JP7679154B2 - Imaging lens - Google Patents

Description

The present invention relates to an imaging lens that forms a subject image on an imaging element such as a CCD sensor or a CMOS sensor.

With the advancement of IoT (Internet of Things) technology, many products and devices, including not only portable information devices such as smartphones and mobile phones, but also game consoles, home appliances, and automobiles, are now connected to networks, and various information is being shared between these things. In an IoT environment, it is possible to provide a variety of services by using image information from cameras built into objects. The amount of image information transmitted over networks is increasing year by year, and the cameras in question are required to be compact and have high resolution.

To obtain clear images with high resolution, it is necessary to effectively correct various aberrations in the imaging lens built into the camera. A lens configuration consisting of eight lenses allows a high degree of freedom in design due to the large number of lenses that make up the imaging lens, and various aberrations can be effectively corrected. Patent Document 1 discloses such an imaging lens consisting of eight lenses.

The imaging lens described in Patent Document 1 is composed of a positive first lens, a negative second lens, a positive third lens, a fourth lens, a fifth lens, a sixth lens, a seventh lens, and a negative eighth lens. Of these, the first lens has a refractive power weaker than the refractive power of the entire lens system. The second lens has a thickness that is determined in relation to the distance between the second lens and the third lens. The third lens has a shape that is limited by the radii of curvature of the object-side surface and the image-plane-side surface. The imaging lens described in Patent Document 1 can correct various aberrations relatively well even though it has a wide angle.

Chinese Patent Publication No. 111007631

In recent years, products and devices equipped with cameras have been rapidly becoming smaller and more functional. For imaging lenses built into cameras, there is a stronger demand than ever before for higher resolution and a lower profile. With the imaging lens described in Patent Document 1 above, it is difficult to achieve a low profile while adequately correcting various aberrations.

The object of the present invention is to provide an imaging lens that is low in height yet capable of effectively correcting various aberrations.

The imaging lens of the present invention is an imaging lens that forms a subject image on an imaging element, and is configured by arranging, in order from the object side to the image surface side, a first lens having positive refractive power, a second lens, a third lens, a fourth lens, a fifth lens, a sixth lens, a seventh lens having positive refractive power, and an eighth lens having negative refractive power.

By making the refractive power of the first lens positive, the imaging lens can be made low-profile. In addition, by placing an eighth lens with negative refractive power on the image surface side of the seventh lens with positive refractive power, the imaging lens can be made low-profile while favorably correcting chromatic aberration. In this specification, low-profile refers to a small optical total length, i.e., the ratio of the distance on the optical axis from the object-side surface of the first lens to the image surface to the diagonal length of the image surface of the imaging element (optical total length/diagonal length=total length diagonal ratio).

In the imaging lens of the above configuration, it is preferable that the second lens is a meniscus lens with a convex surface facing the object side in the paraxial direction. By forming the second lens in such a shape, coma aberration, astigmatism, curvature of field, and chromatic aberration can be effectively corrected.

In the imaging lens having the above configuration, it is preferable that the fourth lens has negative refractive power. Also, it is preferable that the object-side surface of the fourth lens is a concave surface in the paraxial direction.

In the imaging lens of the above configuration, it is desirable that the surface of the fifth lens facing the image plane is a convex surface paraxially. It is also desirable that the surface of the sixth lens facing the object plane is a concave surface paraxially. With such shapes of the fifth lens and the sixth lens, it is possible to satisfactorily correct the field curvature.

In the imaging lens of the above configuration, it is desirable that the object-side surface of the seventh lens is a paraxially concave surface. By forming the object-side surface of the seventh lens into this shape, coma aberration, astigmatism, and field curvature can be effectively corrected.

In the imaging lens of the above configuration, it is desirable that the eighth lens has an aspheric image-side surface with an inflection point. By forming the image-side surface of the eighth lens aspheric with an inflection point, it is possible to satisfactorily correct the field curvature and distortion aberration in the peripheral part of the image while ensuring the back focus. In addition, it is possible to satisfactorily correct paraxial and peripheral aberrations while suppressing the angle of incidence of the light beam emitted from the imaging lens to the image surface of the imaging element within the range of the chief ray angle (CRA: Chief Ray Angle).

Furthermore, in the imaging lens having the above configuration, it is desirable that the surface of the eighth lens on the image side is a concave surface paraxially. By forming the eighth lens in such a shape, it is possible to preferably achieve a low height of the imaging lens while ensuring the back focus.

In addition, in the present invention, a "lens" refers to an optical element that has refractive power. Therefore, optical elements such as prisms and flat filters that change the direction of light travel are not included in the "lens" of the present invention, and these optical elements can be placed in front of or behind the imaging lens or between the lenses as appropriate.

It is desirable for the imaging lens having the above configuration to satisfy the following conditional expression (1).
-50.0<f8/f<-3.0 (1)
however,
f: focal length of the entire lens system,
f8: focal length of the eighth lens,
Let us assume that.

By satisfying conditional expression (1), it is possible to reduce the height of the imaging lens while ensuring back focus. In addition, it is possible to correct field curvature, distortion, and coma in a well-balanced manner.

It is desirable for the imaging lens having the above configuration to satisfy the following conditional expression (2).
0.70<f1/f<1.30 (2)
however,
f: focal length of the entire lens system,
f1: focal length of the first lens,
Let us assume that.

By satisfying conditional expression (2), it is possible to achieve a low-profile imaging lens while ensuring a sufficient back focus.

It is desirable for the imaging lens having the above configuration to satisfy the following conditional expression (3).
-2.20<f2/f3<-0.80 (3)
however,
f2: focal length of the second lens,
f3: focal length of the third lens,
Let us assume that.

By satisfying conditional expression (3), it is possible to effectively correct astigmatism while reducing the height of the imaging lens.

It is desirable for the imaging lens having the above configuration to satisfy the following conditional expression (4).
-6.50<f4/f3<-1.50 (4)
however,
f3: focal length of the third lens,
f4: focal length of the fourth lens,
Let us assume that.

By satisfying conditional expression (4), the imaging lens can be made low-profile while still providing good correction of spherical aberration and astigmatism.

It is desirable for the imaging lens having the above configuration to satisfy the following conditional expression (5).
1.00<f34/f<4.00 (5)
however,
f: focal length of the entire lens system,
f34: composite focal length of the third and fourth lenses,
Let us assume that.

By satisfying conditional expression (5), it is possible to effectively correct astigmatism and field curvature while reducing the height of the imaging lens.

It is desirable for the imaging lens having the above configuration to satisfy the following conditional expression (6).
-15.0<f4/f<-2.0 (6)
however,
f: focal length of the entire lens system,
f4: focal length of the fourth lens,
Let us assume that.

By satisfying conditional expression (6), it is possible to appropriately reduce the height of the imaging lens while ensuring the back focus. In addition, astigmatism and lateral chromatic aberration can be effectively corrected.

It is desirable for the imaging lens having the above configuration to satisfy the following conditional expression (7).
-6.00<f56/f<-1.00 (7)
however,
f: focal length of the entire lens system,
f56: the composite focal length of the fifth and sixth lenses,
Let us assume that.

By satisfying conditional expression (7), the imaging lens can be made low-profile while still providing good correction of spherical aberration and astigmatism.

It is desirable for the imaging lens having the above configuration to satisfy the following conditional expression (8).
2.0<f7/f<30.0 (8)
however,
f: focal length of the entire lens system,
f7: focal length of the seventh lens,
Let us assume that.

By satisfying conditional expression (8), it is possible to preferably achieve a low-profile imaging lens while ensuring a sufficient back focus. In addition, it is possible to achieve good, well-balanced correction of spherical aberration, coma aberration, astigmatism, distortion, and curvature of field.

It is desirable for the imaging lens having the above configuration to satisfy the following conditional expression (9).
-4.00<f7/f8<-0.20 (9)
however,
f7: focal length of the seventh lens,
f8: focal length of the eighth lens,
Let us assume that.

By satisfying conditional expression (9), the imaging lens can be made low-profile while effectively correcting the field curvature. In addition, the back focus can be ensured.

It is desirable for the imaging lens having the above configuration to satisfy the following conditional expression (10).
0.50<f67/f56<8.50 (10)
however,
f56: the composite focal length of the fifth and sixth lenses,
f67: the composite focal length of the sixth lens and the seventh lens,
Let us assume that.

By satisfying conditional expression (10), spherical aberration can be effectively corrected.

It is desirable for the imaging lens having the above configuration to satisfy the following conditional expression (11).
0.10<R2r/R3f<0.85 (11)
however,
R2r: paraxial radius of curvature of the image side surface of the second lens,
R3f: paraxial radius of curvature of the object side surface of the third lens,
Let us assume that.

By satisfying conditional expression (11), it is possible to effectively correct astigmatism and lateral chromatic aberration while reducing the height of the imaging lens.

It is desirable for the imaging lens having the above configuration to satisfy the following conditional expression (12).
-90.0<R4f/D34<-15.0 (12)
however,
R4f: paraxial radius of curvature of the object side surface of the fourth lens,
D34: the distance on the optical axis between the third lens and the fourth lens,
Let us assume that.

By satisfying conditional expression (12), it is possible to effectively correct astigmatism and chromatic aberration while reducing the height of the imaging lens.

It is desirable for the imaging lens having the above configuration to satisfy the following conditional expression (13).
0.15<|R5f/R5r|<1.35 (13)
however,
R5f: paraxial radius of curvature of the object side surface of the fifth lens,
R5r: paraxial radius of curvature of the image side surface of the fifth lens,
Let us assume that.

By satisfying condition (13), spherical aberration can be effectively corrected.

It is desirable for the imaging lens having the above configuration to satisfy the following conditional expression (14).
-2.00<R6f/f<-0.40 (14)
however,
f: focal length of the entire lens system,
R6f: paraxial radius of curvature of the object side surface of the sixth lens,
Let us assume that.

By satisfying conditional expression (14), it is possible to realize a low-profile imaging lens while ensuring the back focus. In addition, it is possible to effectively correct spherical aberration and chromatic aberration of magnification.

It is desirable for the imaging lens having the above configuration to satisfy the following conditional expression (15).
0.80<D34/D45<2.00 (15)
however,
D34: the distance on the optical axis between the third lens and the fourth lens,
D45: the distance on the optical axis between the fourth lens and the fifth lens,
Let us assume that.

By satisfying conditional expression (15), it is possible to effectively correct astigmatism while reducing the height of the imaging lens.

It is desirable for the imaging lens having the above configuration to satisfy the following conditional expression (16).
0.01<D56/f<0.08 (16)
however,
f: focal length of the entire lens system,
D56: the distance on the optical axis between the fifth lens and the sixth lens,
Let us assume that.

By satisfying conditional expression (16), the imaging lens can be made low-profile while still providing good correction for spherical aberration.

It is desirable for the imaging lens having the above configuration to satisfy the following conditional expression (17).
12.0<f3/T3<25.0 (17)
however,
f3: focal length of the third lens,
T3: thickness of the third lens on the optical axis,
Let us assume that.

By satisfying conditional expression (17), the imaging lens can be made low-profile while still providing good correction of spherical aberration and astigmatism.

It is desirable for the imaging lens having the above configuration to satisfy the following conditional expression (18).
0.50<T8/T7<1.50 (18)
however,
T7: thickness of the seventh lens on the optical axis,
T8: thickness of the eighth lens on the optical axis,
Let us assume that.

By satisfying conditional expression (18), it is possible to effectively correct astigmatism while reducing the height of the imaging lens.

It is desirable for the imaging lens having the above configuration to satisfy the following conditional expression (19): By satisfying conditional expression (19), chromatic aberration can be corrected satisfactorily.
35.0<νd4 (19)
however,
νd4: Abbe number of the fourth lens,
Let us assume that.

In order to correct chromatic aberration better, it is desirable for the imaging lens having the above configuration to satisfy the following conditional expression (20).
νd5<35.0 (20)
however,
νd5: Abbe number of the fifth lens,
Let us assume that.

In order to correct chromatic aberration better, it is desirable for the imaging lens having the above configuration to satisfy the following conditional expression (21).
35.0<νd6 (21)
however,
νd6: Abbe number of the sixth lens,
Let us assume that.

In order to correct chromatic aberration better, it is desirable for the imaging lens having the above configuration to satisfy the following conditional expression (22).
35.0<νd7 (22)
however,
νd7: Abbe number of the seventh lens,
Let us assume that.

In order to correct chromatic aberration better, it is desirable for the imaging lens having the above configuration to satisfy the following conditional expression (23).
35.0<νd8 (23)
however,
νd8: Abbe number of the eighth lens,
Let us assume that.

In the imaging lens of the above configuration, it is desirable that the focal length of the eighth lens is longer than twice the focal length of the entire lens system. The seventh and eighth lenses are positioned closest to the image plane of the imaging element among the eight lenses. By forming these two lenses from low-dispersion materials as shown in the above conditional expressions (22) and (23), chromatic aberration can be corrected even better.

However, in an image sensor with a high number of pixels, the light receiving area of each pixel is reduced, so the captured image tends to be dark. One method for correcting this is to improve the light receiving sensitivity of the image sensor using an electric circuit. However, when the light receiving sensitivity is increased, noise components that do not directly contribute to the formation of the image are also amplified. Therefore, in order to obtain a sufficiently bright image without providing an electric circuit or the like, it is desirable for the imaging lens having the above configuration to satisfy the following conditional expression (24).
0.55<Dep/ih (24)
however,
Dep: entrance pupil diameter,
ih: maximum image height on the image plane of the image sensor,
Let us assume that.

In order to suitably reduce the height of the imaging lens of the present invention, it is desirable for the imaging lens to satisfy the overall length to diagonal ratio expressed by the following conditional expression (25).
0.5<TTL/(2×ih)<1.0 (25)
however,
TTL: the distance on the optical axis from the object side surface of the first lens to the image plane,
ih: maximum image height on the image plane of the image sensor,
Let us assume that.

Note that, although an infrared cut filter, cover glass, or other insert is usually placed between the imaging lens and the image plane, in this specification, the distance on the optical axis of these inserts is measured using the air equivalent length.

In the imaging lens of the present invention, it is desirable to arrange each of the lenses from the first lens to the eighth lens with an air gap between them. By arranging each lens with an air gap between them, the imaging lens of the present invention has a lens configuration that does not include any cemented lenses. With such a lens configuration, all eight lenses that make up the imaging lens can be formed from a plastic material, which makes it possible to reduce the manufacturing costs of the imaging lens.

In the imaging lens of the present invention, it is desirable to form both sides of each of the first lens to the eighth lens aspheric. By forming both sides of each lens aspheric, various aberrations can be corrected better from the paraxial to the peripheral parts of the lenses.

When the angle of view of the imaging lens of the present invention is 2ω, it is desirable to satisfy 70°≦2ω. By satisfying this conditional expression, the imaging lens can be made to have a wider angle, so that the imaging lens can be made thinner and have a wider angle.

In this specification, the surface shape of each lens is specified by the sign of the radius of curvature. The radius of curvature is generally defined as positive or negative, i.e., the direction of light travel is positive, and the radius of curvature is positive when the center of the radius of curvature is on the image side as viewed from the lens surface, and negative when the center is on the object side. Therefore, the "object side surface with a positive radius of curvature" refers to a convex surface on the object side, and the "object side surface with a negative radius of curvature" refers to a concave surface on the object side. In addition, the "image side surface with a positive radius of curvature" refers to a concave surface on the image side, and the "image side surface with a negative radius of curvature" refers to a convex surface on the image side. Note that the radius of curvature in this specification refers to the paraxial radius of curvature, and may not match the general shape of the lens in the lens cross-sectional view.

The imaging lens of the present invention provides a low-profile imaging lens that has high resolution with well-corrected aberrations, yet is particularly suitable for incorporation into small cameras.

FIG. 1 is a cross-sectional view showing a schematic configuration of an imaging lens according to Numerical Example 1. 2A to 2C are aberration diagrams showing spherical aberration, astigmatism, and distortion of the imaging lens shown in FIG. 1 . FIG. 11 is a cross-sectional view showing a schematic configuration of an imaging lens according to Numerical Example 2. 4 is an aberration diagram showing spherical aberration, astigmatism, and distortion of the imaging lens shown in FIG. 3. FIG. 11 is a cross-sectional view showing a schematic configuration of an imaging lens according to Numerical Example 3. 6A to 6C are aberration diagrams showing spherical aberration, astigmatism, and distortion of the imaging lens shown in FIG. 5 . FIG. 11 is a cross-sectional view showing a schematic configuration of an imaging lens according to Numerical Example 4. 8A to 8C are aberration diagrams showing spherical aberration, astigmatism, and distortion of the imaging lens shown in FIG. 7. FIG. 11 is a cross-sectional view showing a schematic configuration of an imaging lens according to Numerical Example 5. 10A to 10C are aberration diagrams showing spherical aberration, astigmatism, and distortion of the imaging lens shown in FIG. 9 . FIG. 13 is a cross-sectional view showing a schematic configuration of an imaging lens according to Numerical Example 6. 12A to 12C are aberration diagrams showing spherical aberration, astigmatism, and distortion of the imaging lens shown in FIG. 11 . FIG. 13 is a cross-sectional view showing a schematic configuration of an imaging lens according to Numerical Example 7. 14A to 14C are aberration diagrams showing spherical aberration, astigmatism, and distortion of the imaging lens shown in FIG. 13. FIG. 13 is a cross-sectional view showing a schematic configuration of an imaging lens according to Numerical Example 8. 16A to 16C are aberration diagrams showing spherical aberration, astigmatism, and distortion of the imaging lens shown in FIG. 15. FIG. 13 is a cross-sectional view showing a schematic configuration of an imaging lens according to Numerical Example 9. 18A to 18C are aberration diagrams showing spherical aberration, astigmatism, and distortion of the imaging lens shown in FIG. 17.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will now be described in detail with reference to the drawings.
1, 3, 5, 7, 9, 11, 13, 15, and 17 are cross-sectional views showing schematic configurations of imaging lenses according to Numerical Examples 1 to 9 of this embodiment. Since the basic lens configuration is the same in all of the Numerical Examples, the imaging lens according to this embodiment will be described here with reference to the cross-sectional view of Numerical Example 1.

As shown in FIG. 1, the imaging lens according to this embodiment has, in order from the object side to the image surface side, a first lens L1 having a positive refractive power, a second lens L2, a third lens L3, a fourth lens L4, a fifth lens L5, a sixth lens L6, a seventh lens L7 having a positive refractive power, and an eighth lens L8 having a negative refractive power. Each lens from the first lens L1 to the eighth lens L8 is arranged with an air gap between them. A filter IR is disposed between the eighth lens L8 and the image surface IM of the imaging element. This filter IR can be omitted. In this specification, the refractive power of each lens refers to the refractive power in the paraxial direction unless otherwise specified.

The first lens L1 has a shape in which the radius of curvature r2 of the object side surface and the radius of curvature r3 of the image side surface are both positive. The first lens L1 is shaped to be a meniscus lens with a convex surface facing the object side in the paraxial direction. The shape of the first lens L1 is not limited to the shape according to the present numerical example 1. The shape of the first lens L1 may be a shape in which the refractive power is positive. For example, the shape of the first lens L1 may be a shape in which the radius of curvature r2 is positive and the radius of curvature r3 is negative, so that the lens is a biconvex lens in the paraxial direction, or a shape in which the radius of curvature r2 and the radius of curvature r3 are both negative, so that the lens is a meniscus lens with a concave surface facing the object side in the paraxial direction. From the viewpoint of reducing the height of the imaging lens, it is desirable that the shape of the first lens L1 is a shape in which the radius of curvature r2 is positive, that is, a shape in which the surface on the object side is a convex surface in the paraxial direction.

The second lens L2 has negative refractive power. The refractive power of the second lens L2 is not limited to being negative. The refractive power of the second lens L2 may be positive.

The second lens L2 has a shape in which the radius of curvature r4 of the object side surface and the radius of curvature r5 (=R2r) of the image side surface are both positive. The second lens L2 is shaped to be a meniscus lens with a convex surface facing the object side in the paraxial direction. The shape of the second lens L2 is not limited to the shape according to the present numerical example 1. For example, the shape of the second lens L2 may be a shape in which the radius of curvature r4 is negative and the radius of curvature r5 is positive, and may be a shape that is a biconcave lens in the paraxial direction, or a shape that is a biconvex lens in the paraxial direction. The shape of the second lens L2 may also be a meniscus lens with a concave surface facing the object side in the paraxial direction. From the viewpoint of reducing the height of the imaging lens, it is desirable that the shape of the second lens L2 is a shape in which the object side surface is a convex surface in the paraxial direction, similar to the first lens L1.

The third lens L3 has a positive refractive power. The refractive power of the third lens L3 is not limited to being positive. The refractive power of the third lens L3 may be negative.

The third lens L3 has a shape in which the radius of curvature r6 (=R3f) of the object side surface and the radius of curvature r7 of the image side surface are both positive. The third lens L3 is shaped as a meniscus lens with a convex surface facing the object side in the paraxial direction. The shape of the third lens L3 is not limited to the shape according to the present numerical example 1. The third lens L3 in the numerical example 9 is an example of a shape in which the third lens L3 is a biconvex lens in the paraxial direction. The shape of the third lens L3 may also be a meniscus lens with a concave surface facing the object side in the paraxial direction or a biconcave lens in the paraxial direction. When considering the reduction in height of the imaging lens, it is desirable for the shape of the third lens L3 to be a shape in which the object side surface is a convex surface in the paraxial direction.

The fourth lens L4 has a negative refractive power. The refractive power of the fourth lens L4 is not limited to being negative. The refractive power of the fourth lens L4 may be positive. In addition, the refractive power of the fourth lens L4 may be zero paraxially and may be positive or negative in the peripheral portion.

The fourth lens L4 has a shape in which the radius of curvature r8 (=R4f) of the object side surface and the radius of curvature r9 of the image side surface are both negative. The fourth lens L4 is shaped to be a meniscus lens with a concave surface facing the object side in the paraxial direction. The shape of the fourth lens L4 is not limited to the shape according to the present numerical example 1. The shape of the fourth lens L4 may be any shape that has a negative refractive power. The fourth lens L4 in the numerical examples 4, 5, and 7 to 9 is an example of a shape that is a biconcave lens in the paraxial direction. The shape of the fourth lens L4 may be a meniscus lens with a convex surface facing the object side in the paraxial direction. When considering good correction of various aberrations, it is desirable for the shape of the fourth lens L4 to be a shape in which the object side surface is concave in the paraxial direction.

The fifth lens L5 has a positive refractive power. The refractive power of the fifth lens L5 is not limited to being positive. The imaging lenses according to Numerical Examples 6 to 9 are examples of lens configurations in which the refractive power of the fifth lens L5 is negative. In addition, the refractive power of the fifth lens L5 is zero paraxially, and may be positive or negative in the peripheral portion.

The fifth lens L5 has a shape in which the radius of curvature r10 (= R5f) of the surface on the object side and the radius of curvature r11 (= R5r) of the surface on the image side are both negative. The fifth lens L5 has a shape that forms a meniscus lens with a concave surface facing the object side on the paraxial line. The shape of the fifth lens L5 is not limited to the shape according to Numerical Example 1. The fifth lens L5 in Numerical Example 3 is an example of a shape that forms a biconvex lens on the paraxial line. The shape of the fifth lens L5 may also be a shape that forms a meniscus lens with a convex surface facing the object side on the paraxial line, or a shape that forms a biconcave lens on the paraxial line.

The sixth lens L6 has a negative refractive power. The refractive power of the sixth lens L6 is not limited to being negative. The refractive power of the sixth lens L6 may be positive. In addition, the refractive power of the sixth lens L6 may be zero paraxially and may be positive or negative in the peripheral portion.

The sixth lens L6 has a shape in which the radius of curvature r12 (=R6f) of the object side surface and the radius of curvature r13 of the image side surface are both negative. The sixth lens L6 is shaped as a meniscus lens with a concave surface facing the object side in the paraxial direction. The shape of the sixth lens L6 is not limited to the shape according to the present numerical example 1. The fifth lens L5 in numerical examples 2, 3, and 8 is an example of a shape that becomes a biconcave lens in the paraxial direction. The shape of the sixth lens L6 may be a meniscus lens with a convex surface facing the object side in the paraxial direction, or a biconcave lens in the paraxial direction. In order to reduce the height of the imaging lens while satisfactorily correcting spherical aberration, it is desirable for the shape of the sixth lens to be a shape in which the object side surface is concave in the paraxial direction.

The seventh lens L7 has a shape in which the radius of curvature r14 of the object side surface and the radius of curvature r15 of the image side surface are both positive. The seventh lens L7 is shaped as a meniscus lens with a convex surface facing the object side on the paraxial line. The shape of the seventh lens L7 is not limited to the shape according to Numerical Example 1, and may be any shape that has a positive refractive power. The shape of the seventh lens L7 may be a meniscus lens with a concave surface facing the object side on the paraxial line, or a biconvex lens on the paraxial line.

The eighth lens L8 has a shape in which the radius of curvature r16 of the object side surface and the radius of curvature r17 of the image side surface are both positive. The eighth lens L8 is shaped as a meniscus lens with a convex surface facing the object side in the paraxial direction. The shape of the eighth lens L8 is not limited to the shape according to the present numerical example 1. The shape of the eighth lens L8 may be a shape in which the refractive power is negative. The shape of the eighth lens L8 may be a meniscus lens with a concave surface facing the object side in the paraxial direction, or a shape in which the refractive power is negative in the paraxial direction. In order to reduce the height of the imaging lens and to achieve good correction of various aberrations, it is desirable that the shape of the eighth lens L8 be a shape in which the surface on the image side is concave in the paraxial direction.

Both sides of the seventh lens L7 and the eighth lens L8 are aspheric surfaces with inflection points. Here, the inflection point refers to a point where the sign of the curvature changes on the curve, and refers to a point where the direction of bending of the curve on the lens surface changes. Both sides of the seventh lens L7 and the eighth lens L8 in the imaging lens according to this embodiment are aspheric surfaces with poles. Due to such shapes of the seventh lens L7 and the eighth lens L8, not only the axial chromatic aberration but also the off-axis chromatic aberration of magnification can be well corrected, and the angle of incidence of the light emitted from the imaging lens to the image plane IM can be suitably suppressed within the range of the CRA. Depending on the required optical performance and the degree of reduction in height of the imaging lens, the surfaces of the seventh lens L7 and the eighth lens L8 other than the surface on the image plane side of the eighth lens L8 may be formed as aspheric surfaces without inflection points.

The imaging lens according to this embodiment satisfies the following conditional expressions (1) to (24).
-50.0<f8/f<-3.0 (1)
0.70<f1/f<1.30 (2)
-2.20<f2/f3<-0.80 (3)
-6.50<f4/f3<-1.50 (4)
1.00<f34/f<4.00 (5)
-15.0<f4/f<-2.0 (6)
-6.00<f56/f<-1.00 (7)
2.0<f7/f<30.0 (8)
-4.00<f7/f8<-0.20 (9)
0.50<f67/f56<8.50 (10)
0.10<R2r/R3f<0.85 (11)
-90.0<R4f/D34<-15.0 (12)
0.15<|R5f/R5r|<1.35 (13)
-2.00<R6f/f<-0.40 (14)
0.80<D34/D45<2.00 (15)
0.01<D56/f<0.08 (16)
12.0<f3/T3<25.0 (17)
0.50<T8/T7<1.50 (18)
35.0<νd4 (19)
νd5<35.0 (20)
35.0<νd6 (21)
35.0<νd7 (22)
35.0<νd8 (23)
0.55<Dep/ih (24)
however,
f: focal length of the entire lens system,
f1: focal length of the first lens L1,
f2: focal length of the second lens L2,
f3: focal length of the third lens L3,
f4: focal length of the fourth lens L4,
f7: focal length of the seventh lens L7,
f8: focal length of the eighth lens L8,
f34: the composite focal length of the third lens L3 and the fourth lens L4,
f56: the composite focal length of the fifth lens L5 and the sixth lens L6,
f67: the composite focal length of the sixth lens L6 and the seventh lens L7,
R2r: paraxial radius of curvature of the image-side surface of the second lens L2,
R3f: paraxial radius of curvature of the object side surface of the third lens L3,
R4f: paraxial radius of curvature of the object side surface of the fourth lens L4,
R5f: paraxial radius of curvature of the object side surface of the fifth lens L5,
R5r: paraxial radius of curvature of the image-side surface of the fifth lens L5,
R6f: paraxial radius of curvature of the object side surface of the sixth lens L6,
D34: the distance on the optical axis between the third lens L3 and the fourth lens L4,
D45: the distance on the optical axis between the fourth lens L4 and the fifth lens L5,
D56: the distance on the optical axis between the fifth lens L5 and the sixth lens L6,
T3: thickness of the third lens L3 on the optical axis X,
T7: thickness of the seventh lens L7 on the optical axis X,
T8: thickness of the eighth lens L8 on the optical axis X,
νd4: Abbe number of the fourth lens L4,
νd5: Abbe number of the fifth lens L5,
νd6: Abbe number of the sixth lens L6,
νd7: Abbe number of the seventh lens L7,
νd8: Abbe number of the eighth lens L8,
Dep: entrance pupil diameter,
ih: maximum image height on the image plane IM of the image sensor,
Let us assume that.

The imaging lens according to this embodiment satisfies the total length to diagonal ratio expressed by the following conditional expression (25).
0.5<TTL/(2×ih)<1.0 (25)
however,
TTL: the distance on the optical axis X from the object side surface of the first lens L1 to the image plane IM,
Let us assume that.

Moreover, the imaging lens according to this embodiment satisfies the following conditional expression.
70°≦2ω
however,
ω: Half angle of view,
Let us assume that.

It is not necessary to satisfy all of the above conditional expressions, and the effect corresponding to each conditional expression can be obtained by satisfying each of the above conditional expressions individually.

The imaging lens according to this embodiment exerts more preferable effects by satisfying the following conditional expressions (2a) to (23a).
0.80<f1/f<1.20 (2a)
-2.00<f2/f3<-1.00 (3a)
-6.00<f4/f3<-2.00 (4a)
1.50<f34/f<3.00 (5a)
-12.0<f4/f<-3.0 (6a)
-5.00<f56/f<-1.00 (7a)
3.0<f7/f<28.0 (8a)
-3.00<f7/f8<-0.30 (9a)
0.80<f67/f56<8.00 (10a)
0.25<R2r/R3f<0.70 (11a)
-90.0<R4f/D34<-20.0 (12a)
0.15<|R5f/R5r|<1.25 (13a)
-1.80<R6f/f<-0.50 (14a)
1.00<D34/D45<1.90 (15a)
0.01<D56/f<0.07 (16a)
13.0<f3/T3<23.0 (17a)
0.70<T8/T7<1.40 (18a)
35.0<νd4<95.0 (19a)
13.0<νd5<35.0 (20a)
35.0<νd6<95.0 (21a)
35.0<νd7<95.0 (22a)
35.0<νd8<95.0 (23a)

For the above conditional expressions (2a) to (23a), the lower limit value or upper limit value of the corresponding conditional expressions (2) to (23) may be applied.

In this embodiment, the lens surface of each lens is formed as an aspheric surface. The aspheric surface formula of these aspheric surfaces is shown in the following formula.
however,
Z: distance in the optical axis direction,
H: distance from the optical axis in a direction perpendicular to the optical axis,
C: paraxial curvature (=1/r, r: paraxial curvature radius),
k: conic constant,
An: n-th aspheric coefficient,
Let us assume that.

Next, numerical examples of the imaging lens according to this embodiment are shown. In each table showing basic lens data, f indicates the focal length of the entire lens system, Fno indicates the F-number, ω indicates the half angle of view, ih indicates the maximum image height on the image plane IM, and TTL indicates the distance on the optical axis from the object-side surface of the first lens L1 to the image plane IM. Also, i indicates the surface number counted from the object side, r indicates the paraxial radius of curvature, d indicates the surface-to-surface distance on the optical axis X, nd indicates the refractive index at a reference wavelength of 588 nm, and νd indicates the Abbe number at that reference wavelength. Note that a surface with an asterisk (*) added to the surface number indicates that it is aspheric.

Numerical Example 1
Basic lens data

Figure 2 is an aberration diagram showing spherical aberration (mm), astigmatism (mm), and distortion (%). The astigmatism diagram and distortion diagram show the amount of aberration at a reference wavelength (588 nm). The astigmatism diagram also shows the sagittal image plane (S) and tangential image plane (T), respectively (the same applies to Figures 4, 6, 8, 10, 12, 14, 16, and 18). As shown in Figure 2, the imaging lens of Numerical Example 1 can satisfactorily correct various aberrations.

Numerical Example 2
Basic lens data

As shown in FIG. 4, the imaging lens according to Numerical Example 2 can also effectively correct various aberrations.

Numerical Example 3
Basic lens data

As shown in FIG. 6, the imaging lens according to Numerical Example 3 can also effectively correct various aberrations.

Numerical Example 4
Basic lens data

As shown in FIG. 8, the imaging lens according to Numerical Example 4 can also effectively correct various aberrations.

Numerical Example 5
Basic lens data

As shown in FIG. 10, the imaging lens according to Numerical Example 5 can also effectively correct various aberrations.

Numerical Example 6
Basic lens data

As shown in FIG. 12, the imaging lens according to Numerical Example 6 can also effectively correct various aberrations.

Numerical Example 7
Basic lens data

As shown in FIG. 14, the imaging lens according to Numerical Example 7 can also effectively correct various aberrations.

Numerical Example 8
Basic lens data

As shown in FIG. 16, the imaging lens according to Numerical Example 8 can also effectively correct various aberrations.

Numerical Example 9
Basic lens data

As shown in FIG. 18, the imaging lens according to Numerical Example 9 can also effectively correct various aberrations.

As described above, the imaging lens according to this embodiment can satisfactorily correct various aberrations, even though the total length to diagonal ratio is small. Values corresponding to conditional expressions (1) to (24) of each numerical example according to this embodiment (values corresponding to conditional expressions) are shown below.

Therefore, when the imaging lens according to the above embodiment is applied to the imaging optical system of a camera built into a portable information device such as a smartphone, a mobile phone, or a personal digital assistant, or into a game console, a home appliance, an automobile, etc., it is possible to achieve both high functionality and compactness of the camera.

The present invention can be applied to imaging lenses incorporated into relatively small cameras built into portable information devices such as smartphones, medical devices, game consoles, home appliances, and automobiles.

X Optical axis ST Aperture stop L1 First lens L2 Second lens L3 Third lens L4 Fourth lens L5 Fifth lens L6 Sixth lens L7 Seventh lens L8 Eighth lens IR Filter IM Image surface

Claims (6)

An imaging lens that forms a subject image on an imaging element, the lens including the following elements in order from the object side to the image plane side:
a first lens having a positive refractive power;
a second lens having a negative refractive power ;
a third lens having a positive refractive power ;
a fourth lens having a negative refractive power;
A fifth lens;
a sixth lens having a negative refractive power ;
a seventh lens having a positive refractive power;
and an eighth lens having a negative refractive power,
the fifth lens has a surface on the image side that is convex on the paraxial direction;
the sixth lens has an object-side surface that is paraxially concave;
An imaging lens characterized by satisfying the following conditional expression:
-50.0<f8/f<-3.0
however,
f: focal length of the entire lens system,
f8: the focal length of the eighth lens,
Let us assume that. 2. The imaging lens according to claim 1, wherein the following condition is satisfied:
-6.50<f4/f3<-1.50
however,
f3: the focal length of the third lens,
f4: the focal length of the fourth lens,
Let us assume that. 2. The imaging lens according to claim 1, wherein the following condition is satisfied:
-6.00<f56/f<-1.00
however,
f: focal length of the entire lens system,
f56: a composite focal length of the fifth lens and the sixth lens,
Let us assume that. 2. The imaging lens according to claim 1, wherein the following condition is satisfied:
2.0<f7/f<30.0
however,
f: focal length of the entire lens system,
f7: the focal length of the seventh lens,
Let us assume that. 2. The imaging lens according to claim 1, wherein the following condition is satisfied:
-90.0<R4f/D34<-15.0
however,
R4f: paraxial radius of curvature of the object side surface of the fourth lens,
D34: the distance on the optical axis between the third lens and the fourth lens,
Let us assume that. 2. The imaging lens according to claim 1, wherein the following condition is satisfied:
-2.00<R6f/f<-0.40
however,
f: focal length of the entire lens system,
R6f: paraxial radius of curvature of the object side surface of the sixth lens,
Let us assume that.