JP2008139786A - Photographic lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a photographic lens composed of three lenses, which has a small telephoto ratio, is miniaturized and also secures a sufficient angle of view. <P>SOLUTION: In the photographic lens, an aperture diaphragm 1, a first lens L1, a second lens L2 and a third lens L3 are arranged in order from an object side. The first lens L1 is a positive meniscus lens turning its convex surface to the object side, the second lens L2 is a negative meniscus lens turning its convex surface to an image side, and the third lens L3 is a positive meniscus lens turning its convex surface to the object side. The three lenses are formed of plastic lenses whose both surfaces are aspherical. The photographic lens satisfies following conditional expressions (1) and (2). (1) 0.7<f<SB>1</SB>/f<0.9 and (2) -3.5<f<SB>2</SB>/f<-1.2, provided that f is the focal length of the lens entire system, f<SB>1</SB>is the focal length of the first lens and f<SB>2</SB>is the focal length of the second lens. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a photographing lens having a three-lens configuration in which an aperture stop, a first lens, a second lens, and a third lens are arranged in this order from the object side.

  With the miniaturization of digital cameras and mobile terminals (cell phones, etc.), further miniaturization is required for the photographic optical systems used for these. Imaging devices such as CCDs and CMOSs used in photographing optical systems are also becoming smaller and having higher pixels, and it is necessary to design optical systems corresponding thereto.

  However, if the size of the image sensor is reduced, it becomes difficult to secure the back focus necessary for inserting a cover glass, an infrared cut filter, or the like, and the optical total length tends to be long. In general, when the optical total length is shortened, the telecentric characteristics are deteriorated.

  Thus, conventionally, a triplet lens is known as a small photographic lens, in which an aperture stop is disposed closest to the object side to move the exit pupil position away from the image plane and to ensure good telecentric characteristics. Among these, Patent Documents 1 to 7 listed below are known as prior arts that use a plastic material and have an aspherical shape with an inflection point to ensure optical performance and telecentric characteristics while shortening the optical total length. It has been.

JP-A-2005-004045 JP 2005-227755 A JP 2005-345919 A JP-A-2005-352206 JP 2006-106321 A JP 2006-133270 A JP 2006-163340 A

  However, each of these prior arts has the following problems. In Patent Document 1, the telephoto ratio (ratio of optical total length to focal length: T / f) is as large as 1.35 to 1.48, which is insufficient in terms of miniaturization. In Patent Document 2, there is an example in which the back focus is secured and the telephoto ratio is as small as 1.19, but the angle of view is as small as about 58 °. In Patent Document 3, the telephoto ratio is as large as 1.24 to 1.50. Patent Document 4 also has a large telephoto ratio of 1.38. In Patent Document 5, there is an example in which the telephoto ratio is as small as 1.19, but the angle of view is as small as about 58 °. Patent Document 6 is small with a telephoto ratio of 1.18, but the F-number is 3.85, which is dark. Patent Document 7 also has a large telephoto ratio of about 1.3.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a three-lens photographic lens capable of reducing the telephoto ratio to reduce the size and securing a sufficient angle of view.

In order to solve the above problems, a photographic lens according to the present invention provides:
From the object side, an aperture stop, a first lens, a second lens, a third lens arranged in this order,
The first lens is a positive meniscus lens having a convex surface facing the object side,
The second lens is a negative meniscus lens having a convex surface facing the image side,
The third lens is a positive meniscus lens having a convex surface facing the object side,
These three lenses are formed by plastic lenses with aspheric surfaces on both sides.
The following conditional expressions (1) and (2) are satisfied.
_{0.7 <f 1 / f <0.9} ···· (1)
−3.5 <f ₂ /f<−1.2 (2)
Here, f is the focal length of the entire lens system, f ₁ is the focal length of the first lens, and f ₂ is the focal length of the second lens.

  The operation and effect of the photographic lens having such a configuration will be described. The photographing optical system of this photographing lens has a three-lens configuration (triplet lens) of first, second, and third lenses. The three lenses are all made of plastic and all have both aspheric surfaces. In addition, by using the first, second, and third lenses as described above, the telephoto ratio can be suppressed, the size can be reduced, and the telecentric characteristics can be corrected well. Further, by satisfying both of the above conditional expressions (1) and (2), as can be seen from each of the examples described later, an adequate angle of view is secured while suppressing the telephoto ratio, and appropriate aberrations are corrected. Optical performance was also secured.

Incidentally, if f ₁ / f is 0.7 or less, the refractive power of the first lens becomes too large, and higher-order spherical aberration and coma aberration become worse. When f ₁ / f is 0.9 or more, the positive optical power of the first lens becomes weak, so that the total optical length becomes long. On the other hand, if f ₂ / f is −3.5 or less, the refractive power of the second lens becomes too small, and it becomes difficult to correct lateral chromatic aberration. If f ₂ / f is −1.2 or more, the field aberration is deteriorated and the aberration cannot be corrected satisfactorily.

In the present invention, when d ₂ is the distance between the first lens and the second lens,
_{0.10 <d 2 / f <0.25} ···· (3)
It is preferable to satisfy the following conditional expression.

When d ₂ / f is 0.10 or less, the distance between the first lens having positive refractive power and the second lens having negative refractive power becomes too narrow, and the balance of off-axis performance is deteriorated so that various aberrations can be corrected well. Disappear. When d ₂ / f is 0.25 or more, the distance between the first lens and the second lens is widened, so that the diameter of the third lens is increased and the miniaturization is impaired. In addition, coma flare increases.

In the present invention, when R ₁ is the object side radius of curvature of the first lens and R ₂ is the image side radius of curvature of the first lens,
_{0.20 <R 1 / R 2 <} 0.55 ···· (4)
It is preferable to satisfy the following conditional expression.

When R ₁ / R ₂ is 0.20 or less, spherical aberration increases and the back focus becomes longer than necessary. When R ₁ / R ₂ is 0.55 or more, the back focus is shortened and the telecentric characteristics are also deteriorated.

In the present invention, when R ₃ is the object side radius of curvature of the second lens and R ₄ is the image side radius of curvature of the second lens,
0.6 <R ₃ / R ₄ <0.8 (5)
It is preferable to satisfy the following conditional expression.

When R ₃ / R ₄ is 0.6 or less, field curvature and coma are overcorrected and performance deteriorates. When R ₃ / R ₄ is 0.8 or more, both the field curvature and coma are insufficiently corrected, and off-axis performance is deteriorated.

In the present invention, when R ₅ is the object side radius of curvature of the third lens,
0.6 <R ₅ /f<1.1 (6)
It is preferable to satisfy the following conditional expression.

If R ₅ / f is 0.6 or less, the radius of curvature of the third lens object side surface becomes too small, and the peripheral portion of the third lens falls into the image plane side, making it difficult to secure the lens back. Also, with such a lens shape, a strong ghost is likely to occur due to total reflection between the surfaces. When R ₅ / f is 1.1 or more, the telecentric characteristic is deteriorated, and coma aberration is deteriorated when this is corrected.

In the present invention, when f ₁₂ is the combined focal length of the first lens and the second lens,
_{1.0 <f 12 / f <1.4} ···· (7)
It is preferable to satisfy the following conditional expression.

If f ₁₂ / f is 1.0 or less, the combined refractive power of the first and second lenses becomes too strong and the back focus is shortened. In addition, the telecentric characteristics are also deteriorated. If f ₁₂ / f is 1.4 or more, the exit pupil position is away from the image plane, so that the telecentric characteristic is improved but the angle of view is narrowed.

In the present invention, when f ₃ is the focal length of the third lens,
_{-0.5 <f 2 / f 3 <} 0.0 ···· (8)
It is preferable to satisfy the following conditional expression.

If f ₂ / f ₃ is −0.5 or less, the positive refractive power of the third lens becomes too strong with respect to the negative refractive power of the second lens, and the lateral chromatic aberration cannot be balanced. When f ₂ / f ₃ is 0.0 or more, the positive refractive power of the third lens is lost and the telecentric characteristics are deteriorated.

  A preferred embodiment of a lens system according to the present invention will be described with reference to the drawings. FIGS. 1 to 8 show lens configuration diagrams (FIGS. 1A to 8A), optical system characteristic data, and aberration diagrams (FIGS. 1B to 8B) from Example 1 to Example 8. FIGS. 9A to 9C are diagrams showing aspheric data of each example. The three-lens imaging lens according to the present invention has a configuration particularly suitable as an optical system built in a digital camera or a portable device (for example, a cellular phone).

<Lens configuration diagram>
1A to 8A show the arrangement of the optical system in each example. In each embodiment, the photographing lens system includes an aperture stop 1, a first lens L1, a second lens L2, a third lens L3, a plane parallel glass 2, and an image plane 3 arranged in this order from the object side along the optical axis. Has been.

  The first lens L1 is a positive meniscus lens having a convex surface facing the object side, the second lens L2 is a negative meniscus lens having a convex surface facing the image side, and the third lens L3 has a convex surface facing the object side. This is a positive meniscus lens. All of these lenses are formed by plastic lenses having both aspheric surfaces. The plane parallel glass 2 has a function as an IR (infrared) cut filter, and an imaging element such as a CCD or a CMOS is disposed on the image plane 3.

  Since each lens L1-L3 is shape | molded with a plastic material, compared with the case where a glass lens is used, material cost can be reduced and the management cost of material can also be reduced. In addition, by using plastic, as in the present invention, all surfaces of each lens can be aspherical, and desired optical performance can be achieved with a small number of lenses of three. In addition, about a plastic material, it is preferable to employ | adopt cheap materials, such as a polycarbonate, and it is preferable to select material with a refractive index smaller than 1.6 about each lens L1-L3.

<Lens specifications and aberration diagrams>
1B to 8B will be described. The focal length f, F number F, and field angle 2ω are shown as lens specifications at the top of the figure. The focal length f is the focal length (mm) of the entire lens system. Below that, 1, 2,... 8 indicate the surface numbers in order from the object side. Surface numbers 1 and 2 are the first lens L1, surface numbers 3 and 4 are the second lens L2, and surface numbers 5 and 6 are the third lens L3. Since the surface numbers 7 and 8 are the parallel flat glass 2, the radius of curvature r is ∞. In addition, an aperture stop 1 is disposed at the position of surface number 1 of the first lens L1. In addition, the curvature radius r has shown the curvature radius (mm) on a paraxial. d is a numerical value indicating the surface separation (mm). nd is a refractive index and νd is an Abbe number.

  As aberration diagrams, spherical aberration, astigmatism, and distortion are shown. Each figure is data about the d-line, and astigmatism shows both data about the sagittal image plane (S) and data about the meridional image plane (M). As can be seen from these aberration diagrams, it can be seen that the aberration has been corrected to a level where there is no practical problem.

Each lens has an aspheric shape on both sides. The aspherical shape is expressed by X = (1 when A, B, C, D, E are aspherical coefficients and the displacement X in the optical axis direction at the position of the height H from the optical axis is expressed with respect to the surface vertex. ^{/ R) H 2 / [1+} {1- (1 + K) (H / R) 2} 1/2]
+ AH ⁴ + BH ⁶ + CH ⁸ + DH ¹⁰ + EH ¹²
It becomes. R is a paraxial radius of curvature, and K is a conical coefficient. An aspherical coefficient such as E-03 means 10 ⁻³ .

In the present invention, when f is the focal length of the entire lens system and f ₁ is the focal length of the first lens L1,
0.7 <f ₁ /f<0.9 (1)
It is preferable to satisfy the following conditional expression.

If f ₁ / f is 0.7 or less, the refractive power of the first lens L1 becomes too large, and higher-order spherical aberration and coma aberration deteriorate. When f ₁ / f is 0.9 or more, the positive optical power of the first lens L2 becomes weak, so that the total optical length becomes long.

In the present invention, when f is the focal length of the entire lens system and f ₂ is the focal length of the second lens L2,
−3.5 <f ₂ /f<−1.2 (2)
It is preferable to satisfy the following conditional expression.

If f ₂ / f is −1.2 or more, the field aberration is deteriorated and the aberration cannot be corrected satisfactorily.

In the present invention, when d ₂ is the distance between the first lens L1 and the second lens L2,
_{0.10 <d 2 / f <0.25} ···· (3)
It is preferable to satisfy the following conditional expression.

When d ₂ / f is 0.10 or less, the distance between the first lens L1 having a positive refractive power and the second lens L2 having a negative refractive power becomes too narrow, and the balance of off-axis performance is deteriorated and various aberrations are improved. Cannot be corrected. When d ₂ / f is 0.25 or more, the distance between the first lens L1 and the second lens L2 is widened, so that the diameter of the third lens L3 is increased and the downsizing is impaired. In addition, coma flare increases.

In the present invention, when R ₁ is the object-side radius of curvature of the first lens L1, and R ₂ is the image-side radius of curvature of the first lens L1,
_{0.20 <R 1 / R 2 <} 0.55 ···· (4)
It is preferable to satisfy the following conditional expression.

When R ₁ / R ₂ is 0.20 or less, spherical aberration increases and the back focus becomes longer than necessary. When R ₁ / R ₂ is 0.55 or more, the back focus is shortened and the telecentric characteristics are also deteriorated.

In the present invention, when R ₃ is the object side radius of curvature of the second lens L2, and R ₄ is the image side radius of curvature of the second lens L2,
0.6 <R ₃ / R ₄ <0.8 (5)
It is preferable to satisfy the following conditional expression.

When R ₃ / R ₄ is 0.6 or less, field curvature and coma are overcorrected and performance deteriorates. When R ₃ / R ₄ is 0.8 or more, both the field curvature and coma are insufficiently corrected, and off-axis performance is deteriorated.

In the present invention, when R ₅ is the object-side radius of curvature of the third lens L3,
0.6 <R ₅ /f<1.1 (6)
It is preferable to satisfy the following conditional expression.

When R ₅ / f is 0.6 or less, the radius of curvature of the third lens object side surface becomes too small, and the peripheral portion of the third lens L3 falls into the image plane side, making it difficult to secure the lens back. Also, with such a lens shape, a strong ghost is likely to occur due to total reflection between the surfaces. When R ₅ / f is 1.1 or more, the telecentric characteristic is deteriorated, and coma aberration is deteriorated when this is corrected.

In the present invention, when f ₁₂ is the combined focal length of the first lens L1 and the second lens L2,
_{1.0 <f 12 / f <1.4} ···· (7)
It is preferable to satisfy the following conditional expression.

If f ₁₂ / f is 1.0 or less, the combined refractive power of the first and second lenses L1 and L2 becomes too strong, and the back focus is shortened. In addition, the telecentric characteristics are also deteriorated. If f ₁₂ / f is 1.4 or more, the exit pupil position is away from the image plane, so that the telecentric characteristic is improved but the angle of view is narrowed.

In the present invention, when f ₃ is the focal length of the third lens L3,
_{-0.5 <f 2 / f 3 <} 0.0 ···· (8)
It is preferable to satisfy the following conditional expression.

When f ₂ / f ₃ is −0.5 or less, the positive refractive power of the third lens L3 is too strong with respect to the negative refractive power of the second lens L2, and the lateral chromatic aberration cannot be balanced. When f ₂ / f ₃ is 0.0 or more, the positive refractive power of the third lens L3 is lost, and the telecentric characteristics are deteriorated.

The above points are shown in Table 1 together with parameters concerning the conditional expressions for each example.

  As can be seen from Table 1 above, according to the photographic lens of the present invention, the telephoto ratio T / f (T is the length from the object side surface (surface number 1) of the first lens L1 to the imaging surface). It could be 1.15 or less, and the size could be reduced as compared with the prior art. As for the telecentric characteristics, α (maximum incident angle to the image pickup device) could be 25.0 ° or less, which was ensured satisfactorily. By the way, with regard to telecentric characteristics, it was necessary to emit the lens system with the chief ray tilt angle within 15 ° and make it incident on the image sensor, but now technical progress in the field of image sensor (changing the shape of the microlens) As a result, the angle of 25 ° or less is allowed. Further, the angle of view is 63 ° or more, and a sufficient size can be secured.

  As described above, according to the present invention, it is understood that the telephoto ratio can be reduced, and the size is suitable for mounting on a digital camera or a portable device.

FIG. 5 is a diagram illustrating a lens configuration of Example 1. Aberration diagram and optical system characteristic data of Example 1 FIG. 5 is a diagram illustrating a lens configuration of Example 2. Aberration diagram and optical system characteristic data of Example 2 FIG. 5 is a diagram illustrating a lens configuration of Example 3. Aberration diagram and optical system characteristic data of Example 3 FIG. 5 is a diagram illustrating a lens configuration of Example 4. Aberration diagram and optical system characteristic data of Example 4 FIG. 10 is a diagram illustrating a lens configuration of Example 5. Aberration diagram and optical system characteristic data of Example 5 FIG. 6 shows a lens configuration of Example 6. Aberration diagram and optical system characteristic data of Example 6 FIG. 10 shows a lens configuration of Example 7. Aberration diagram and optical system characteristic data of Example 7 FIG. 10 shows a lens configuration of Example 8. Aberration diagram and optical system characteristic data of Example 8 The figure which shows the aspherical surface coefficient of Example 1- Example 3. The figure which shows the aspherical surface coefficient of Example 4-Example 6. The figure which shows the aspherical surface coefficient of Example 7-8.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Aperture stop 2 Parallel plane glass 3 Imaging surface L1 1st lens L2 2nd lens L3 3rd lens

Claims (7)

From the object side, an aperture stop, a first lens, a second lens, a third lens arranged in this order,
The first lens is a positive meniscus lens having a convex surface facing the object side,
The second lens is a negative meniscus lens having a convex surface facing the image side,
The third lens is a positive meniscus lens having a convex surface facing the object side,
These three lenses are formed by plastic lenses with aspheric surfaces on both sides.
A photographic lens characterized by satisfying the following conditional expressions (1) and (2):
_{0.7 <f 1 / f <0.9} ···· (1)
−3.5 <f ₂ /f<−1.2 (2)
Here, f is the focal length of the entire lens system, f ₁ is the focal length of the first lens, and f ₂ is the focal length of the second lens. Furthermore, the following conditional expression (3) is satisfied, The taking lens according to claim 1 characterized by things.
_{0.10 <d 2 / f <0.25} ···· (3)
Here, d ₂ is the distance between the first lens and the second lens. Furthermore, the following conditional expression (4) is satisfied, The taking lens according to claim 1 or 2 characterized by things.
_{0.20 <R 1 / R 2 <} 0.55 ···· (4)
Here, R ₁ is the object-side radius of curvature of the first lens, and R ₂ is the image-side radius of curvature of the first lens. Furthermore, the following conditional expression (5) is satisfied, The taking lens according to any one of claims 1 to 3 characterized by things.
0.6 <R ₃ / R ₄ <0.8 (5)
Here, R ₃ is the object-side radius of curvature of the second lens, and R ₄ is the image-side radius of curvature of the second lens. Furthermore, the following conditional expression (6) is satisfied, The taking lens according to any one of claims 1 to 4 characterized by things.
0.6 <R ₅ /f<1.1 (6)
Here, R ₅ is the object-side radius of curvature of the third lens. Furthermore, the following conditional expression (7) is satisfied, The photographic lens according to claim 1, wherein:
_{1.0 <f 12 / f <1.4} ···· (7)
Here, f ₁₂ is the combined focal length of the first lens and the second lens. Furthermore, the following conditional expression (8) is satisfied, The taking lens according to any one of claims 1 to 6 characterized by things.
_{-0.5 <f 2 / f 3 <} 0.0 ···· (8)
Here, f ₃ is the focal length of the third lens.

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