JP2007140006A - Photographic lens and bifocal photographic lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a bifocal photographic lens having high performance while shortening the overall optical length, and to provide a photographic lens available for the bifocal photographic lens. <P>SOLUTION: The photographic lens includes a first lens system 13 having, in order from the object side: a first lens 1 having a positive refractive power; a second lens 2 having a negative refractive power; an aperture diaphragm 10; and a third lens 3 having a positive refractive power. The first lens system 13 satisfies the following conditional expressions: (1) TL<SB>t</SB>/f<SB>t</SB><1.0; (2) 3.0<f<SB>t</SB>/f<SB>t1</SB><3.2; (3) 1.9<f<SB>t</SB>/f<SB>t3</SB><2.1; and (4) 20<v<SB>t1</SB>-v<SB>t2</SB>. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a photographic lens mounted on a camera such as a digital still camera, and more particularly to a bifocal photographic lens capable of selecting two focal lengths by switching between a telephoto side lens system and a wide angle side lens system.

2. Description of the Related Art A bifocal photographing lens that can select two focal lengths by switching between a telephoto side lens system and a wide angle side lens system is known (for example, Patent Document 1). On the other hand, with the spread of camera-equipped mobile phones and the like, there is an increasing demand for downsizing of photographing lenses.
JP-A-8-320517

  However, the conventional bifocal photographing lens as described above has the following problems. That is, the bifocal photographing lens of Patent Document 1 has a problem that it has a long optical total length and is not suitable for miniaturization. Further, in the bifocal photographing lens of Patent Document 1, the zoom ratio (focal length ratio) is 1.74 times, but there is also a demand for further higher magnification.

  The present invention has been made to solve such problems of the prior art, and provides a high-performance bifocal photographic lens and a photographic lens that can be used for the bifocal photographic lens while shortening the total optical length. Objective.

  A photographic lens according to the present invention is a photographic lens including a first lens system including a first lens, a second lens, an aperture stop, and a third lens in order from the object side, wherein the first lens is , A lens having a positive refractive power, the second lens is a lens having a negative refractive power, the third lens is a lens having a positive refractive power, and the first lens system is The following conditions are satisfied.

TL _t / _ft <1.0 (1)
3.0 < _ft / _ft1 <3.2 (2)
1.9 < _ft / _ft3 <2.1 (3)
20 <ν _t1 −ν _t2 (4)
However, TL _t is the distance on the optical axis to the image side focal point of the lens system from the object side lens surface of the first lens, f _t is the focal length of the first lens system, the f _t1 focal length of the first lens, f _t3 is the focal length of the third lens, ν _t1 is the Abbe number of the first lens, and ν _t2 is the Abbe number of the second lens.

  In the photographic lens according to the above configuration, a lens system is configured by the first lens having positive refractive power, the third lens, and the second lens having negative refractive power. The aperture stop is provided behind the second lens (on the imaging surface side).

  Here, the conditional expression (1) in the present invention shows the relationship between the total length of the photographing lens and the focal length, and shows the condition for accommodating telephoto photography while being small.

  That is, when the optical total length ratio with respect to the focal length of the lens is larger than the upper limit of the conditional expression (1), the total length of the photographing lens becomes large and telephoto shooting becomes difficult.

  In addition, the conditional expression (2) in the present invention defines the refractive power of the first lens, and indicates a condition for obtaining a compact and good aberration performance.

  That is, when the ratio of the focal length of the lens system to the focal length of the first lens is smaller than the lower limit value of the conditional expression, the positive refractive power of the first lens is excessively lowered, and the downsizing of the overall length of the photographing lens is maintained. I can't. On the other hand, when the value exceeds the upper limit value of the conditional expression, the positive refractive power of the first lens is excessively increased, resulting in an extremely small curvature, which makes it difficult to process.

  In addition, conditional expression (3) in the present invention defines the refractive power of the third lens. The positive refractive power is appropriately set on the object side image surface of the third lens, and the field curvature correction is good. The conditions for performing are shown.

  That is, when the ratio of the focal length of the lens system to the focal length of the third lens becomes smaller than the lower limit value of the conditional expression, the positive refractive index of the third lens becomes too small and off-axis coma aberration occurs. On the other hand, if the upper limit of the conditional expression is exceeded, the positive refractive power of the third lens cannot be maintained, and the negative Pebbard sum increases, making it difficult to correct field curvature.

  Conditional expression (4) in the present invention defines the relationship between the Abbe number of the first lens and the Abbe number of the second lens, and indicates the condition for obtaining good chromatic aberration correction.

  That is, if the difference between the Abbe number of the first lens and the Abbe number of the second lens is below the lower limit value of the conditional expression, it is difficult to correct axial chromatic aberration and lateral chromatic aberration.

In this way, while adopting a very simple lens system of 3 groups and 3 elements as the lens system, and satisfying conditional expression (1), the total length is short and it can be used for telephoto shooting. Satisfactory aberration performance can be obtained by satisfying (2) to (4). In addition, as an application of the photographic lens, it may be used as a single telephoto lens,
It may be used as a telephoto side lens system of a bifocal photographing lens as described below.

  The bifocal photographing lens according to the present invention includes, in order from the object side, a telephoto side lens system and a wide angle side lens system having a shorter focal length than the telephoto side lens system, and the telephoto side lens system and the wide angle side lens A bifocal photographic lens capable of selecting two focal lengths by switching between systems, wherein the photographic lens is applied as the telephoto side lens system, and satisfies the following conditions with the wide angle side lens system: It is what.

TL _w <d _tbf (5)
Where TL _w is the distance on the optical axis from the aperture stop to the image-side focal point of the lens system when the wide-angle lens system is selected, and d _tbf is the lens surface closest to the imaging surface in the telephoto side lens system to the image-side focal point. The distance on the optical axis (back focus).

  The bifocal photographing lens according to the above configuration includes a telephoto side lens system having a long focal length and a wide angle side lens system having a short focal length, and is configured to be switchable between the telephoto side lens system and the wide angle side lens system. The focal length can be arbitrarily selected. The above-described photographing lens is used for the telephoto lens system.

  Here, the conditional expression (5) in the present invention is a causal relationship between the telephoto side lens system and the wide angle side lens system, specifically, the relationship between the back focus of the telephoto side lens system and the optical total length of the wide angle side lens system. It prescribes.

  In other words, if the above conditional expression is not satisfied, the back focus of the telephoto side lens system becomes shorter than the total optical length of the wide angle side lens system, and it becomes difficult to put in and out the telephoto side lens system and the wide angle side lens system alternately.

  As described above, in the bifocal photographing lens capable of switching two focal lengths, the conditional expression (5) is satisfied, whereby the telephoto side lens system and the wide angle side lens system can be switched smoothly. In addition, by applying a photographic lens having the first lens system as the telephoto side lens system, it is possible to suppress the overall length to be short and to obtain good aberration performance.

  Preferably, the wide-angle side lens system includes, in order from the object side, a second lens system including the aperture stop, a fourth lens, a fifth lens, and a sixth lens, and the fourth lens is a resin material. The fifth lens is a meniscus lens having a positive refractive power and convex on the object side, and the fifth lens is a meniscus lens using a resin material and having a negative refractive power on the object side, and the sixth lens. Is a meniscus lens that uses a resin material and is convex on the object side and has a positive refractive power, and the second lens system is configured to satisfy the following conditions.

TL _w / f _w <1.22 (6)
1.1 <f _w / f _w1 <1.5 (7)
−1.6 <f _w / f _w2 <−0.5 (8)
−3.0 <(N _w2 −1) f / r _w3 <−1.6 (9)
ν _w1 −ν _w2 > 20 (10)
Where f _w is the focal length of the second lens system, f _w1 is the focal length of the fourth lens, f _w2 is the focal length of the fifth lens, N _w2 is the refractive index of the fifth lens, and r _w3 is the fifth lens. Is the radius of curvature of the object side lens surface, ν _w1 is the Abbe number of the fourth lens, and ν _w2 is the Abbe number of the fifth lens.

  In this case, the fourth lens which is a meniscus lens having a positive refractive power on the object side, the fifth lens which is a meniscus lens having a negative refractive power on the object side, and a positive refractive power which is convex on the object side. The second lens system is configured by three lenses including a sixth lens that is a meniscus lens. The fourth, fifth and sixth lenses are made of a resin material. The aperture stop is provided in front of the fourth lens (on the object side). A lens system having such a configuration is used as the wide-angle side lens system.

  Here, the conditional expression (6) in the present invention defines the total length of the photographing lens (wide-angle side lens system) and indicates the condition for achieving miniaturization.

That is, when the ratio of the optical total length to the focal length of the second lens system is larger than the upper limit value of the conditional expression (6), the total length of the photographing lens is increased.

  In addition, conditional expression (7) in the present invention defines the refractive power of the fourth lens, and shows conditions for obtaining a compact and good aberration performance.

  That is, when the ratio of the focal length of the second lens system to the focal length of the fourth lens is smaller than the lower limit value of the conditional expression (7), the positive refractive power of the fourth lens is excessively reduced, and the total length of the photographing lens is reduced. The miniaturization cannot be maintained. On the other hand, when the value exceeds the upper limit value of conditional expression (7), the positive refractive power of the fourth lens increases excessively, resulting in an extremely small radius of curvature, resulting in poor workability.

  In addition, the conditional expression (8) in the present invention defines the refractive power of the fifth lens, and indicates conditions for obtaining a compact and good aberration performance.

  That is, when the ratio of the focal length of the lens system to the focal length of the fifth lens becomes smaller than the lower limit value of the conditional expression (8), the negative refractive power of the fifth lens increases and the downsizing of the overall length of the photographing lens is maintained. I can't. On the other hand, if the upper limit of conditional expression (8) is exceeded, the negative refractive power of the fifth lens will be excessively reduced, and spherical aberration, axial chromatic aberration, and distortion occurring in the fourth lens will be corrected satisfactorily. I can't.

  In addition, the conditional expression (9) in the present invention indicates a condition for appropriately setting the negative refractive power on the object side surface of the fifth lens and performing the field curvature correction satisfactorily.

That is, the function (N _w2 −1) f _w / r _w3 regarding the refractive index of the fifth lens, the focal length of the second lens system, and the radius of curvature of the object side lens surface of the fifth lens is expressed by the conditional expression (9). If it becomes smaller than the lower limit value, the negative refractive power becomes too large on the object side lens surface of the fifth lens, and off-axis coma aberration occurs. On the other hand, if the upper limit of conditional expression (9) is exceeded, the negative refractive power of the fifth lens cannot be maintained, the positive Pebbard sum increases, and it becomes difficult to correct field curvature. In addition, spherical aberration and coma generated in the fourth lens cannot be corrected well.

  Conditional expression (10) in the present invention defines the relationship between the Abbe number of the fourth lens and the Abbe number of the fifth lens, and indicates the condition for obtaining good chromatic aberration correction.

  That is, by making the difference between the Abbe number of the fourth lens and the Abbe number of the fifth lens larger than the lower limit value of the conditional expression (10), it is possible to satisfactorily correct the axial chromatic aberration and the lateral chromatic aberration.

  As described above, the lens system having a very simple configuration of 3 elements in 3 groups is adopted as the wide-angle side lens system, while satisfying the conditional expressions (6) to (10), the total length is short and the lens is sufficiently wide-angle. At the same time, good aberration performance can be obtained. Further, since all the lenses of the second lens system use the resin material, the cost is low and the production can be easily performed.

  Alternatively, the wide-angle side lens system includes, in order from the object side, a third lens system including the aperture stop, the seventh lens, and the eighth lens, and the seventh lens is convex and positive on the object side. The eighth lens is a meniscus lens having a positive refractive power on the image side, and the third lens system is configured to satisfy the following condition.

0.5 <f _w1 / f _w <1.5 (11)
1.0 <f _w / r _w1 <4.0 (12)
Where f _w1 is the focal length of the seventh lens, f _w is the focal length of the third lens system, and r _w1 is the radius of curvature of the object side lens surface of the seventh lens.

  In this case, the second lens is a seventh lens which is a meniscus lens having a positive refractive power having a convex surface on the object side lens surface, and the eighth lens which is a meniscus lens having a positive refractive power having a convex surface on the image side lens surface. A third lens system is configured by the single lens. The aperture stop is provided in front of the first lens (on the object side) or behind (on the image side). A lens system having such a configuration is used as the wide-angle side lens system.

  Here, conditional expression (11) in the present invention prescribes the power arrangement of the seventh lens (front group) with respect to the third lens system, and corrects various aberrations satisfactorily while achieving miniaturization. It shows the conditions to obtain.

  That is, when the ratio of the focal length of the seventh lens to the focal length of the third lens system exceeds the upper limit of the conditional expression (11), the power arrangement of the front group with respect to the third lens system becomes weak, and the third lens The total length of the system becomes longer. On the other hand, if the lower limit of conditional expression (11) is exceeded, the power distribution of the front group with respect to the third lens system becomes strong, so that various aberrations occurring in the front group cannot be corrected by the rear group (eighth lens). .

  In addition, the conditional expression (12) in the present invention defines the power arrangement of the object side lens surface of the seventh lens with respect to the third lens system, and improves the workability while having high lens performance. The conditions that can be used are shown.

  That is, when the ratio of the curvature radius of the object side lens surface of the seventh lens to the focal length of the third lens system exceeds the upper limit of the conditional expression (12), the curvature of the object side lens surface of the seventh lens becomes too large. This makes it difficult to process the seventh lens. On the other hand, if the lower limit of conditional expression (12) is exceeded, the amount of distortion correction increases, and the angle of incidence of the image plane at the maximum field angle increases, so in particular for a photographic lens using a solid-state image sensor. Shading (peripheral dimming, etc.) occurs, and the lens performance cannot be increased.

  In this way, the wide-angle lens system is composed of two lenses, and each parameter of the lens system satisfies the conditional expression (11), thereby achieving miniaturization and satisfying the conditional expressions (11) and (12). Accordingly, various aberrations can be corrected satisfactorily, and in addition, the workability can be improved by satisfying conditional expression (12).

  According to the bifocal photographic lens of the present invention, in the bifocal photographic lens capable of switching between two focal lengths, switching between the telephoto side lens system and the wide angle side lens system is satisfied by satisfying conditional expression (5). It can be done smoothly. In addition, by adopting a very simple lens system of 3 elements in 3 groups as the telephoto side lens system, and satisfying conditional expression (1), the total length is short and it can be used for telephoto shooting. Satisfactory aberration performance can be obtained by satisfying the expressions (2) to (4).

  Furthermore, while adopting a lens system with a very simple configuration of 3 elements in 3 groups as the wide-angle side lens system, by satisfying conditional expressions (6) to (10), the overall length is short and sufficiently wide, Good aberration performance can be obtained. Further, since all the lenses of the second lens system use the resin material, the cost is low and the production can be easily performed.

  Alternatively, the wide-angle lens system is composed of two lenses, and each parameter of the lens system satisfies the conditional expression (11), thereby achieving miniaturization and satisfying the conditional expressions (11) and (12). Thus, various aberrations can be corrected satisfactorily, and in addition, the workability can be improved by satisfying conditional expression (12).

  The best mode for carrying out the present invention will be described below with reference to the accompanying drawings.

<First Embodiment>
FIG. 1 is a cross-sectional view of the bifocal photographing lens in the first embodiment of the present invention when switching the telephoto side lens system, and FIG. 2 is a cross-sectional view of the bifocal photographing lens of FIG. .

  The bifocal photographing lens according to the present embodiment includes, in order from the object side, a telephoto side lens system and a wide angle side lens system having a shorter focal length than the telephoto side lens system, and the telephoto side lens system and the wide angle side lens system This is a bifocal photographing lens that can select two focal lengths by switching between the two. The telephoto side lens system is a photographic lens including a first lens system 13 including a first lens 1, a second lens 2, an aperture stop 10, and a third lens 3 in order from the object side. The first lens 1 is a lens having a positive refractive power, the second lens 2 is a lens having a negative refractive power, and the third lens 3 is a lens having a positive refractive power. is there.

  Further, the wide-angle lens system includes a second lens system 14 including an aperture stop 11, a fourth lens 4, a fifth lens 5, and a sixth lens 6 in order from the object side, and the fourth lens 4. Is a meniscus lens in which a resin material is used and the object side is convex and has a positive refractive power. The fifth lens 5 is a meniscus lens in which a resin material is used and the object side is concave and has a negative refractive power. The sixth lens 6 is a meniscus lens made of a resin material, convex on the object side and having a positive refractive power.

  Each parameter value of the photographic lens in the present embodiment having the above configuration is as follows.

Here, r _tj and r _wj are curvature radii (mm) at j-th surface numbers R _tj and R _wj in order from the object side, d _tj and d _wj are j-th surface center distances (mm) in order from the object side, N _td, N _wd is the refractive index of the d-th lens, ν _td, ν _wd is the Abbe number of d-th lens, f _t, f _w is the focal length of the lens system (mm), Fno. Is the open F number, and 2Y is the diagonal length (mm) of the effective screen.

Further, the aspherical shape is such that the distance (sag amount) in the optical axis direction from the tangent plane of the surface vertex is x, the height h from the optical axis, r is the paraxial radius of curvature, κ is the conic constant, and A _m ( When m = 4, 6, 8,..., 12) is the m-th aspherical coefficient, it is expressed by the following equation.

x = {(1 / r) h ² } / [1+ {1− (1 + κ) (1 / r) ² h ² } ^1/2 ]
+ A ₄ h ⁴ + A ₆ h ⁶ + A ₈ h ⁸ + A ₁₀ h ¹⁰ + A ₁₂ h ¹² (X)
Therefore, in the following, it shows a value of A _m in the above formula (X), and specifies a non-spherical shape.

  First, each parameter value of the telephoto lens system shown in FIG. 1 is as shown below.

  The parameter values of the wide-angle lens system shown in FIG. 2 are as shown below.

In the present embodiment, the distance TL _t on the optical axis from the object side lens surface of the first lens to the image side focal point of the lens system is 9.03, and the focal length f _{t of} the first lens system 13 is satisfied. = 9.3, so TL _t / f _t = 0.971 <1.0, which satisfies the conditional expression (1). Further, since the focal length f _t1 = 3.06 of the first _{lens, 3.0 <f t / f t1} = 3.039 <3.2 , and the thus satisfying the conditional expression (2). Further, since the focal length f _t3 = 4.88 of the third lens, satisfies _{1.9 <f t / f t3 =} 1.95 <2.1 , and the conditional expression (3). Further, since the Abbe number ν _t1 = 56.4 of the first lens and the Abbe number ν _t2 = 29.9 of the second lens, 20 <ν _t1 −ν _t2 = 26.5, and the conditional expression (4 ) Is satisfied.

  FIG. 3 is an aberration diagram in the telephoto side lens system of FIG. 3A shows spherical aberration, FIG. 3B shows astigmatism, and FIG. 3C shows distortion.

  As shown in FIG. 3, the lens system has a very simple configuration of 3 elements in 3 groups and satisfies conditional expression (1), so that the total length is short and it can be used for telephoto shooting. Satisfying conditional expressions (2) to (4) makes it possible to obtain good aberration performance.

In the telephoto lens system described in the present embodiment, lenses having aspherical surfaces are used in the lens surfaces R _t1 , R _t2 , R _t3 , R _t4 , R _t5, and R _t6 , but the present invention is not limited to this. It is not limited to these, and can be selected and adopted as appropriate.

Further, when the wide-angle side lens system is selected, the distance TL _w on the optical axis from the aperture stop to the image side focal point of the lens system is 4.56, and the image from the lens surface closest to the imaging surface in the telephoto side lens system is obtained. Since the distance on the optical axis to the side focal point (back focus) d _tbf = d _t7 = 6.01, 4.56 = TL _w <d _tbf = 6.01 is satisfied, and the conditional expression (5) is satisfied. Yes.

As described above, in the bifocal photographing lens capable of switching two focal lengths, by satisfying the conditional expression (5), each lens is used as the optical axis when switching to the telephoto side lens system or the wide angle side lens system. By simply moving in the orthogonal direction, the two lenses can be switched smoothly without contacting each other. Therefore, an increase in size and complexity of such a bifocal photographing lens switching mechanism can be prevented, and a more compact and inexpensive bifocal photographing lens can be realized. In the present embodiment, d _tbf −TL _w = 1.45 has a margin, and a shutter device or the like can be provided in this margin.

Further, in the present embodiment, since the focal length f _w = 3.82 of the wide-angle side lens system, TL _w / f _w = 1.20 <1.22, which satisfies the conditional expression (6). . Further, since the focal length f _w1 of the fourth lens 4 is 3.08, 1.1 <f _w / f _w1 = 1.24 <1.5, which satisfies the conditional expression (7). Since the focal length f _w2 of the fifth lens 5 is 2.77, −1.6 <f _w / f _w2 = −1.38 <−0.5, which satisfies the conditional expression (8). Yes. Further, since the refractive index N _{w2 of} the fifth lens 5 is 1.58 and the radius of curvature r _w3 of the object side surface R _w3 of the fifth lens 5 is −1.128, −3.0 <(N _w2 − 1) f _w / r _w3 = −2.06 <−0.5, which satisfies the conditional expression (9). Finally, since the Abbe number ν _w1 of the fourth lens 4 is 56.40 and the Abbe number ν _w2 of the fifth lens 5 is 29.90, ν _w1 −ν _w2 = 26.50> 20. Expression (10) is satisfied.

  FIG. 4 is an aberration diagram in the wide-angle side lens system of FIG. 4A shows spherical aberration, FIG. 4B shows astigmatism, and FIG. 4C shows distortion.

  As shown in FIG. 4, while satisfying the conditional expressions (6) to (10), the lens system has a very simple configuration of 3 elements in 3 groups, and the overall length is short and a sufficiently wide angle is good. Aberration performance can be obtained. Further, since all the lenses use a resin material, the cost is low and the lens can be easily produced.

  In the bifocal photographing lens as described above, the zoom magnification (ratio of the focal lengths of the wide-angle side lens system and the telephoto side lens system) is 2.5 times, compared with the conventional example (1.74 times in Patent Document 1). A sufficiently high magnification can be ensured.

Second Embodiment
FIG. 5 is a cross-sectional view of the bifocal photographing lens in the second embodiment of the present invention when the wide-angle lens system is switched.

  In the second embodiment, the telephoto side lens system is exactly the same as that of the first embodiment, and the wide angle side lens system is composed of the same lens system. Each parameter value of the wide angle side lens system shown in FIG. 5 is as follows.

In the present embodiment, since the distance TL _w on the optical axis from the aperture stop to the image side focal point is 4.565, and the focal length f _w of the wide angle side lens system is 3.89, TL _w / f _w = 1.18 <1.22, which satisfies the conditional expression (6). Since the focal length f _w1 of the fourth lens 4 is 2.90, 1.1 <f _w / f _w1 = 1.34 <1.5, which satisfies the conditional expression (7). Since the focal length f _w2 of the fifth lens 5 is 4.58, −1.6 <f _w / f _w2 = −0.85 <−0.5, which satisfies the conditional expression (8). Yes. Further, since the refractive index N _{w2 of} the fifth lens 5 is 1.5855 and the radius of curvature r _w3 of the object side surface R _w3 of the fifth lens 5 is −1.095, −3.0 <(N _w2 − 1) f _w / r _w3 = −2.1 <−0.5, which satisfies the conditional expression (9). Finally, since the Abbe number ν _w1 of the fourth lens 4 is 56.40 and the Abbe number ν _w2 of the fifth lens 5 is 29.90, ν _w1 −ν _w2 = 26.50> 20. Expression (10) is satisfied.

  FIG. 6 is an aberration diagram in the wide-angle side lens system of FIG. 6A shows spherical aberration, FIG. 6B shows astigmatism, and FIG. 6C shows distortion.

  Also in this embodiment, as shown in FIG. 6, although the lens system has a very simple configuration of three elements in three groups, the total length is sufficiently short by satisfying conditional expressions (6) to (10). In addition to a wide angle, good aberration performance can be obtained. Further, since all the lenses use a resin material, the cost is low and the lens can be easily produced.

<Third Embodiment>
FIG. 7 is a cross-sectional view of the bifocal photographing lens in the third embodiment of the present invention when the wide-angle lens system is switched.

  Also in the third embodiment, the telephoto side lens system is exactly the same as that of the first embodiment, and the wide angle side lens system is composed of the same lens system. Each parameter value of the photographic lens in this embodiment is as follows.

In the present embodiment, the distance TL _w on the optical axis from the aperture stop to the image side focal point is TL _w = 4.
561 and the focal length of the lens system f _w = 3.85, so TL _w / f _w = 1.19.
<1.22 is achieved, thereby satisfying the expression (6). Further, the focal length f _w1 of the fourth lens 4 =
Since 3.40, 1.1 <f _w / f _w1 = 1.13 <1.5, which satisfies the conditional expression (7). Since the focal length f _w2 of the fifth lens 5 is 4.81, −1.6 <f _w
/ F _w2 = −0.8 <−0.5, which satisfies the conditional expression (8). Further, the refractive index N _{w2 of} the fifth lens 5 is 1.5855, and the radius of curvature r _w3 of the object side surface R _w3 of the fifth lens 5 is −−.
Since 1.396, −3.0 <(N _w2 −1) f _w / r _w3 = −1.6 <−0.5, which satisfies the conditional expression (9). Finally, since the Abbe number ν _w1 of the fourth lens 4 is 56.40 and the Abbe number νw2 of the fifth lens 5 is 29.90, νw1−ν _w2 = 26.50.
> 20, which satisfies the conditional expression (10).

  FIG. 8 is an aberration diagram in the wide-angle side lens system of FIG. FIG. 8A shows spherical aberration, FIG. 8B shows astigmatism, and FIG. 8C shows distortion.

  Also in this embodiment, as shown in FIG. 8, although the lens system has a very simple configuration of 3 elements in 3 groups, the total length is sufficiently short by satisfying conditional expressions (6) to (10). In addition to a wide angle, good aberration performance can be obtained. Further, since all the lenses use a resin material, the cost is low and the lens can be easily produced.

In the wide-angle side lens system described in the above embodiments, lenses that are aspherical at the lens surfaces R _w1 , R _w2 , R _w3 , R _w4 , R _w5, and R _w6 are used. The present invention is not limited to this, and can be selected and adopted as appropriate.

<Fourth embodiment>
FIG. 9 is a cross-sectional view of the bifocal photographing lens according to the fourth embodiment of the present invention when the wide-angle lens system is switched.

Also in the fourth embodiment, the telephoto side lens system is exactly the same as in the first embodiment. On the other hand, as shown in FIG. 9, the wide-angle side lens system includes a third lens system 15 including a seventh lens 7, an aperture stop 12, and an eighth lens 8 in order from the object side. The seventh lens 7 is a positive meniscus lens having a convex surface on the object side lens surface R _w1 , and the eighth lens 8 is a positive meniscus lens having a convex surface on the image side lens surface R _w4 . An optical member 9 such as a face plate or a filter in a solid-state imaging device such as a CCD is disposed between the image side lens surface R _{w4 of} the eighth lens 8 and the imaging surface IMG.

  Each parameter value of the wide-angle side lens system shown in FIG. 9 is as follows.

In the present embodiment, the focal length f _w1 of the seventh lens 7 is 3.542 mm,
Since the focal length f _{w of} the third lens system 15 is 2.628 mm, 0.5 <f _w1 / f _w =
1.34 <1.5 is achieved, thereby satisfying the conditional expression (11). Further, since the curvature radius r _w1 of the object-side lens surface R _w1 of the seventh lens 7 is 0.700 mm, 1.0 <f _w / r _w1 = 3.7
5 <4.0, which satisfies the conditional expression (12). The total length L of the third lens system 15
The focal length fw in normalized values L / f _w = 1.26 next to the third lens system 15, and very 1.3 times or less of the focal length f _w of the total length L of the third lens system 15 is a lens system Realizes a compact photographic lens.

  FIG. 10 is an aberration diagram in the wide-angle side lens system of FIG. FIG. 9 (a) shows spherical aberration, FIG. 9 (b) shows astigmatism, and FIG. 9 (c) shows distortion.

  In the present embodiment, each aberration is corrected well in spite of realizing a very compact photographing lens as compared with the prior art.

<Fifth Embodiment>
FIG. 11 is a cross-sectional view of the bifocal photographing lens in the fifth embodiment of the present invention when the wide angle side lens system is switched.

  The photographing lens according to this embodiment is different from the fourth embodiment in that the position of the aperture stop 12 is in front of the seventh lens 7 (object side), as shown in FIG.

Each parameter value of the photographic lens in the present embodiment having the above configuration is as follows.

In the present embodiment, the focal length f _w1 = 2.669mm of the seventh lens 7, since the focal length f _w = 2.323mm of the third lens system 15, 0.5 <f _w1 / f _w = 0.86 <1.5 is achieved, thereby satisfying the conditional expression (11). Further, since the curvature radius r _w1 of the object-side lens surface R _w1 of the seventh lens 7 is 0.782 mm, 1.0 <f _w / r _w1 = 2.97 <4.0 is satisfied, and the conditional expression (12) Is satisfied. The third of the overall length L of the lens system 15 the third lens system 15 the focal length f _w in normalized values L / f _w = 1.13 next to, the overall length L is the lens system of the third lens system 15 This realizes a very compact photographic lens with a focal length f _w of 1.3 times or less.

  FIG. 12 is an aberration diagram of the photographing lens of FIG. 12A shows spherical aberration, FIG. 12B shows astigmatism, and FIG. 12C shows distortion.

  In the present embodiment as well, each aberration is favorably corrected in spite of realizing a very compact photographing lens as compared with the prior art.

In any of the third lens systems 15 of the above two embodiments, lenses having aspherical surfaces are used in the lens surfaces R _w1 , R _w2 , R _w3 , and R _w4 , but the present invention is based on this. It is not limited and can be selected as appropriate.

  In the bifocal photographing lens according to the present invention, in the bifocal photographing lens capable of switching two focal lengths, the conditional expression (5) is satisfied, thereby smoothly switching between the telephoto side lens system and the wide angle side lens system. It can be carried out. In addition, while satisfying the conditional expression (1) while adopting a lens system with a very simple configuration of three elements in three groups as the telephoto side lens system, the overall length can be shortened and the size can be reduced, and the conditional expression (2 ) To (4), satisfactory aberration performance can be obtained, and conditional expression (6) is adopted while adopting a lens system having a very simple configuration of three elements in three groups as the wide-angle side lens system. By satisfying () to (10), the overall length is short and the lens is sufficiently wide-angled, and good aberration performance can be obtained. In addition, since all the lenses of the second lens system use a resin material, the cost is low and the lens can be easily produced. Alternatively, the wide-angle lens system is composed of two lenses, and the lens Each parameter of the system satisfies the conditional expression (11), thereby achieving miniaturization. By satisfying the conditional expressions (11) and (12), various aberrations can be corrected favorably. By satisfying conditional expression (12), workability can be improved.

  As described above, the bifocal photographing lens according to the present invention has the above-described effects, and switches between a photographing lens mounted on a camera such as a digital still camera, in particular, a telephoto side lens system and a wide angle side lens system. This is useful as a bifocal photographing lens that can select two focal lengths.

  The bifocal photographing lens according to the present invention can also be applied to applications such as technical fields that require the above-described specific effects.

Sectional drawing at the time of the telephoto side lens system switching of the bifocal photographing lens in 1st Embodiment of this invention Sectional view when switching the wide-angle lens system of the bifocal photographing lens of FIG. Aberration diagram in the telephoto lens system of FIG. Aberration diagram in the wide-angle lens system of FIG. Sectional drawing at the time of wide-angle side lens system switching of the bifocal photographing lens in 2nd Embodiment of this invention Aberration diagram in the wide-angle lens system of FIG. Sectional drawing at the time of wide-angle side lens system switching of the bifocal photographing lens in 3rd Embodiment of this invention Aberration diagram in the wide-angle lens system of FIG. Sectional drawing at the time of the wide-angle side lens system switching of the bifocal photographing lens in 4th Embodiment of this invention Aberration diagram in the wide-angle lens system of FIG. Sectional drawing at the time of wide-angle side lens system switching of the bifocal photographing lens in 5th Embodiment of this invention Aberration diagram of the photographic lens of FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 1st lens 2 2nd lens 3 3rd lens 4 4th lens 5 5th lens 6 6th lens 7 7th lens 8 8th lens 10, 11, 12 Aperture stop 13 1st lens system 14 2nd lens System 15 Third lens system

Claims (5)

In order from the object side, a photographic lens composed of a first lens system including a first lens, a second lens, a third lens, and an aperture stop,
The first lens is a lens having a positive refractive power;
The second lens is a lens having negative refractive power,
The third lens is a lens having a positive refractive power;
The first lens system satisfies the following conditions.
TL _t / _ft <1.0 (1)
3.0 < _ft / _ft1 <3.2 (2)
1.9 < _ft / _ft3 <2.1 (3)
20 <ν _t1 −ν _t2 (4)
However, TL _t is the distance on the optical axis to the image side focal point of the lens system from the object side lens surface of the first lens, f _t is the focal length of the first lens system, the f _t1 focal length of the first lens, f _t3 is the focal length of the third lens, ν _t1 is the Abbe number of the first lens, and ν _t2 is the Abbe number of the second lens. A telephoto lens system and a wide-angle lens system that has a shorter focal length than the telephoto lens system, and can select two focal lengths by switching between the telephoto lens system and the wide-angle lens system. A lens,
The photographic lens according to claim 1 is applied as the telephoto side lens system,
A bifocal photographing lens characterized by satisfying the following conditions with the wide-angle side lens system.
TL _w <d _tbf (5)
Where TL _w is the distance on the optical axis from the aperture stop to the image-side focal point of the lens system when the wide-angle lens system is selected, and d _tbf is the lens surface closest to the imaging surface in the telephoto side lens system to the image-side focal point. The distance on the optical axis. The wide-angle side lens system includes, in order from the object side, a second lens system including the aperture stop, the fourth lens, the fifth lens, and the sixth lens.
The fourth lens is a meniscus lens that uses a resin material and has a positive refractive power on the object side,
The fifth lens is a meniscus lens made of a resin material, concave on the object side and having negative refractive power,
The sixth lens is a meniscus lens made of a resin material, convex on the object side and having positive refractive power,
The bifocal photographing lens according to claim 2, wherein the second lens system satisfies the following condition.
TL _w / f _w <1.22 (6)
1.1 <f _w / f _w1 <1.5 (7)
−1.6 <f _w / f _w2 <−0.5 (8)
−3.0 <(N _w2 −1) f / r _w3 <−1.6 (9)
ν _w1 −ν _w2 > 20 (10)
Where f _w is the focal length of the second lens system, f _w1 is the focal length of the fourth lens, f _w2 is the focal length of the fifth lens, N _w2 is the refractive index of the fifth lens, and r _w3 is the fifth lens. Is the radius of curvature of the object side lens surface, ν _w1 is the Abbe number of the fourth lens, and ν _w2 is the Abbe number of the fifth lens. The wide-angle side lens system is a photographing lens composed of a lens system including, in order from the object side, a seventh lens, an aperture stop, and an eighth lens,
The seventh lens is a positive meniscus lens having a convex surface on the object side lens surface;
The eighth lens is a positive meniscus lens having a convex surface on the image side lens surface;
The bifocal photographing lens according to claim 2, wherein the lens system satisfies the following condition.
0.5 <f _w1 / f _w <1.5 (11)
1.0 <f _w / r _w1 <4.0 (12)
Where f _w1 is the focal length of the seventh lens, f is the focal length of the third lens system, and r _w1 is the radius of curvature of the object side lens surface of the seventh lens. The wide-angle side lens system includes, in order from the object side, a third lens system including the aperture stop, a seventh lens, and an eighth lens.
The seventh lens is a meniscus lens having a positive refractive power that is convex on the object side,
The eighth lens is a meniscus lens having a positive refractive power and a convex image side,
The bifocal photographing lens according to claim 2, wherein the third lens system satisfies the following condition.
0.5 <f _w1 / f _w <1.5 (11)
1.0 <f _w / r _w1 <4.0 (12)
Where f _w1 is the focal length of the seventh lens, f _w is the focal length of the third lens system, and r _w1 is the radius of curvature of the object side lens surface of the seventh lens.

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