JP4744357B2 - Viewfinder lens - Google Patents

Description

  The present invention relates to a finder lens, and in particular, a small digital still camera using an image sensor such as a CCD, a mobile camera mounted on a portable information terminal device such as a mobile phone, a portable personal computer, a portable music player, or the like. The present invention relates to a thin reverse Galileo finder lens mounted on a digital video camera or the like.

  2. Description of the Related Art In recent years, due to remarkable technological progress of image sensors used in digital still cameras, digital video cameras, etc., higher density and higher pixels have been advanced, and accordingly, image sensors such as CCDs have been downsized. Further, in portable information terminal devices such as digital still cameras and mobile phones, as a result of the downsizing of the image sensor, the photographing lens used therefor is naturally becoming smaller and thinner. In particular, some recent digital still cameras have a camera body with a thickness of 11.5 mm or less and a thin single focus lens mounted thereon, and accordingly, the finder lens needs to be thinned.

In recent years, there has been a movement to mount an optical viewfinder on a mobile phone or the like in order to reduce power consumption as much as possible (for example, Patent Document 1, Patent Document 2, and Patent Document 3).
In this way, a thin digital still camera with a single focus lens in the photographic optical system, a mobile camera installed in a mobile phone terminal, etc. employ a virtual image finder, that is, a reverse Galileo finder lens, especially as a finder. Are known (for example, Patent Document 4 and Patent Document 5).

Although these finder lenses are formed by two lenses, a first lens having a negative refractive power and a second lens having a positive refractive power, the total lens length from the front surface of the first lens to the rear surface of the second lens. Is as long as about 18 mm to about 20 mm, and cannot be used particularly for a thin digital still camera in recent years.
In addition, when the distance between the first lens and the second lens is shortened to reduce the size of the two lenses, it is necessary to increase the refractive power of each lens. However, the curvature of the first lens and the second lens is required. Becomes too large, and aberration correction and lens processing become very difficult.

Also, three reverse Galileo finder lenses are known, but the total lens length is also as long as about 19 mm to 21 mm, and it is difficult to adopt in recent thin digital still cameras (for example, Patent Documents). 6).
Furthermore, in the case of a real image type optical finder, a method of enlarging an image formed by an objective lens with an eyepiece lens is adopted, and an optical system for inverting the image formed by the objective lens is necessary. The configuration becomes complicated, and it is difficult to reduce the overall length of the lens and the size of the unit.

JP 2001-320454 A Japanese Patent Laid-Open No. 2002-320001 JP 2006-42291 A JP 2002-214542 A Japanese Patent Laid-Open No. 10-282408 JP-A-5-215965

  The present invention has been made in view of the above circumstances, and the object of the present invention is to reduce the size and thickness, that is, to shorten the total length of the lens (for example, about 9 mm), An object of the present invention is to provide a small and thin reverse Galileo finder lens suitable for use with a mobile camera mounted on a portable information terminal device such as a telephone, a portable personal computer, or a portable music player.

The finder lens of the present invention includes, in order from the object side to the pupil side, a biconcave first lens having negative refractive power, and a meniscus first lens having negative refractive power and having a concave surface facing the object side. Two lenses and a biconvex third lens having positive refractive power, and the first lens is formed such that the absolute value of the refractive power of the pupil side surface is larger than the object side surface, The second lens is formed so that the absolute value of the refractive power of the object side surface is larger than the pupil side surface, and the third lens is the absolute value of the refractive power of the pupil side surface relative to the object side surface. The surface on the pupil side of the first lens has an aspherical surface whose negative refractive power decreases toward the periphery, and the object side surface of the second lens is The surface of the third lens on the pupil side has an aspherical surface whose negative refractive power increases as it goes to the periphery. The aspherical surface having a shape in which the positive refractive power decreases as it goes to the lens, the combined focal length of the first lens and the second lens is f12, the focal length of the third lens is f3, and the focal length of the first lens is If f1, the focal length f2 of the second lens, and the distance on the optical axis from the object-side surface of the first lens to the pupil-side surface of the third lens are T, conditional expressions (1), (2), (3),
(1) 0.3 <| f12 / f3 | <0.5
(2) 1.3 <f2 / f1 <3
(3) 8.0 mm ≦ T ≦ 11.5 mm
It is characterized by satisfying .
According to this configuration, by adopting an inverted Galilean lens arrangement and forming the objective lens with two lenses, the first lens and the second lens, the total lens length can be shortened, and various aberrations can be corrected well. can do.
In general, in order to shorten the total length of the finder lens, it is only necessary to increase the refractive power of the objective lens and the eyepiece lens, but this increases the amount of aberration. Therefore, by making the first lens a biconcave shape with concave surfaces facing the object side and the pupil side, distortion can be corrected well, and the second lens has a meniscus shape with the concave surface facing the object side. As a result, various off-axis aberrations can be corrected particularly well. Further, the first lens surface on the pupil side, the second lens surface on the object side, and the third lens surface on the pupil side are formed to have a large refractive power, and these surfaces have at least a large refractive power. By making the lens aspherical, the total lens length can be shortened (that is, downsizing and thinning) and various aberrations can be corrected well.
In particular, the first lens has an aspheric surface whose negative refractive power decreases as the pupil side surface moves toward the periphery, and the second lens object side surface decreases toward the periphery. Since the aspherical surface has a shape in which the refractive power of the third lens increases, and the pupil side surface of the third lens has an aspherical shape in which the positive refractive power decreases as it goes toward the periphery, various aberrations are improved. This can be corrected, and the amount of aberration generated in each lens can be reduced.
Furthermore, by satisfying the above conditional expression (1) that defines the finder magnification, it is possible to prevent the curvature radii of the first lens and the second lens from becoming small, thereby facilitating processing, and the third lens. It is possible to reduce the outer diameter of the lens by reducing the angle of the light beam emitted from the lens to the pupil side, and the conditional expression (2) regarding the ratio of the focal lengths of the first lens and the second lens forming the objective lens ), In particular, various off-axis aberrations can be corrected satisfactorily. Further, by satisfying conditional expression (3) regarding the overall length of the finder lens, various aberrations can be corrected satisfactorily. Further, it is possible to achieve miniaturization and thinning while preventing the lens curvature radius from becoming too small and achieving easy processing.

In the above configuration, the first lens, the second lens, and the third lens may be plastic lenses.
According to this configuration, since all plastic lenses are used, the degree of freedom of shape is higher than that of a glass lens, and it is easy to form a shape that is difficult to process with a glass lens, for example, the shape of the outer periphery of the lens. The finder unit including the lens frame (case) for holding the lens can be downsized. Further, even a lens having a reduced edge thickness (lens edge thickness) can be easily processed by changing the shape outside the effective beam diameter. Furthermore, the weight can be reduced at a lower cost than using a glass aspheric lens.

In the above configuration, the plastic lens can be formed of the same material.
According to this configuration, the plastic lens can be manufactured with a single injection molding machine, and the material can be used efficiently and the material procurement can be simplified, thereby reducing the cost. can do.

In the above configuration, a configuration in which a protective coat is applied to at least one of the object-side surface of the first lens and the pupil-side surface of the third lens can be employed.
According to this configuration, the lens surface of the first lens or the third lens exposed to the outside is protected in a state in which the finder lens is mounted on a digital camera or the like, thereby preventing the adhesion of dirt or the generation of scratches. Can do.

  According to the finder lens having the above-described configuration, it is possible to achieve a reduction in size and thickness (a reduction in the total lens length), and a digital still camera, or a portable information terminal device such as a mobile phone, a portable personal computer, a portable music player, or the like. It is possible to obtain a small and thin reverse Galileo finder lens suitable for use with a mobile camera mounted on the camera.

DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the accompanying drawings.
FIG. 1 is a structural view of a finder lens according to the present invention, and FIGS. 2 to 4 are aberration diagrams in an example based on specific numerical values.
As shown in FIG. 1, this finder lens includes, in order from the object side to the pupil side (pupil position EP), a biconcave first lens 1 having a negative refractive power, and an object having a negative refractive power. This is a reverse Galileo finder lens including a second meniscus lens 2 having a concave surface facing the side, and a biconvex third lens 3 having positive refractive power.

Here, in the first lens 1 to the third lens 3, as shown in FIG. 1, each surface is Si (i = 1 to 6), and the curvature radius of each surface Si is Ri (i = 1 to 6). ), The refractive index of the first lens 1 to the third lens 3 with respect to the d-line is represented by Ni (i = 1 to 3) and the Abbe number is represented by νi (i = 1 to 3). The distance (thickness, air interval) on the optical axis L from the first lens 1 to the pupil position (EP) is represented by Di (i = 1 to 6).
Further, the focal length of the first lens 1 is f1, the focal length of the second lens 2 is f2, the combined focal length of the first lens 1 and the second lens 2 is f12, and the first lens 1 from the object-side surface S1 The distance on the optical axis L to the pupil side surface S6 of the three lenses 3 is represented by T.

The first lens is preferably made of a plastic material, and as shown in FIG. 1, a biconcave lens having negative refractive power in which the object-side surface S1 is concave and the pupil-side surface S2 is concave. It is. Thus, by making the first lens 1 have a biconcave shape with concave surfaces facing the object side and the pupil side, distortion can be corrected well.
In addition, the first lens 1 has a radius of curvature R2 of the pupil side surface S2 smaller than the curvature radius R1 of the object side surface S1 (| R1 |> R2), that is, than the surface S1 of the object side. The absolute value of the refractive power of the pupil-side surface S2 is formed to be large, and the surface S2 having a large refractive power is formed to be at least an aspherical surface. Thereby, the total lens length can be shortened (that is, downsizing and thinning), and various aberrations can be corrected well.
Furthermore, the pupil-side surface S2 of the first lens 1 is formed as an aspheric surface having a negative refractive power that decreases toward the peripheral portion, whereby various aberrations can be favorably corrected. The amount of aberration generated in each lens can be reduced.

The second lens 2 is preferably formed of a plastic material, and as shown in FIG. 1, a meniscus lens having negative refractive power in which the object-side surface S3 is a concave surface and the pupil-side surface S4 is a convex surface. It is. Thus, by making the second lens 2 have a meniscus shape with the concave surface facing the object side, various off-axis aberrations can be corrected particularly well.
Further, in the second lens 2, the curvature radius R3 of the object side surface S3 is smaller than the curvature radius R4 of the pupil side surface S4 (| R4 |> | R3 |), that is, the pupil side surface S4. Further, the absolute value of the refractive power of the object side surface S3 is larger than that of the object side surface S3. Further, the surface S3 on the side where the refractive power is larger is at least aspherical. Thereby, the total lens length can be shortened (that is, downsizing and thinning), and various aberrations can be corrected well.
Furthermore, the surface S3 on the object side of the second lens 2 is formed as an aspheric surface having a negative refractive power that increases toward the peripheral portion, whereby various aberrations can be favorably corrected. The amount of aberration generated in each lens can be reduced.

The third lens 3 is preferably made of a plastic material, and has a biconvex shape having a positive refractive power in which the object-side surface S5 is a convex surface and the pupil-side surface S6 is a convex surface, as shown in FIG. It is a lens.
The third lens 3 has a radius of curvature R6 of the pupil-side surface S6 smaller than the radius of curvature R5 of the object-side surface S5 (R5> | R6 |), that is, is larger than that of the object-side surface S5. The absolute value of the refractive power of the surface S6 on the pupil side is increased, and the surface S6 on the higher refractive power side is at least aspherical. Thereby, the total lens length can be shortened (that is, downsizing and thinning), and various aberrations can be corrected well.
Furthermore, the pupil-side surface S6 of the third lens 3 is formed into an aspheric surface having a shape in which the positive refractive power decreases as it goes toward the periphery, so that various aberrations can be corrected favorably. The amount of aberration generated in each lens can be reduced.

Here, the first lens 1, the second lens 2, and the third lens 3 are all plastic lenses, so that the degree of freedom of shape is higher than that of the glass lens, and the shape that is difficult to process with the glass lens, for example, the outer periphery of the lens It is easy to make the shape of the part quadrangular, and the finder unit including the lens frame (case) that holds the lens can be downsized. In addition, even a lens with a reduced edge thickness (lens edge thickness) can be easily processed by changing the shape outside the effective beam diameter, and can be made cheaper and lighter than using a glass aspheric lens.
In addition, by forming the first lens 1, the second lens 2, and the third lens 3 from the same plastic material, it becomes possible to manufacture the lens with a single injection molding machine. Efficient use and simplified material procurement can be achieved, and further cost reduction can be achieved.
As the plastic material for the first lens 1, the second lens 2, and the third lens 3, acrylic resin (methacrylic resin, PMMA), polycarbonate resin (PC), etc. can be used. Various plastic materials such as cycloolefin polymer, norbornene resin, and fluorene polyester, which have improved properties, hygroscopicity, birefringence and the like, can be used.

The aspheric surfaces of the first lens 1, the second lens 2, and the third lens 3 are defined by the following formula.
Z = Cy ² / [1+ (1-εC ² y ² ) ^1/2 ] + Dy ⁴ + Ey ⁶ + Fy ⁸ + Gy ¹⁰
Where Z: distance from the tangent plane at the apex of the aspheric surface to a point on the aspheric surface whose height from the optical axis L is y, y: height from the optical axis, C: curvature at the apex of the aspheric surface ( = 1 / R), ε: conical constant, D, E, F, G: aspherical coefficients.

In the finder lens configured as described above, the combined focal length f12 of the first lens 1 and the second lens 2 and the focal length f3 of the third lens 3 are preferably conditional expression (1),
(1) 0.3 <| f12 / f3 | <0.5
It is formed so as to satisfy.
Conditional expression (1) defines the range of the finder magnification (f12 / f3). If the value of | f12 / f3 | exceeds the lower limit value of the conditional expression (1), the radii of curvature R1 to R4 of the first lens 1 and the second lens 2 constituting the objective lens become too small, which makes processing difficult. Therefore, it is not preferable. On the other hand, if the value of | f12 / f3 | exceeds the upper limit value of the conditional expression (1), the angle of light emitted from the third lens 3 to the pupil side becomes tight. Can't be achieved. That is, when the value of | f12 / f3 | satisfies the conditional expression (1), it is possible to prevent the radii of curvature R1 to R4 of the first lens 1 and the second lens 2 from being reduced and to facilitate the processing. In addition, the angle of the light beam emitted from the third lens 3 toward the pupil can be moderated to reduce the outer diameter of the lens.

In the finder lens configured as described above, the focal length f1 of the first lens 1 and the focal length f2 of the second lens 2 are preferably conditional expression (2).
(2) 1.3 <f2 / f1 <3
It is formed so as to satisfy.
Conditional expression (2) defines the range of the focal length ratio (f2 / f1) of the first lens 1 and the second lens 2 constituting the objective lens. That is, when the value of f2 / f1 satisfies the conditional expression (2), various off-axis aberrations can be particularly favorably corrected.

In the finder lens configured as described above, preferably, a protective coat is provided on at least one of the object-side surface S1 of the first lens 1 and the pupil-side surface S6 of the third lens 3. .
This protects the lens surface of the first lens 1 or the third lens 3 exposed to the outside in a state in which the finder lens is mounted on a digital camera or the like, and prevents dirt from being attached or scratches. it can.

In the finder lens having the above configuration, the distance T (mm) on the optical axis from the object-side surface S1 of the first lens to the pupil-side surface S6 of the third lens 3 is preferably conditional expression (3).
(3) 8.0 mm ≦ T ≦ 11.5 mm
It is formed so as to satisfy.
Conditional expression (3) defines the total length T of the finder lens. If the value of T exceeds the lower limit value of conditional expression (3), the refractive power of each lens becomes too small, making it difficult to correct aberrations well, and the radius of curvature of each lens becomes too small. Therefore, processing becomes difficult. On the other hand, if the value of T exceeds the upper limit value of conditional expression (3), the total length of the finder lens becomes too long, and it becomes impossible to achieve downsizing and thinning. That is, when the value of T satisfies the conditional expression (3), various aberrations can be corrected satisfactorily, and the lens radius of curvature can be prevented from becoming too small, facilitating processing, and downsizing. Thinning can be achieved.

  Next, specific numerical examples of the inverse Galileo finder lens will be described below as Example 1, Example 2, and Example 3.

The numerical data of the conditional expressions (1) to (3), the main specifications, and various numerical data (setting values) in the first embodiment are as follows. Further, the spherical aberration, astigmatism, and distortion in Example 1 are the results shown in FIG. In the astigmatism in FIG. 2, S indicates an aberration on the sagittal plane, and M indicates an aberration on the meridional plane.
<Value of conditional expression>
(1) 0.3 <| f12 / f3 | <0.5 → | f12 / f3 | = 0.40
(2) 1.3 <f2 / f1 <3 → f2 / f1 = 2.12
(3) 8.0 mm ≦ T ≦ 11.5 mm → T = 8.7 mm

<Specification specifications>
The focal length f1 of the first lens 1 = −6.05 mm, the focal length f2 of the second lens 2 = −12.84 mm, the focal length f3 of the third lens 3 = 9.04 mm, the first lens 1 and the second lens 2. Composite focal length f12 = −3.64 mm, finder magnification β = 0.40, finder lens half angle of view ω = 24.0 °, pupil diameter φ = 5.0 mm
<Curvature radius>
R1 = −13.425 mm (aspheric surface), R2 = 4.116 mm (aspheric surface), R3 = −3.944 mm (aspheric surface), R4 = −11.051 mm (aspheric surface), R5 = 57.796 mm, R6 = -4.921mm (aspherical surface)

<Spacing on the optical axis>
D1 = 1.140 mm, D2 = 2.800 mm, D3 = 1.300 mm, D4 = 0.700 mm, D5 = 2.760 mm, D6 = 15,000 mm
<Refractive index (Nd)>
N1 = 1.50914, N2 = 1.50914, N3 = 1.50914
<Abbe number (νd)>
ν1 = 56.4, ν2 = 56.4, ν3 = 56.4

<Numerical data of aspheric coefficient>
<S1 surface>
ε = 0.650000, D = 0.15552710 × 10 ⁻³ , E = −0.1756700 × 10 ⁻⁴ , F = 0.3906800 × 10 ⁻⁵ , G = 0.0000000
<S2 surface>
ε = 0.0000000, D = −0.1655530 × 10 ⁻⁶ , E = −0.2521250 × 10 ⁻⁵ , F = 0.164440 × 10 ⁻⁵ , G = 0.0000000
<S3 surface>
ε = 2.0094000, D = −0.6682670 × 10 ⁻³ , E = 0.2552850 × 10 ⁻⁴ , F = 0.6719390 × 10 ⁻⁵ , G = 0.8772710 × 10 ⁻⁸
<S4 surface>
ε = 1.2,000,000, D = −0.3298950 × 10 ⁻³ , E = 0.47448030 × 10 ⁻⁴ , F = −0.9506340 × 10 ⁻⁷ , G = 0.0000000
<S6 surface>
ε = 0.7590000, D = −0.2002920 × 10 ⁻⁴ , E = 0.1698300 × 10 ⁻⁵ , F = 0.163163130 × 10 ⁻⁷ , G = 0.0000000

  In Example 1, a small and thin finder lens having a total lens length T of 8.7 mm and various aberrations corrected satisfactorily as shown in FIG. 2 was obtained.

The numerical data of the conditional expressions (1) to (3), the main specification specifications, and various numerical data (setting values) in the second embodiment are as follows. Further, the spherical aberration, astigmatism, and distortion in Example 2 are the results shown in FIG. In the astigmatism in FIG. 3, S indicates an aberration on the sagittal plane, and M indicates an aberration on the meridional plane.
<Value of conditional expression>
(1) 0.3 <| f12 / f3 | <0.5 → | f12 / f3 | = 0.40
(2) 1.3 <f2 / f1 <3 → f2 / f1 = 2.12
(3) 8.0 mm ≦ T ≦ 11.5 mm → T = 8.7 mm

<Specification specifications>
The focal length f1 of the first lens 1 = −6.05 mm, the focal length f2 of the second lens 2 = −12.84 mm, the focal length f3 of the third lens 3 = 9.04 mm, the first lens 1 and the second lens 2. Composite focal length f12 = −3.64 mm, finder magnification β = 0.40, finder lens half angle of view ω = 24.0 °, pupil diameter φ = 5.0 mm
<Curvature radius>
R1 = -14.554 mm (aspherical surface), R2 = 4.802 mm (aspherical surface), R3 = −3.944 mm (aspherical surface), R4 = −11.051 mm (aspherical surface), R5 = 57.796 mm, R6 = -4.921mm (aspherical surface)

<Spacing on the optical axis>
D1 = 1.140 mm, D2 = 2.800 mm, D3 = 1.300 mm, D4 = 0.700 mm, D5 = 2.760 mm, D6 = 15,000 mm
<Refractive index (Nd)>
N1 = 1.58385, N2 = 1.50914, N3 = 1.50914
<Abbe number (νd)>
ν1 = 30.3, ν2 = 56.4, ν3 = 56.4

<Numerical data of aspheric coefficient>
<S1 surface>
ε = −7.4856600, D = 0.7613331 × 10 ⁻³ , E = −0.103626 × 10 ⁻³ , F = 0.71718050 × 10 ⁻⁵ , G = 0.0000000
<S2 surface>
ε = 0.0000000, D = 0.1208728 × 10 ⁻² , E = −0.1093354 × 10 ⁻³ , F = 0.7032872 × 10 ⁻⁵ , G = 0.0000000
<S3 surface>
ε = 2.0094000, D = −0.6682670 × 10 ⁻³ , E = 0.2552850 × 10 ⁻⁴ , F = 0.6719390 × 10 ⁻⁵ , G = 0.8772710 × 10 ⁻⁸
<S4 surface>
ε = 1.2,000,000, D = −0.3298950 × 10 ⁻³ , E = 0.47448030 × 10 ⁻⁴ , F = −0.9506340 × 10 ⁻⁷ , G = 0.0000000
<S6 surface>
ε = 0.7590000, D = −0.2002920 × 10 ⁻⁴ , E = 0.1698300 × 10 ⁻⁵ , F = 0.163163130 × 10 ⁻⁷ , G = 0.0000000

  In Example 2, a small and thin finder lens having a total lens length T of 8.7 mm and various aberrations corrected satisfactorily as shown in FIG. 3 was obtained.

The numerical data of the conditional expressions (1) to (3), the main specification specifications, and various numerical data (setting values) in Example 3 are as follows. In addition, the spherical aberration, astigmatism, and distortion in Example 3 are the results shown in FIG. In the astigmatism in FIG. 4, S represents the aberration on the sagittal plane, and M represents the aberration on the meridional plane.
<Value of conditional expression>
(1) 0.3 <| f12 / f3 | <0.5 → | f12 / f3 | = 0.40
(2) 1.3 <f2 / f1 <3 → f2 / f1 = 2.10.
(3) 8.0 mm ≦ T ≦ 11.5 mm → T = 8.7 mm

<Specification specifications>
The first lens 1 has a focal length f1 = −6.05 mm, the second lens 2 has a focal length f2 = −12.71 mm, the third lens 3 has a focal length f3 = 9.04 mm, the first lens 1 and the second lens 2. Composite focal length f12 = −3.64 mm, finder magnification β = 0.40, finder lens half angle of view ω = 24.0 °, pupil diameter φ = 5 mm
<Curvature radius>
R1 = −13.425 mm (aspherical surface), R2 = 4.116 mm (aspherical surface), R3 = −4.132 mm (aspherical surface), R4 = −10.401 mm (aspherical surface), R5 = 57.796 mm, R6 = -4.921mm (aspherical surface)

<Spacing on the optical axis>
D1 = 1.140 mm, D2 = 2.800 mm, D3 = 1.300 mm, D4 = 0.700 mm, D5 = 2.760 mm, D6 = 15,000 mm
<Refractive index (Nd)>
N1 = 1.50914, N2 = 1.58385, N3 = 1.50914
<Abbe number (νd)>
ν1 = 56.4, ν2 = 30.3, ν3 = 56.4

<Numerical data of aspheric coefficient>
<S1 surface>
ε = 0.650000, D = 0.15552710 × 10 ⁻³ , E = −0.1756700 × 10 ⁻⁴ , F = 0.3906800 × 10 ⁻⁵ , G = 0.0000000
<S2 surface>
ε = 0.0000000, D = −0.1655530 × 10 ⁻⁶ , E = −0.2521250 × 10 ⁻⁵ , F = 0.164440 × 10 ⁻⁵ , G = 0.0000000
<S3 surface>
ε = 2.1090470, D = −0.59999029 × 10 ⁻³ , E = −0.61833060 × 10 ⁻⁴ , F = 0.24343464 × 10 ⁻⁴ , G = −0.2622152 × 10 ⁻⁵
<S4 surface>
ε = −11.2921290, D = −0.1780159 × 10 ⁻² , E = 0.9596692 × 10 ⁻⁴ , F = −0.2369840 × 10 ⁻⁵ , G = 0.0000000
<S6 surface>
ε = 0.7590000, D = −0.2002920 × 10 ⁻⁴ , E = 0.1698300 × 10 ⁻⁵ , F = 0.163163130 × 10 ⁻⁷ , G = 0.0000000

  In Example 3, a small and thin finder lens having a total lens length T of 8.7 mm and various aberrations corrected satisfactorily as shown in FIG. 4 was obtained.

  As described above, since the finder lens of the present invention can achieve a reduction in size and thickness (a reduction in the total lens length), such as a digital still camera, a mobile phone, a portable personal computer, a portable music player, etc. As a finder lens for a mobile camera or the like mounted on a portable information terminal device, in particular, a fixed point lens having a half angle of view ω of 24 ° to 35 ° (or an angle of view 2ω of 48 ° to 70 °) is employed. It can be used as a finder lens for photographic equipment or as a finder lens for photographic equipment using an image sensor such as a CCD, and is also useful as a finder lens for other digital cameras or optical equipment that are not particularly required to be miniaturized. It is.

It is a block diagram which shows one Embodiment of the finder lens which concerns on this invention. FIG. 3 is an aberration diagram showing spherical aberration, astigmatism, and distortion at an infinite object distance in Example 1. FIG. 6 is an aberration diagram showing spherical aberration, astigmatism, and distortion at an infinite object distance in Example 2. FIG. 9 is an aberration diagram showing spherical aberration, astigmatism, and distortion at an infinite object distance in Example 3.

Explanation of symbols

L optical axis EP pupil position 1 first lens 2 second lens 3 third lens f1 focal length f2 of the first lens focal length f3 of the second lens focal length f12 of the third lens synthetic focus of the first lens and the second lens Distance T Distance from the object side surface of the first lens to the pupil side surface of the third lens

Claims (4)

In order from the object side to the pupil side, a biconcave first lens having negative refractive power, a meniscus second lens having negative refractive power and having a concave surface facing the object side, and positive refraction A biconvex third lens having force,
The first lens is formed such that the absolute value of the refractive power of the pupil side surface is larger than the object side surface;
The second lens is formed such that the absolute value of the refractive power of the object side surface is larger than the pupil side surface;
The third lens is formed such that the absolute value of the refractive power of the pupil side surface is larger than the object side surface,
The pupil side surface of the first lens has an aspherical surface having a negative refractive power that decreases toward the periphery.
The object side surface of the second lens has an aspherical surface whose negative refractive power increases toward the periphery,
The pupil-side surface of the third lens has an aspherical surface with a positive refractive power that decreases toward the periphery.
The combined focal length of the first lens and the second lens is f12, the focal length of the third lens is f3, the focal length of the first lens is f1, the focal length f2 of the second lens, and the object of the first lens When the distance on the optical axis from the side surface to the pupil side surface of the third lens is T, conditional expressions (1), (2), (3),
(1) 0.3 <| f12 / f3 | <0.5
(2) 1.3 <f2 / f1 <3
(3) 8.0 mm ≦ T ≦ 11.5 mm
A finder lens characterized by satisfying The first lens, the second lens, and the third lens are plastic lenses.
The finder lens according to claim 1. The plastic lenses are all made of the same material,
The finder lens according to claim 2. At least one of the object side surface of the first lens and the pupil side surface of the third lens is provided with a protective coat.
The finder lens according to any one of claims 1 to 3, wherein:

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