 0.7318 r 5 = ∞ ( _{stop) d 5 = 0.8534 r 6 =} -8.2735 d 6 = 3.9341 n 3 = 1.80518 ν 3 _{= 25.43 r 7 = 8.4104 d 7} = 0.3383 r 8 = 12.9741 d 8 = 1.5702 n 4 = 1.69680 ν 4 = 56.49 r 9 = -7.3532 d 9 = 2.0000 r 10 = ∞ d 10 = 14.5000 n 5 = 1.51633 ν 5 = 64.15 r ₁₁ = ∞ Aspherical surface coefficient E = −0.39129 × 10 ⁻³ , F = 0.34753 × 10 ⁻⁵ G = −0.30953 × 10 ⁻⁶ f _I /f=-1.890, f _II /f=1.945 D / f = 3.731, | Δ | / f _I = 1.554 × 10 ^-2 d ₆ /f=0.605 Example 4 f = 6.5, F / 2.0, 2ω = 63.2 ゜ r ₁ = 59.2315 (aspherical surface) d ₁ = 1.0000 n ₁ = 1.51728 ν ₁ = 69.56 r ₂ = 6.6227 d ₂ = 18.5374 r _{3 = 14.8114 d 3 = 2.0000 n} 2 = 1.64769 ν 2 = 33.80 r 4 = -18.2812 d 4 = 2.7178 r 5 = ∞ ( _{stop) d 5 = 1.9192 r 6 =} -7.4648 d 6 = 6.2190 n 3 = 1.84666 ν 3 _{= 23.88 r 7 = 18.3866 d 7} = 0.8449 r 8 = 534.2400 d 8 = 1.9279 n 4 = 1.58900 ν 4 = 48.61 r 9 = -8.8610 d 9 = 0.4000 r 10 = 17.3529 d 10 = 2.0000 n 5 = 1.61700 ν 5 = 62.79 r ₁₁ = -16.3303 d ₁₁ = 2.0000 r ₁₂ = ∞ d ₁₂ = 14.5000 n ₆ = 1.51633 ν ₆ = 64.15 r ₁₃ = ∞ Aspherical coefficient E = 0.12444 × 10 ^-3 , F = 0.79347 × 10 ^-6 G = −0.10730 × 10 ⁻⁷ f _I /f=−2.232, f _II /f=2.389 D / f = 5.490, | Δ | / f _I = 1.925 × 10 ⁻² d ₆ /f=0.957 Example 5 f = 6.5, F / 1.8, 2ω = 63.2 ゜ r ₁ = 19.8204 (aspheric surface) d ₁ = 1.0000 n ₁ = 1.79216 ν ₁ = 54.68 r ₂ = 5.5540 d ₂ = 11.8153 r ₃ = 19.1492 d ₃ = 2.4000 n ₂ = 1.69895 v ₂ = 30.12 r ₄ = -14.0501 d ₄ = 3.5275 r ₅ = ∞ (aperture) d ₅ = 1.0000 r ₆ = -8.1613 d ₆ = 6.8387 n ₃ = 1.84666 v _{3 = 23.88 r 7 = 18.9347 d 7} = 0.6000 r 8 = 77.6404 d 8 = 1.9969 n 4 = 1.51821 ν 4 = 65.04 r 9 = -8.9511 d 9 = 0.3283 r 10 = 17.4949 d 10 = 2.3492 n 5 = 1.78650 ν 5 = 50.00 r ₁₁ = -22.9835 d ₁₁ = 2.0000 r ₁₂ = ∞ d ₁₂ = 14.5000 n ₆ = 1.51633 ν ₆ = 64.15 r ₁₃ = ∞ Aspherical coefficient E = 0.18771 × 10 ^-3 , F = 0.99216 × 10 ^-6 G = −0.86876 × 10 ⁻⁸ f _I /f=−1.678, f _II /f=2.288 D / f = 4.448, | Δ | / f _I = 1.322 × 10 ⁻² d _{6 /} f = 1.052 where r ₁ , r ₂ , ... are the radii of curvature of the respective surfaces of the lens, d ₁ , d ₂ , ... are the wall thickness and air space of each lens, and n ₁ , n ₂ , ... are the respective lenses. refractive index, ν _1, ν _2, ... is the Abbe's number of each lens.

In the first embodiment, the first lens (the lens closest to the object) has an aspherical surface on the object side in the lens configuration shown in FIG. Focusing is performed by the positive lens closest to the image in the first lens unit or the second lens unit. Example 1
FIG. 6 shows the state of aberration at infinity.
FIG. 7 shows the state of aberration at an imaging magnification of 0.0305 when focusing is performed with the first lens, and the imaging magnification of 0.03 when focusing is performed with the lens closest to the image.
The aberration situation at 14 × is shown in FIG.

Embodiment 2 has a lens configuration shown in FIG. 2, in which a plastic lens is used as the first lens and its image-side surface is made aspherical. Focusing is performed by the positive lens closest to the image in the first lens unit or the second lens unit. FIG. 9 shows the aberration situation at infinity in this embodiment.

In the third embodiment, the second lens group includes three lenses in the lens configuration shown in FIG. 3, and the image-side surface of the first lens is aspheric. Focusing is performed by the first lens. The aberration situation at infinity in this embodiment is as shown in FIG.

In the fourth embodiment, the object side surface of the first lens is made aspherical in the lens configuration shown in FIG. This embodiment is the first
Focusing is performed by the lens or the positive lens closest to the image in the second lens group. The state of aberrations at infinity in this embodiment is as shown in FIG.

Example 5 has a lens configuration shown in FIG. 5, in which the object-side surface of the first lens is aspherical, and the aperture ratio is 1.8.
Focusing in this embodiment is performed by the positive lens closest to the image in the first lens unit or the second lens unit. The aberration situation at infinity in this embodiment is as shown in FIG.

In these examples, the glass block disposed on the image side of the second lens group assumes an optical member such as a low-pass filter or a glass prism for obtaining a half mirror for splitting a light beam into a finder.

〔The invention's effect〕

Although the wide-angle lens of the present invention is constituted by a small number of lenses such as five or four, the angle of view (2
ω) is about 64 °, the aperture ratio is about 1.8 to 2.8, the lens system has a long back focus of 2 to 2.5 times the focal length, and each aberration is corrected well.

[Brief description of the drawings]

1 to 5 show Embodiments 1 to 5 of the present invention, respectively.
6 to 8 are aberration curve diagrams of the first embodiment,
9 is an aberration curve diagram of the second embodiment, FIG. 10 is an aberration curve diagram of the third embodiment, FIG. 11 is an aberration curve diagram of the fourth embodiment, and FIG. 12 is an aberration curve diagram of the fifth embodiment.

Claims (1)

(57) [Claims] 1. A first lens comprising one negative lens in order from the object side.
And a second lens group including four lenses of a positive lens, a negative lens, a positive lens, and a positive lens, or a second lens group including three lenses of a positive lens, a negative lens, and a positive lens. A wide-angle lens having a long back focus, wherein at least one surface has an aspheric surface having a shape in which a negative refractive power becomes weaker toward the periphery, and satisfies the following conditions. (1) −4 <f _I /f<−1.5 (2) 1.5 <f _II / f <4 (3) 2 <D / f <8 (4) 1 × 10 ⁻⁵ <| Δ | / f _I < 1 × 10 ⁻¹ (y = y _EC ) where f is the focal length of the entire system, f _I is the focal length of the first lens group, f _II is the composite focal length of the second lens group, and D is the focal length of the first lens group. The distance between the rear principal point and the front principal point of the second lens group, Δ is the amount of deviation of the aspheric surface from the reference spherical surface, y is the height from the optical axis,
y _EC is the principal ray height at the maximum angle of view. (2) When the thickness of the negative lens in the second lens group and d _6, Batsukufuokasu long wide-angle lens of claims (1), characterized by satisfying the following condition (5). (5) 0.2 <d ₆ / f <2 (3) Focusing is performed by extending one negative lens in the first lens group or one positive lens closest to the image in the second lens group. A wide-angle lens having a long back focus in the range (1) or (2).