JP4628554B2 - Single focus lens - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a single focus lens used as a photographing lens of an electronic camera, for example.
[0002]
[Prior art]
2. Description of the Related Art In recent years, electronic cameras that capture a subject using a solid-state imaging device such as a CCD (charge coupled device) instead of a silver salt film have become widespread. Electronic cameras generally include a still camera for taking a still image and a video camera for taking a moving image. Conventionally, examples of photographing lenses used in such electronic cameras include those described in Japanese Patent Laid-Open Nos. 8-152555 and 9-90213. Japanese Patent Application Laid-Open No. 8-152555 describes an invention relating to a photographic lens having a configuration using six glass lenses. On the other hand, Japanese Patent Application Laid-Open No. 9-90213 discloses an invention relating to a five-lens photographing lens using four glass lenses and one plastic lens. In the photographing lens described in Japanese Patent Laid-Open No. 9-90213, an aspherical surface is used for a plastic lens.
[0003]
[Problems to be solved by the invention]
By the way, there is a demand for downsizing of an electronic camera, like a camera using a silver salt film. For this reason, it is desirable that the taking lens used in the electronic camera is also reduced in size with a simple configuration and a reduced overall length. In addition, it is desirable that the manufacturing cost is kept low. However, the conventional photographing lens has a problem that such a condition is not sufficiently satisfied. For example, the photographing lens described in Japanese Patent Application Laid-Open No. 8-152555 has a lens configuration with a relatively large number of lenses, ie, six, and uses a glass material for all the lenses. There is a problem of becoming high. Further, for example, in the photographing lens described in Japanese Patent Laid-Open No. 9-90213, although the number of lenses is five and a plastic lens is used for one of the five, the cost is reduced, Has a problem that it is long and lacks compactness.
[0004]
In order to reduce the number of lenses and shorten the overall length of the photographic lens, it is conceivable to employ a configuration in which aspheric surfaces are positively used for a plurality of lens surfaces. At this time, it is desirable in terms of cost and manufacturability to use an optical resin (plastic) as a lens material for a lens using an aspheric surface rather than glass. On the other hand, plastic lenses have a large change in optical performance with respect to environmental changes such as temperature and humidity compared to glass lenses, so it is better to reduce the power in order to reduce this change in optical performance. Conceivable. In consideration of the above, when a plurality of aspheric surfaces are used in the photographing lens, it is desirable to use a plastic lens and to perform appropriate power distribution in consideration of environmental changes for the plastic lens.
[0005]
However, in the past, those that sufficiently satisfy conditions such as appropriate power distribution, simple configuration, cost, and short overall length have not been developed. In particular, conventionally, development of a lens in consideration of cost is insufficient.
[0006]
The present invention has been made in view of such problems, and an object of the present invention is to provide a single focus lens that has an extremely low cost and a simple configuration, has a short overall length, and can obtain optical performance that is mainly suitable for a photographing lens. It is to provide.
[0007]
[Means for Solving the Problems]
A single focus lens according to the present invention includes, in order from the object side, a meniscus first lens having a positive refractive power, a meniscus second lens having at least one aspherical surface, and an image plane. A third lens having a positive refractive power with a convex surface facing the side, and a meniscus fourth lens with a concave surface facing the image surface side and at least one surface made up of an aspherical surface. It will be.
[0008]
Here, in the single focus lens according to the present invention, it is desirable that the second lens and the fourth lens are made of an optical resin mainly from the viewpoint of manufacturability of an aspheric surface.
[0009]
In addition, the single focus lens according to the present invention has the following conditions in order to limit the refractive power of the second lens and the fourth lens using an optical resin, and to reduce deterioration of optical performance mainly due to environmental changes. It is desirable to satisfy the expressions (1) and (2).
−0.2 <f / f ₂ <0.2 (1)
−0.2 <f / f ₄ <0.2 (2)
However,
f ₂ : focal length of the second lens f ₄ : focal length of the fourth lens
Furthermore, it is desirable that the single focus lens according to the present invention satisfies the following conditional expressions (3) to (5) in order to mainly correct chromatic aberration.
1.70 <Nd1 (3)
35> νd1 (4)
50 <νd3 (5)
However,
Nd1: Refractive index of the first lens with respect to the d-line νd1: Abbe number of the first lens with respect to the d-line νd3: Abbe number of the third lens with respect to the d-line
In the single focus lens according to the present invention, since aspheric surfaces are used for a plurality of lens surfaces, it becomes easy to obtain good optical performance with a very small number of lenses while shortening the overall length.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0013]
FIG. 1 shows a configuration of a single focus lens according to an embodiment of the present invention, and shows a cross-sectional structure of each lens element in a single plane including an optical axis Z ₀ . In FIG. 1, the side indicated by the symbol Z _OBJ is the object side, that is, the side on which, for example, the subject for photographing is present. In FIG. 1, the side indicated by the symbol Z _IMG is the image forming side (image plane side), that is, the side on which the object image on the object side is formed. In FIG. 1, reference symbol Ri indicates the radius of curvature of the i-th lens surface that sequentially increases toward the image surface side, with the most object-side lens surface being the first. The symbol Di indicates the surface interval on the optical axis between the i-th lens surface and the i + 1-th lens surface. In FIG. 1, the part indicated by the symbol St represents the stop of the lens system. The single focus lens 1 according to the present embodiment is used as, for example, a photographing lens of an electronic camera, and an imaging surface of an imaging element such as a CCD is disposed on the imaging surface.
[0014]
As shown in FIG. 1, the single focus lens 1 according to the present embodiment includes a first meniscus lens L1 having positive refractive power (power), and at least one surface is an aspherical surface. The second lens L2 having a meniscus shape, the third lens L3 having a positive refractive power with the convex surface facing the image surface side, the concave surface facing the image surface side, and at least one surface is configured as an aspheric surface. And a meniscus fourth lens L4. The first lens L1 to the fourth lens L4 are sequentially arranged from the object side in this order. The stop St is located between the second lens L2 and the third lens L3. The third lens L3 is preferably made of a material having a strong convex surface facing the image surface side and a large Abbe number.
[0015]
The single focus lens 1 further includes a cover glass La disposed on the image forming side of the fourth lens L4. The cover glass La is for protecting the imaging surface of an imaging element such as a CCD. The surface on the image plane side of the cover glass La is disposed so as to coincide with the image plane, for example. When the surface on the image plane side of the cover glass La coincides with the image plane, the imaging surface of the image sensor comes into contact with the surface on the image plane side of the cover glass La.
[0016]
In the single focus lens 1, the second lens L2 and the fourth lens L4 are preferably composed of a plastic lens made of an optical resin mainly considering the manufacturability of aspherical processing. Here, examples of the optical resin that can be used as the plastic lens include acrylic resin, epoxy resin, and polycarbonate. However, since it is considered that the optical resin used as the plastic lens can obtain optical performance with higher resolution when the birefringence is smaller, the second lens L2 and the fourth lens L4 are compared in birefringence. It is desirable to use a small acrylic resin.
[0017]
In the single focus lens 1, it is desirable that the second lens L2 and the fourth lens L4 have a relatively weak power compared to other lens elements. Specifically, the focal length of the entire lens system f, and the focal length of the second lens L2 f _2, a focal length of the fourth lens L4 is taken as f _4, the following conditional expressions (1), It is desirable to be configured to satisfy (2).
[0018]
−0.2 <f / f ₂ <0.2 (1)
−0.2 <f / f ₄ <0.2 (2)
[0019]
The single focus lens 1 further has a refractive index Nd1 for the _d -line (wavelength λ _d = 587.6 nm) of the first lens L1, an Abbe number νd1 for the d-line of the first lens L1, and a third lens L3. It is desirable that the following conditional expressions (3) to (5) are satisfied when the Abbe number with respect to the d-line is νd3.
[0020]
1.70 <Nd1 (3)
35> νd1 (4)
50 <νd3 (5)
[0021]
Next, optical actions and effects brought about by the single focus lens 1 configured as described above will be described.
[0022]
In the single focus lens 1 according to the present embodiment, since the second lens L2 has an aspheric surface, mainly correction of spherical aberration and coma aberration is advantageously performed. Further, by disposing the second lens L2 behind the first lens L1 made of glass material, even if the second lens L2 is made of an optical resin, the lens surface can be prevented from being damaged. .
[0023]
Further, in this single focus lens 1, a strong convex surface is directed to the image surface side, and a positive third lens L3 made of a material having a large Abbe number is disposed, thereby mainly correcting chromatic aberration and curvature of field. It can be done well. Further, in this single focus lens 1, the fourth lens L4 arranged at a position close to the image plane has an aspheric surface and has a meniscus shape with a concave surface facing the image plane side. Distortion and field curvature correction are advantageously performed.
[0024]
Conditional expression (1) and conditional expression (2) described above limit the refractive powers of the second lens L2 and the fourth lens L4, respectively. In general, a lens using an optical resin is likely to change its optical performance such as focal length due to changes in environment such as temperature and humidity. This change in optical performance becomes more pronounced as the refractive power of the lens increases. Therefore, when optical resin is used for the second lens L2 and the fourth lens L4, the refractive power of the lens is limited so as to satisfy the ranges of the conditional expressions (1) and (2). Degradation of optical performance against environmental changes can be kept low. If the range of conditional expression (1) and conditional expression (2) is exceeded, the refractive power of the second lens L2 and the fourth lens L4 becomes strong, and is strongly affected by temperature and humidity, etc. There is a risk of performance degradation.
[0025]
Conditional expressions (3) to (5) are for limiting optical characteristics of the lens material (glass) in the lenses (first lens L1 and third lens L3) not using the optical resin. This mainly contributes to correction of chromatic aberration. When a glass lens exceeding the range of conditional expressions (3) to (5) is used, it is difficult to mainly correct chromatic aberration.
[0026]
As described above, according to the single focus lens 1 of the present embodiment, since aspheric surfaces are used for a plurality of lens surfaces, a good optical performance can be obtained with a very small number of lenses of four while shortening the overall length. Performance can be obtained. Further, since an optical resin is used for the aspheric lenses (second lens L2 and fourth lens L4), it becomes easy to perform aspheric processing, and it becomes easy to obtain desired optical performance. Further, the cost can be reduced in terms of aspherical processing. Furthermore, according to the present embodiment, for the aspheric lens, since appropriate power distribution is performed in consideration of the environmental change, for example, the shape or refractive index of the aspheric lens changes as the temperature changes. However, the change in the focal length of the entire lens system can be reduced. That is, the degree of deterioration of the optical performance with respect to environmental changes can be reduced.
[0027]
As described above, according to the single focus lens 1 of the present embodiment, by satisfying the above-described configuration and conditional expressions, the overall length is short, and for example, optical performance optimal for a photographing lens for an electronic camera can be achieved at a very low cost. Can be obtained.
[0028]
[Example]
Next, specific numerical examples of the single focus lens of the present embodiment will be described.
[0029]
<Example 1>
The cross-sectional structure of the single focus lens 1-1 of this embodiment is the same as that of the single focus lens 1 shown in FIG.
[0030]
2A and 2B show specific numerical examples of the single focus lens 1-1 according to the present embodiment. Surface numbers Si in FIGS. 2A and 2B indicate lens surface numbers that sequentially increase toward the image surface side, with the lens surface closest to the object side being the first. Refractive index Ndi and Abbe number νdi indicate values for the d-line, respectively. The curvature radius Ri indicates the curvature radius of the i-th lens surface from the object side, similarly to the symbol Ri shown in FIG. The surface distance Di is also the same as the symbol Di shown in FIG. 1, and indicates the distance on the optical axis between the i-th lens surface Si and the i + 1-th lens surface Si + 1 from the object side. The unit of the value of the curvature radius Ri and the surface interval Di is millimeter (mm). In the drawing, a lens surface having a radius of curvature Ri of 0 (zero) indicates that the surface shape is a plane. Further, FIG. 2A shows the focal length f (= 1.00 mm), F number (Fno. = 5.6) and angle of view 2ω (= 51.0) of the entire system in the single focus lens 1-1. The value of °) is also shown.
[0031]
In FIG. 2A, the symbol “*” attached to the left side of the surface number indicates that the lens surface is an aspherical surface. In the present embodiment, the lens surfaces S3 and S4 of the second lens L2 and the lens surfaces S8 and S9 of the fourth lens L4 are aspherical. Polymethylmethacrylate (PMMA) is used as a lens material for the second lens L2 and the fourth lens L4 having an aspheric surface. In FIG. 2A, numerical values of the radius of curvature near the optical axis are shown as the radius of curvature of these aspheric surfaces.
[0032]
FIG. 2B shows values of aspheric coefficients K, A ₄ , A ₆ , A ₈ and A ₁₀ for representing the aspheric shapes of the lens surfaces S3 and S4 and the lens surfaces S8 and S9. These aspheric coefficients are coefficients in an aspheric polynomial expressed by the following equation (A). Aspheric polynomial of equation (A) are those in the direction perpendicular to the optical axis Z ₀ taking the h-axis representing the aspherical shape. In the aspheric polynomial of the formula (A), h represents the distance (height) (unit: mm) from the optical axis Z ₀ to the lens surface. Z (h) represents the sag amount of the lens surface at the height h. More specifically, Z (h) is the length of a perpendicular line drawn from a point on the aspheric surface at a height h from the optical axis Z ₀ to the tangential plane (plane perpendicular to the optical axis) of the apex of the aspheric surface. (Unit: mm). C is the reciprocal (1 / R) of the paraxial radius of curvature R of the lens surface in the vicinity of the optical axis. K represents the eccentricity, and A ₄ , A ₆ , A ₈ , and A ₁₀ represent the fourth-order, sixth-order, eighth-order, and tenth-order aspheric coefficients, respectively. In the numerical value representing the aspheric coefficient shown in FIG. 2B, the symbol “E” indicates that the numerical value that follows is an exponent with 10 as the base, and an exponent function with 10 as the base. The numerical value represented by “E” is multiplied by the numerical value before “E”.
For example, “1.0E-02” indicates “1.0 × 10 ⁻² ”.
[0033]
Z (h) = Ch ² / {1+ (1−K · C ² · h ² ) ^1/2 }
+ A ₄ h ⁴ + A ₆ h ⁶ + A ₈ h ⁸ + A ₁₀ h ¹⁰ (A)
[0034]
FIG. 10 collectively shows values corresponding to the conditional expressions (1) to (5) described above for each of the present embodiment and later-described embodiments 2 and 3. In the single focus lens 1-1 of this example, the value of “f / f ₂ ” is “−0.012”, and the value of “f / f ₄ ” is “0.014”. The conditions (1) and (2) are satisfied. Further, since the refractive index Nd1 of the first lens L1 with respect to the d-line is “1.84666”, the condition of the conditional expression (3) is satisfied. Further, since the Abbe number νd1 of the first lens L1 with respect to the d-line is “23.8” and the Abbe number νd3 of the third lens L3 with respect to the d-line is “58.1”, the conditional expressions (4) and (4) The condition of 5) is satisfied.
[0035]
3A to 3D show various aberrations of the single focus lens 1-1 according to the present embodiment. More specifically, FIG. 3 (A) shows spherical aberration, FIG. 3 (B) shows astigmatism, FIG. 3 (C) shows distortion (distortion aberration), and FIG. 3 (D) shows lateral chromatic aberration. Show. In these figures, each aberration is based on the e-line. In each aberration diagram, the curves with the symbols g, e, and C indicate the aberrations for the g-line, e-line, and C-line, respectively. The wavelengths of g-line, e-line, and C-line are 435.8 nm, 546.1 nm, and 656.3 nm, respectively. In FIG. 3B, the solid line shows the aberration with respect to the sagittal image surface, and the dotted line shows the aberration with respect to the tangential (meridional) image surface. In each aberration diagram, ω represents a half angle of view.
[0036]
<Example 2>
Next, a second example of the single focus lens 1 according to the present embodiment will be described.
[0037]
5A and 5B show specific numerical examples of the single focus lens 1A according to the present embodiment. FIG. 4 shows the cross-sectional structure of the single focus lens 1A corresponding to the numerical example shown in FIG. The meanings of the numerical values shown in FIGS. 5A and 5B are the same as those in the first embodiment (FIGS. 2A and 2B).
[0038]
In the present embodiment, as in the first embodiment, the lens surfaces S3 and S4 of the second lens L2 and the lens surfaces S8 and S9 of the fourth lens L4 are aspherical. PMMA is used as a lens material for the second lens L2 and the fourth lens L4.
[0039]
Also in this embodiment, as shown in FIG. 10, the value of “f / f ₂ ” is “0.001” and the value of “f / f ₄ ” is “0.040”. The conditions of conditional expressions (1) and (2) are satisfied. Further, since the refractive index Nd1 of the first lens L1 with respect to the d-line is “1.84666”, the condition of the conditional expression (3) is satisfied. Further, since the Abbe number νd1 of the first lens L1 with respect to the d-line is “23.8” and the Abbe number νd3 of the third lens L3 with respect to the d-line is “81.6”, the conditional expressions (4) and (4) The condition of 5) is satisfied.
[0040]
6A to 6D show various aberrations of the single focus lens 1A according to the present example. More specifically, FIG. 6 (A) shows spherical aberration, FIG. 6 (B) shows astigmatism, FIG. 6 (C) shows distortion, and FIG. 6 (D) shows lateral chromatic aberration. In FIG. 6B, the solid line shows the aberration with respect to the sagittal image surface, and the dotted line shows the aberration with respect to the tangential image surface. The meaning of each symbol attached to these aberration diagrams is the same as in the case of Example 1 (FIGS. 3A to 3D).
[0041]
<Example 3>
Next, a third example of the single focus lens 1 according to the present embodiment will be described.
[0042]
8A and 8B show specific numerical examples of the single focus lens 1B according to the present embodiment. FIG. 7 shows the cross-sectional structure of the single focus lens 1B of the present example corresponding to the numerical example shown in FIG. The meanings of the numerical values shown in FIGS. 8A and 8B are the same as those in the first embodiment (FIGS. 2A and 2B).
[0043]
Also in this embodiment, the lens surfaces S3 and S4 of the second lens L2 and the lens surfaces S8 and S9 of the fourth lens L4 are aspherical. PMMA is used as a lens material for the second lens L2 and the fourth lens L4.
[0044]
Also in this embodiment, as shown in FIG. 10, the value of “f / f ₂ ” is “−0.015” and the value of “f / f ₄ ” is “0.018”. Conditional expressions (1) and (2) are satisfied. In addition, since the refractive index Nd1 of the first lens L1 with respect to the d-line is “1.992286”, the condition of the conditional expression (3) is satisfied. Further, since the Abbe number νd1 of the first lens L1 with respect to the d-line is “20.9” and the Abbe number νd3 of the third lens L3 with respect to the d-line is “58.1”, the conditional expressions (4), ( The condition of 5) is satisfied.
[0045]
9A to 9C show various aberrations of the single focus lens 1B according to the present example. More specifically, FIG. 9 (A) shows spherical aberration, FIG. 9 (B) shows astigmatism, FIG. 9 (C) shows distortion, and FIG. 9 (D) shows lateral chromatic aberration. In FIG. 9B, the solid line shows the aberration with respect to the sagittal image surface, and the dotted line shows the aberration with respect to the tangential image surface. The meaning of each symbol attached to these aberration diagrams is the same as in the case of Example 1 (FIGS. 3A to 3D).
[0046]
As described above, for all the single focus lenses of the examples, various aberrations are satisfactorily corrected in a state where the above conditional expressions are satisfied. For example, the optical performance optimum for a photographing lens for an electronic camera is obtained. You can see that
[0047]
In addition, this invention is not limited to the said embodiment, A various deformation | transformation implementation is possible.
For example, the values of the radius of curvature R, the surface interval D, the refractive index N, and the Abbe number ν of each lens component are not limited to the values shown in the above numerical examples, and may take other values.
[0048]
Further, the present invention is not limited to a photographing lens for an electronic camera but can be applied to a photographing lens for a camera using a so-called silver salt film.
[0049]
【The invention's effect】
As described above, according to the single focus lens of any one of claims 1 to 4, in order from the object side, the first meniscus lens having positive refractive power and at least one surface A second meniscus lens having an aspherical surface, a third lens having a positive refractive power with a convex surface facing the image surface side, a concave surface facing the image surface side, and at least one surface being non-surface Because it has a configuration in which a fourth meniscus lens made of spherical surfaces is arranged, the total number of lenses is small, the overall length is short, and the optical performance is optimal for photographic lenses. Can be obtained.
[0050]
In particular, according to the single focus lens according to claim 2, in the single focus lens according to claim 1, the second lens and the fourth lens having an aspheric surface are made of optical resin. It becomes easy to process and it becomes easy to obtain desired optical performance at low cost.
[0051]
In particular, according to the single focus lens of the third aspect, for the second lens and the fourth lens using the optical resin, “−0.2 <f / f ₂ <0.2” and “−0”. Since the conditional expressions (1) and (2) represented by .2 <f / f ₄ <0.2 ”are satisfied, the refractive powers of the second lens and the fourth lens are limited to be low. Therefore, it is possible to suppress degradation of optical performance with respect to environmental changes such as temperature and humidity.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing a configuration of a single focus lens according to an embodiment of the present invention.
FIG. 2 is an explanatory diagram showing a first specific numerical example (Example 1) of a single focus lens according to an embodiment of the present invention;
3 is an aberration diagram showing spherical aberration, astigmatism, distortion and lateral chromatic aberration in the single focus lens of Example 1 shown in FIG. 2; FIG.
FIG. 4 is a cross-sectional view showing a configuration of a second specific numerical example (Example 2) of the single focus lens according to the embodiment of the present invention.
FIG. 5 is an explanatory diagram showing a second specific numerical example of the single focus lens according to the embodiment of the present invention.
6 is an aberration diagram showing spherical aberration, astigmatism, distortion, and lateral chromatic aberration in the single focus lens of Example 2 shown in FIG. 5; FIG.
FIG. 7 is a cross-sectional view showing a configuration of a third specific numerical example (Example 3) of the single focus lens according to the embodiment of the present invention.
FIG. 8 is an explanatory diagram showing a third specific numerical example of a single focus lens according to an embodiment of the present invention.
9 is an aberration diagram showing spherical aberration, astigmatism, distortion, and lateral chromatic aberration in the single focus lens of Example 3 shown in FIG. 7; FIG.
FIG. 10 is an explanatory diagram showing condition values satisfied by a single focus lens of each example.
[Explanation of symbols]
La ... cover glass, L1 ... first lens, L2 ... second lens, L3 ... third lens, L4 ... fourth lens, Z ₀ ... optical axis, St ... diaphragm, 1, 1A, 1B ... single Focus lens.

Claims (2)

From the object side,
A meniscus first lens having positive refractive power;
A meniscus second lens having at least one surface formed of an aspheric surface;
A third lens having a positive refractive power with a convex surface facing the image surface side;
A single focus lens having a concave surface facing the image plane and a meniscus fourth lens having at least one aspherical surface ,
The second lens and the fourth lens are made of optical resin,
Furthermore, the single focus lens characterized by satisfying the following conditional expressions (1) and (2) .
−0.2 <f / f ₂ <0.2 (1)
−0.2 <f / f ₄ <0.2 (2)
However,
f: Focal length of the entire system
f ₂ : focal length of the second lens
f ₄ : focal length of the fourth lens Furthermore, the following conditional expression (3) to (5) single focus lens according to claim 1, characterized by being configured to satisfy the.
1.70 <Nd1 (3)
35> νd1 (4)
50 <νd3 (5)
However,
Nd1: Refractive index for the d-line of the first lens νd1: Abbe number for the d-line of the first lens νd3: Abbe number of the third lens for the d-line

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