WO2023004794A1

WO2023004794A1 - Imaging optics and imaging device

Info

Publication number: WO2023004794A1
Application number: PCT/CN2021/109809
Authority: WO
Inventors: Sota Miyatani; Ryotaro Izumi; Yingqing LIU; Takuya Anzawa; Qing TONG
Original assignee: Huawei Technologies Co.,Ltd.
Priority date: 2021-07-30
Filing date: 2021-07-30
Publication date: 2023-02-02
Also published as: CN117677882A

Abstract

A periscope-type optical system with a shortened total length of optics is disclosed. The optical system includes a reflective optical element (P) for bending an optical path by 90 degrees from an object side toward an image side and a rear group of lenses (G_rear) on the optical path following the reflective optical element (P), wherein the rear group of lenses (G_rear) has a positive refractive power as a whole. The reflective optical element (P) has a convex curvature on a first surface (S1) of the object side and a concave curvature on a third surface (S3) of the image side.

Description

IMAGING OPTICS AND IMAGING DEVICE

TECHNICAL FIELD

The present invention relates to imaging optics, and more particularly to imaging optics and imaging devices having reflective optical elements.

The present invention relates to imaging lenses to focus an image of an object on an individual imaging device, such as a CCD, a CMOS sensor, etc. The present invention specifically relates to photographing lenses mounted on portable devices, such as smartphones, game machines, PCs, IP cameras, home appliances, automobiles, unmanned aerial vehicles, and photographing devices, etc.

BACKGROUND

With the spread of smartphones in recent years, the needs for imaging lenses are diversifying. Since the size of the imaging lens module is directly related to the product size to be mounted, it is desirable for the imaging lens to enhance the lens performance while maintaining the small module thickness. Specifically, this includes the enlargement of the imaging lens, telescoping, enlargement of the diameter, and improvement of optical performance, etc.

In recent years, multi-camera systems have become mainstream, and telephoto lenses play an important role in differentiating smartphone products.

For example, there are numerous opportunities that users can enjoy, such as viewing and shooting distant objects at athletic meets and the like, and taking pictures of scenery and celestial bodies.

Currently, for smartphone products, periscope-type imaging optics utilizing a right-angle prism is used as the imaging optics, which is used in the imaging lens module having such a telephoto lens function, for the purpose of reducing the height of the lens unit.

In the prior art, a right-angle prism having no refractive power by itself is used in the periscope-type imaging optics. Because this configuration adds the right-angle prism into the conventional telescopic optics, it is possible to reduce the module size in its thickness direction by bending the optical path. However, the total length of the optics is increased due to the addition of the prism.

Further, in general, an anti-vibration mechanism is used for the product. This is known as an Optical Image Stabilizer (OIS) , which can sense the vibration during shooting by using a sensor and correct the optical axis. Specifically, the OIS is performed by physically tilting or shifting the periscope prism or the imaging lens relative to the optical axis using a piezoelectric element or the like.

SUMMARY

For an imaging lens module having a telephoto lens function, it is possible to shorten the overall length of the optical system. Additionally, an imaging lens module can be provided which is able to offer sufficient anti-vibration performance without adopting an additional configuration as an anti-vibration mechanism.

In the periscope-type imaging optics, a right-angle prism having a refractive power by itself is employed. By adding the refractive power to the prism, the overall length of the optical system can be shortened. Here, the refractive power refers to a degree of refraction caused by an optical system, such as a lens, and it is also referred to simply as a power. With the power prism effect, it is possible to reduce the overall length of the optical system and achieve a low profile as an imaging lens module while maintaining the anti-vibration configuration.

According to a first aspect, an embodiment of the present invention provides an optical system of periscope-type. The optical system comprises: a reflective optical element (P) for bending an optical path by 90 degrees from an object side toward an image side; and a rear group of lenses on the optical path following the reflective optical element. Here, the rear group of lenses has a positive refractive power as a whole, and the reflective optical element has a convex curvature on a first surface (S1) of the object side and a concave curvature on a third surface (S3) of the image side.

With reference to the first aspect, in one possible implementation, the following conditional expressions (1) , (2) , and (3) are satisfied:

FOV≤38° (1)

0.25≤P_S1/f≤5.0 (2)

-0.3≤f/Pf≤0.3 (3)

where

FOV: field angle of the optical system (unit: degrees) ,

f: a focal length of the optical system (unit: mm) ,

P_S1: proximal axis radius of curvature on the object side of the reflective optical element (in mm) , and

Pf: a focal length of the reflective optical element (unit: mm) .

With reference to the first aspect, in one possible implementation, a first lens (L1) included in the rear group of lenses and arranged closest to the object side has a positive refractive power, and wherein the following conditional expression (4) is satisfied:

0.18≤L1f/f≤1.0 (4)

where

L1f: a focal length of the optical system of the rear group of lenses (unit: mm) .

With reference to the first aspect, in one possible implementation, the following conditional expressions (1) ’, (2) ’, and (3) ’ are further satisfied:

FOV≤34° (1) ’

0.4≤P_S1/f≤3.5 (2) ’

-0.15≤f/Pf≤0.15 (3) ’.

With reference to the first aspect, in one possible implementation, the reflective optical element is tilted to provide an optical vibration isolation.

With reference to the first aspect, in one possible implementation, the reflective optical element is moved in an optical axis direction and/or in a direction perpendicular to the optical axis to provide an optical vibration isolation.

With reference to the first aspect, in one possible implementation, the rear group of lenses consists of a plurality of lens elements including at least four lens elements, and

wherein a part of the plurality of lens elements is moved in an optical axis direction to provide a focusing.

According to a second aspect, an embodiment of the present invention provides a smartphone comprising the optical system according to any one of the possible implementations of the first aspect described above.

According to a third aspect, an embodiment of the present invention provides a digital camera comprising the optical system according to any one of the possible implementations of the first aspect described above.

BREIF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

In order to more clearly describe embodiments of the present invention, the accompanying drawings as required will be briefly described below. Obviously, in the following description, the accompanying drawings show only some embodiments of the present invention, and other drawings from these accompanying drawings can be drawn by a skilled person in the art without creative effort.

Fig. 1 is a schematic diagram of an imaging lens according to an embodiment of the present invention.

Fig. 2 is a diagram illustrating parameters of lenses in the imaging lens according to the present invention.

Fig. 3 is a graph illustrating spherical aberration, astigmatism, and distortion of an imaging lens according to an embodiment of the present invention.

Fig. 4 is a schematic diagram of an imaging lens according to another embodiment of the present invention.

Fig. 5 is a graph illustrating spherical aberration, astigmatism, and distortion of an imaging lens according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The imaging lens according to the present invention is composed of a reflective optical element (P) for bending an optical path by 90 degrees from an object side toward an image side, followed by a rear group of lenses (G_rear) . The reflective optical element (P) is a non-planar prism composed of a first surface (S1) with a convex face toward the object side, a second surface (S2) with a reflective face, and a third surface (S3) with a concave curvature toward the object side.

By using such a non-planar reflective optical element as a periscope-type optical system, it is possible to obtain the effect of sufficiently shortening the overall length of the optical system for the imaging lens while maintaining the anti-vibration performance and the reduction of the height which are equivalent to conventional ones. This can solve the aforementioned technical problems.

Hereinafter, embodiments according to the present invention will be described in detail with reference to the drawings.

Fig. 1 is a schematic view illustrating the structure of an imaging lens according to a first embodiment of the present invention. In the imaging lens according to this embodiment, there is a reflective optical element (P) for bending the optical path from the object side to the image side by 90 degrees, followed by a rear group lens (G_rear) . P is a nonplanar prism composed of a first surface (S1) with convex curvature toward the object side, a second surface (S2) with reflective surface, and a third surface (S3) with curvature of the same sign (in the same direction) as the first surface, i.e., concave curvature. The rear group lens (G_rear) is composed of six lenses, which are a first lens (L1) , a second lens (L2) , a third lens (L3) , a fourth lens (L4) , a fifth lens (L5) , and a sixth lens (L6) from the side of P toward the imaging surface (IMG) . G_rear has a positive refractive power as a whole. Further, immediately before the image surface (IMG) , a filter (IR) , such as an infrared-cut filter or a cover glass, is arranged. The filter (IR) may be omitted.

A shortening of the overall optical system length will be described below. Here, it is assumed that the reflected optical element (P) is a right-angled prism. A prism generally consists of flat surfaces comprising a first surface (S1) , a second surface (S2) and a third surface (S3) , and the prism does not have refractive power as a whole. On the other hand, the first surface (S1) of the prism P used in the optical system of the present invention is a curved surface having a predetermined curvature with a convex direction toward the object side, and P as a whole has a positive refractive power. This allows the overall length of the optical system to be shortened while continuing to use the conventional rear group lens (G_rear) .

Here, if the convex curvature of S1 is increased, the effect of shortening the overall length of the optical system is also increased. For example, by increasing the curvature, the total length of the optical system can be reduced by about 13%. However, if the curvature increases too much, the aberration generated in S1 also increases, making it difficult to obtain good imaging performance at the IMG. Therefore, in the third surface (S3) of the prism P, it is necessary to adjust the refractive power of the prism P to an appropriate range by providing a curvature that is concave (i.e. a concave curvature towards the object side) , which is in contrast to S1.

In addition, when the overall length of the optical system is shortened by the positive refractive power of P as a whole, deflection of the image due to camera shake cause by a user becomes relatively large, and it is necessary to increase the efficiency of the Optical Image Stabilizer (OIS) . Generally, OIS is performed by shifting, namely moving, a portion or all of the imaging lens vertically and/or horizontally relative to the optical path (or optical axis) .

Since the prism P of the present invention has a positive refractive power by itself, by tilting, namely inclining the entire prism P relative to the optical path (or the optical axis) the OIS can be implemented more efficiently compared to vertical and/or horizontal moving.

In the first embodiment as described above, the optical system performs the OIS by tilting P on the optical path and/or by shifting L5 and L6 of G_rear in the optical axis direction.

In the first embodiment described above, it is desirable that the optical system satisfies the following conditional expressions (1) , (2) , and (3) :

(1) FOV≤38°

(2) 0.25≤P_S1/f≤5.0

(3) -0.3≤f/Pf≤0.3.

Here, as shown in Fig. 2, each parameter indicated by the above-described conditional expression is as follows. FOV is a viewing angle of the optical system (in degrees) , f is a focal length of the optical system (in mm) , P_S1 is a proximal axis radius of curvature on the object side of the reflective optical element (in mm) , and Pf is a focal length of the reflective optical element (in mm) .

The conditional expression (1) appropriately sets an imaging field angle of the optical system. When exceeding this range, the field angle becomes too large to maintain the periscope-type structure, thus making it difficult to reduce the height of the image lens module. Specifically, a “vignetting” of peripheral field angle occurs in the reflection optical element P.

The conditional expression (2) appropriately sets a proximal axis radius of curvature of the first surface (S1) of the reflected optical element P. When below this range, the aberration generated in S1 becomes too large and it becomes difficult to obtain good imaging performance. On the other hand, when exceeding this range, the effect of shortening the overall length of the optical system becomes insufficient.

The conditional expression (3) appropriately sets the refractive power (i.e., 1/focal length f) of the reflected optical element P. When outside this range, the refractive power of the reflected optical element P increases, and the performance degradation during optical vibration isolation using the reflected optical element P becomes a problem.

In addition, when outside the ranges of the above-described conditional expressions (in particular, for the conditional expression (3) ) , the problem of performance deterioration during the optical vibration isolation becomes obvious, and in order to avoid this, effects of the optical vibration isolation by shifting or tilting the optical element P in the optical path may become insufficient. In this case, another configuration is required to be adopted, resulting in a larger size of the imaging lens module.

On the other hand, in the aforementioned first embodiment, the first lens (L1) included in the G_rear and arranged closest to the object side has a positive refractive power, and selectively satisfies the following conditional expression (4) :

(4) 0.18≤L1f/f≤1.0.

Here, as shown in Fig. 2, L1f is a focal length of the optical system of the rear group lens (in mm) .

The conditional expression (4) sets the appropriate range for the focal length of L1. When below this range, the refractive power of L1 becomes stronger, and it becomes difficult to maintain good resolution performance during the vibration isolation. Further, when below this range, the total length of the optical system of the imaging lens module becomes longer, resulting in a problem in that it is impossible to fully utilize the effect of shortening of the total length of the optical system obtained by using the reflective optical element P.

In addition, in the aforementioned first embodiment, it is desirable that the optical system further satisfies the following conditional expressions (1) ’, (2) ’, and (3) ’:

(1)’ FOV≤34°

(2) ’0.3≤P_S1/f≤3.5

(3) ’-0.15≤f/Pf≤0.15.

In the optical system of the aforementioned first embodiment, it is desirable that G_rear comprises a plurality of lenses including at least four lens elements. The focusing is achieved by shifting, namely moving, some parts of the lens elements in the direction of the optical axis.

Thus, G_rear needs to be configured properly. The small number of G_rear lens elements makes it difficult to maintain good resolution performance. In the present invention, compared with a conventional planar right-angle prism, using a nonplanar prism facilitates the expansion of Fno (F-number, or F-value) , which has been difficult to do in the past. However, as the Fno expands, the need for the aberration correction required of the optical system increases, so that it is difficult to achieve sufficient resolution performance in a configuration using a small number of lens elements.

In addition, in the conventional planar right-angle prism, parallel light is incident from the object side to G_rear. Therefore, a method of focusing by shifting the entire G_rear in the direction of the optical axis is more efficient for macro performance. On the other hand, in the present invention, since convergent light is incident to the G_rear, it is difficult to ensure the macro performance by shifting the entire G_rear. This can be a side effect in achieving a shortening of the total length of the optical system. However, to accommodate this, the desired macro performance can be readily secured by employing an inner focus type configuration, in which some parts of the lens elements comprising the G_rear are used for focusing.

Fig. 3 shows a spherical aberration plot, an astigmatism plot, and a distortion plot for the imaging lens according to the aforementioned first embodiment. Here, the spherical aberration plot shows the amount of aberration for the F-line (486.1 nm) , d-line (587.6 nm) , and C-line (656.3 nm) wavelengths, respectively, as solid lines. The astigmatism plot shows the amount of aberration of the d-line in the sagittal image plane S as a solid line and the amount of aberration of the d-line in the tangential image plane T as a dashed line. Where, F refers to F value and IH refers to the image height. The distortion plot shows the aberration of the d-line as a solid line. Accordingly, it is clear that the imaging lens has sufficient desired optical performance.

Table 1 and Table 2 below show lens data for an imaging lens according to the aforementioned first embodiment. Where, i is the surface number counted from the object side to the image side, Radius is the radius of curvature, Thickness is the thickness of the center of the lens or the air gap, Refractive Index (Nd) is the refractive index of d-line, Abbe Number (vd) is Abbe’s number to d-line, and Stop is the aperture. Aspherical surfaces are indicated by adding the sign of * (asterisk) after surface number i. Further, the aspherical shape adopted on the aspherical surface of the lens surface is defined by Equation 1 listed below, where z is the distance from the apex of the lens surface in the optical axis direction (sag amount) , H is the height in the direction perpendicular to the optical axis direction, c is the near axis curvature at the apex of the lens (inverse of the radius of curvature) , k is the cone constant (Conic constant) , and A4, A6, A8, A10, A12, A14, A16, A18, and A20 are the fourth, sixth, eighth, tenth, twelfth, twelfth, fourteenth, fourteenth, eighteenth, and twentieth aspheric coefficients, respectively.

[Equation 1]

[Table 1]

surface		Radius (mm)	Thickness (mm)	Refractive Index (Nd)	Abbe Number (vd)
P	S1*	35.933	7.743	1.545	56.019
	S2*	29.203	0.455
Stop	-	Infinity	0.100
L1	S1*	7.206	1.665	1.545	56.019
	S2*	-11.426	0.040
L2	S1*	-21.985	0.483	1.614	25.692
	S2*	16.083	0.040
L3	S1*	17.151	1.854	1.535	55.686
	S2*	-21.692	1.045
L4	S1*	-36.483	1.134	1.671	19.226
	S2*	-22.794	1.011
L5	S1*	-15.477	1.100	1.671	19.226
	S2*	-19.747	1.010
L6	S1*	-14.649	1.714	1.535	55.686
	S2*	7.720	2.696
Optical Glass	S1	Infinity	0.210	1.517	64.174
	S2	Infinity	0.300

[Table 2]

Fig. 4 is a schematic view illustrating the structure of an imaging lens according to a second embodiment of the present invention. Compared to the aforementioned first embodiment, the overall configuration of the imaging lens is the same, but the parameters of the prism and each lens element are different.

Fig. 5 shows a spherical aberration plot, an astigmatism plot, and a distortion plot for the imaging lens according to the aforementioned second embodiment. Thus, it can be seen that the imaging lens also has sufficiently desirable optical performance.

Table 3 and Table 4 below show lens data for the imaging lens according to the aforementioned second embodiment.

[Table 3]

surface		Radius (mm)	Thickness (mm)	Refractive Index (Nd)	Abbe Number (vd)
P	S1*	12.685	8.177	1.545	56.019
	S2*	10.464	0.791
Stop	-	Infinity	0.100
L1	S1*	9.609	1.108	1.535	55.686
	S2*	-9.522	0.040
L2	S1*	-15.822	0.442	1.671	19.226
	S2*	22.904	0.041
L3	S1*	27.136	1.049	1.535	55.686
	S2*	-24.658	0.547
L4	S1*	-24.500	1.190	1.614	25.692
	S2*	-7.238	1.192
L5	S1*	-10.194	1.011	1.650	21.534
	S2*	-10.122	0.769
L6	S1*	-26.796	1.701	1.525	56.253
	S2*	4.929	4.201
Optical Glass	S1	Infinity	0.210	1.517	64.174
	S2	Infinity	0.300

[Table 4]

	ASP	Conic Constant	4th	6th	8th	10th	12th	14th	16th	18th	20th
P	S1*	2.022E-01	-8.700E-05	-2.115E-07	6.421E-10	2.950E-10	0.000E+00	0.000E+00	0.000E+00	0.000E+00	0.000E+00
	S2*	1.092E+01	-1.224E-03	-4.000E-05	-1.000E-06	-1.000E-06	0.000E+00	0.000E+00	0.000E+00	0.000E+00	0.000E+00
L1	S1*	-7.019E+00	-2.095E-03	-3.074E-04	-5.000E-06	-5.000E-06	0.000E+00	0.000E+00	0.000E+00	0.000E+00	0.000E+00
	S2*	7.709E+00	-1.665E-03	-7.000E-06	2.000E-06	-2.000E-06	0.000E+00	0.000E+00	0.000E+00	0.000E+00	0.000E+00
L2	S1*	6.606E+00	-3.495E-03	-1.800E-05	7.000E-06	2.000E-06	0.000E+00	0.000E+00	0.000E+00	0.000E+00	0.000E+00
	S2*	-2.436E+01	-4.839E-03	1.600E-05	3.700E-05	-1.000E-06	0.000E+00	0.000E+00	0.000E+00	0.000E+00	0.000E+00
L3	S1*	-4.868E+01	-2.482E-03	5.316E-04	4.800E-05	-4.000E-06	0.000E+00	0.000E+00	0.000E+00	0.000E+00	0.000E+00
	S2*	5.000E+01	-6.535E-03	3.623E-04	2.400E-05	-2.000E-06	0.000E+00	0.000E+00	0.000E+00	0.000E+00	0.000E+00
L4	S1*	4.455E+01	-7.809E-03	-2.048E-04	2.400E-05	2.000E-06	0.000E+00	0.000E+00	0.000E+00	0.000E+00	0.000E+00
	S2*	4.958E-01	-1.844E-03	-1.452E-04	1.400E-05	2.000E-06	0.000E+00	0.000E+00	0.000E+00	0.000E+00	0.000E+00
L5	S1*	-2.692E+01	4.035E-03	3.401E-04	-2.100E-05	2.000E-06	0.000E+00	0.000E+00	0.000E+00	0.000E+00	0.000E+00
	S2*	-2.674E+01	-1.289E-03	1.222E-03	-1.145E-04	8.000E-06	0.000E+00	0.000E+00	0.000E+00	0.000E+00	0.000E+00
L6	S1*	-3.149E+01	-2.289E-02	2.365E-03	-1.907E-04	3.000E-06	0.000E+00	0.000E+00	0.000E+00	0.000E+00	0.000E+00
	S2*	-1.149E+01	-7.696E-03	8.359E-04	-5.900E-05	2.000E-06	0.000E+00	0.000E+00	0.000E+00	0.000E+00	0.000E+00

Table 5 shows the parameters for the imaging lens according to the first and second embodiments described above. Thus, it can be seen that the imaging lens according to the aforementioned first and second embodiments satisfies the conditional expressions (1) - (4) as shown in Table 5.

[Table 5]

	Example 1	Example 2
(1) FOV	30.3	27.0
(2) P_S1/f	2.492	0.762
(3) f/Pf	-0.030	0.046
(4) L1/f	0.579	0.546

The above description shows embodiments provided by this application, however, it is not intended to limit the present invention. Any modification, equivalent substitution, or improvement made without departing from the spirit and principle of the present invention is to be included within the scope of protection of this application.

Description of Symbols

P: Reflective optical element

L1: first lens

S1: first surface of the reflective optical element in the object side

IR: filter

IMG: imaging surface

Claims

An optical system of periscope-type, comprising:

a reflective optical element (P) configured to bend an optical path by 90 degrees from an object side toward an image side; and

a rear group of lenses on the optical path following the reflective optical element, the rear group of lenses having a positive refractive power as a whole;

wherein the reflective optical element has a convex curvature on a first surface (S1) of the object side and a concave curvature on a third surface (S3) of the image side.
The optical system according to claim 1, wherein the following conditional expressions (1) , (2) , and (3) are satisfied:

FOV≤38° (1)

0.25≤P_S1/f≤5.0 (2)

-0.3≤f/Pf≤0.3 (3)

where

FOV: field angle of the optical system (unit: degrees) ,

f: a focal length of the optical system (unit: mm) ,

P_S1: proximal axis radius of curvature on the object side of the reflective optical element (unit: mm) , and

Pf: a focal length of the reflective optical element (unit: mm) .
The optical system according to claim 2, wherein a first lens (L1) included in the rear group of lenses and arranged closest to the object side has a positive refractive power, and wherein the following conditional expression (4) is satisfied:

0.18≤L1f/f≤1.0 (4)

where

L1f: a focal length of the optical system of the rear group of lenses (unit: mm) .
The optical system according to claim 2 or 3, wherein the following conditional expressions (1) ’ , (2) ’ , and (3) ’ are further satisfied:

FOV≤34° (1) ’

0.4≤P_S1/f≤3.5 (2) ’

-0.15≤f/Pf≤0.15 (3) ’ .
The optical system according to claim 2 or 3, wherein the following conditional expression (4) ’ is further satisfied:

0.25≤L1f/f≤0.72 (4) ’ .
The optical system according to any one of claims 1-5, wherein the reflective optical element is tilted to provide an optical vibration isolation.
The optical system according to any one of claims 1-5, wherein the reflective optical element is moved in an optical axis direction and/or in a direction perpendicular to the optical axis to provide an optical vibration isolation.
The optical system according to any one of claims 1-7, wherein the rear group of lenses consists of a plurality of lens elements including at least four lens elements, and

wherein a part of the plurality of lens elements is moved in an optical axis direction to provide a focusing.
A smartphone comprising the optical system according to any one of claims 1-8.
A digital camera comprising the optical system according to any one of claims 1-8.