JPWO2020194503A1 - Imaging optics - Google Patents

Abstract

Provided is a five-lens imaging optical system that does not use a junction lens to realize an endoscope that is sufficiently small, sufficiently wide-angle, and sufficiently high resolution. The imaging optical system is a first lens having a negative refractive force and having a plane or a convex surface on the object side, which are arranged in order from the object side to the image side, and a biconvex lens having a positive refractive force. A second lens, an aperture, a third lens which has a positive refractive power and is a biconvex lens, a fourth lens which has a negative refractive power and is a biconcave lens, and a third lens which has a positive refractive power. It is an imaging optical system of 5 lenses including 5 lenses, each lens is not a junction lens, and the focal distance of the 5th lens is represented by f5 and the total focal distance is represented by f, 2.6 <f5 /. Satisfy f <7.

Description

The present invention relates to an imaging optical system, particularly an imaging optical system for an endoscope.

Endoscopes used in the medical field include insertion type endoscopes and capsule type endoscopes. In a normal insertion type endoscope, an image pickup optical system at a tip portion, that is, an image pickup element located at a distant position from an objective lens is connected by a fiber or a relay lens. The imaging optical system of such a normal insertion type endoscope is required to have telecentricity in order to reduce the amount of light loss. Further, also in the insertion type endoscope, there is an electronic endoscope having an image pickup optical system and an image pickup element at the tip portion and displaying an image on a display device located at a distant position. The capsule type endoscope includes an image pickup optical system and an image pickup element in the capsule. Therefore, telecentricity is not required for the imaging optical system of the capsule type endoscope and the electronic endoscope. On the other hand, all types of endoscopes are required to be small, wide-angle, and high-resolution. In order to achieve high resolution, it is necessary to reduce the aberration of the imaging optical system. Moreover, it is desirable not to use a bonded lens from the viewpoint of cost.

Patent Document 1 discloses an imaging optical system for an endoscope having a five-lens lens that does not use a bonded lens. However, the astigmatism of the above-mentioned imaging optical system is relatively large, and its resolution is not sufficiently satisfactory.

As described above, a five-lens imaging optical system that does not use a junction lens has not been developed in order to realize an endoscope that is sufficiently small, sufficiently wide-angle, and sufficiently high resolution.

Japanese Unexamined Patent Publication No. 2006-276779

Therefore, there is a need for a five-lens imaging optical system that does not use a junction lens in order to realize an endoscope that is sufficiently small, sufficiently wide-angle, and sufficiently high resolution. An object of the present invention is to provide a five-lens imaging optical system that does not use a junction lens in order to realize an endoscope that is sufficiently small, sufficiently wide-angle, and sufficiently high in resolution.

The imaging optical system according to the present invention has a first lens which has a negative refractive force and whose surface on the object side is a flat or convex surface, which is arranged in order from the object side to the image side, and has a positive refractive force. A second lens that is a biconvex lens, an aperture, a third lens that has a positive refractive power, a third lens that is a biconvex lens, a fourth lens that has a negative refractive power and is a biconcave lens, and a positive refractive power. Each lens is a five-lens imaging optical system that is not a junction lens, and the focal distance of the fifth lens is represented by f5, and the total focal distance is represented by f.
2.6 <f5 / f <7
Meet.

The imaging optical system according to the present invention is a five-lens optical system in which each lens is not a junction lens. By not using a bonded lens, it is possible to provide a compact and low-cost imaging optical system.

The imaging optical system according to the present invention has a first lens which is arranged from the object side to the image side and has a negative refractive power, and the surface on the object side is a flat or convex surface, and has a positive refractive power. It includes a second lens which is a convex lens, an aperture, and a third lens which has a positive refractive power and is a biconvex lens. A configuration including a first lens having a negative refractive power, a second lens having a positive refractive power, and a third lens having a positive refractive power arranged from the object side to the image side is wide-angle. It is easy to correct various aberrations.

Since the surface of the first lens on the object side is a flat surface or a convex surface, it is possible to prevent droplets from accumulating on the lens surface and thereby reducing the resolution.

The chromatic aberration of magnification of the second lens having a positive refractive power before the aperture and the chromatic aberration of magnification of the third lens having a positive refractive power after the aperture cancel each other out, and the chromatic aberration of magnification is reduced.

The imaging optical system according to the present invention includes a fourth lens having a negative refractive power and being a biconcave lens, and a fifth lens having a positive refractive power, and the focal length of the fifth lens is represented by f5. The total focal length is represented by f,
2.6 <f5 / f <7
Meet.

By making f5 / f smaller than the upper limit of the above equation, the refractive power of the fifth lens can be maintained relatively large, and an increase in curvature of field and a decrease in the peripheral illumination ratio can be suppressed. By making f5 / f larger than the lower limit of the above equation, it is possible to prevent the refractive power of the fifth lens from becoming excessive and suppress the increase in astigmatism.

In the imaging optical system of the first embodiment of the present invention, all surfaces of the first lens other than the surface on the object side are aspherical surfaces.

Aberration can be reduced by making all surfaces of the first lens other than the surface on the object side aspherical, and good resolution can be obtained.

In the imaging optical system of the second embodiment of the present invention, the fifth lens is a biconvex lens.

By using the fifth lens as a biconvex lens, it is possible to obtain good resolution by reducing curvature of field while suppressing a decrease in the peripheral illumination ratio.

The imaging optical system of the third embodiment of the present invention is further described.
4 <f5 / f <7
Meet.

f5 / f, formula
4 <f5 / f <7
By making it larger than the lower limit of, it is easy to set the distance from the exit pupil position of the optical system to the image plane to about 1.5 mm, corresponding to a small and high-resolution sensor with a large incident angle of light rays at the periphery. Become.

In the imaging optical system of the fourth embodiment of the present invention, the Abbe number of the second lens is ν2, the Abbe number of the third lens is ν3, and the Abbe number of the fourth lens is ν4.
｜ ν2-ν3 ｜ <10
ν4 <ν2
ν4 <ν3
Meet.

When the above conditions are satisfied, chromatic aberration can be reduced by using only the material of the fourth lens as a highly dispersed resin having a small Abbe number. Therefore, the number of lenses made of highly dispersed resin is reduced, which is advantageous in terms of cost.

In the imaging optical system of the fifth embodiment of the present invention, the distance from the apex of the surface of the first lens on the object side to the image plane is set as TTL.
5.5 <TTL / f <6.5
Meet.

By making TTL / f larger than the lower limit value of the above equation, it becomes easy to make the peripheral illumination ratio equal to or more than a predetermined value, and by making it smaller than the upper limit value, it becomes easy to make the optical system compact.

In the imaging optical system of the fifth embodiment of the present invention, the F number is Fno.
4.0 <Fno <6.5
Meet.

By making Fno larger than the lower limit of the above equation, the depth of field of the optical system becomes deeper and it is possible to take a wide range of images. By making Fno smaller than the upper limit of the above equation, it is possible to maintain a resolution compatible with a sensor having a small pixel size, and it is also possible to cope with the miniaturization of the sensor.

It is a figure which shows the structure of the image pickup optical system of Example 1. FIG. It is a figure which shows the spherical aberration of the image pickup optical system of Example 1. It is a figure which shows the distortion of the image pickup optical system of Example 1. FIG. It is a figure which shows the astigmatism of the imaging optical system of Example 1. It is a figure which shows the Magnification Chromatic Aberration of the image pickup optical system of Example 1. FIG. It is a figure which shows the structure of the imaging optical system of Example 2. It is a figure which shows the spherical aberration of the imaging optical system of Example 2. It is a figure which shows the distortion of the image pickup optical system of Example 2. FIG. It is a figure which shows the astigmatism of the imaging optical system of Example 2. It is a figure which shows the Magnification Chromatic Aberration of the image pickup optical system of Example 2. FIG. It is a figure which shows the structure of the imaging optical system of Example 3. It is a figure which shows the spherical aberration of the image pickup optical system of Example 3. It is a figure which shows the distortion of the image pickup optical system of Example 3. FIG. It is a figure which shows the astigmatism of the imaging optical system of Example 3. It is a figure which shows the Magnification Chromatic Aberration of the image pickup optical system of Example 3. FIG. It is a figure which shows the structure of the imaging optical system of Example 4. It is a figure which shows the spherical aberration of the image pickup optical system of Example 4. It is a figure which shows the distortion of the image pickup optical system of Example 4. It is a figure which shows the astigmatism of the imaging optical system of Example 4. It is a figure which shows the Magnification Chromatic Aberration of the image pickup optical system of Example 4. It is a figure which shows the structure of the imaging optical system of Example 5. It is a figure which shows the spherical aberration of the imaging optical system of Example 5. It is a figure which shows the distortion of the image pickup optical system of Example 5. It is a figure which shows the astigmatism of the imaging optical system of Example 5. It is a figure which shows the Magnification Chromatic Aberration of the image pickup optical system of Example 5.

FIG. 1 is a diagram showing a configuration of an imaging optical system according to an embodiment of the present invention (Example 1 described later). The imaging optical system is a first lens 101 having a negative refractive power and having a plane or a convex surface on the object side arranged from the object side to the image side, and a biconvex lens having a positive refractive power. A second lens 102, an aperture 103, a third lens 104 which has a positive refractive power and is a biconvex lens, a fourth lens 105 which has a negative refractive power and is a biconcave lens, and a positive refraction. A fifth lens 106 having power is provided. The imaging optical system is a five-lens optical system in which each lens is not a junction lens. The luminous flux that has passed through the lens is focused on the image plane 108 after passing through the optical member 107. The optical member 107 is a cover glass of a sensor or the like. In the present specification and the scope of patent claims, a lens having a negative refractive power is a lens that is incident on the optical system within a range of an angle of view and has a negative refractive power with respect to a light beam passing through the pupil. The lens having a positive refractive power means a lens having a positive refractive power with respect to the above-mentioned light beam.

The features of the imaging optical system according to the embodiment of the present invention will be described below.

The imaging optical system of the embodiment of the present invention is a five-lens optical system in which each lens is not a junction lens. By not using a bonded lens, it is possible to provide a compact and low-cost imaging optical system.

The imaging optical system of the embodiment of the present invention has a first lens arranged from the object side to the image side and having a negative refractive power, and the surface on the object side is a flat or convex surface, and has a positive refractive power. A second lens, which is a biconvex lens, an aperture, and a third lens, which has a positive refractive power and is a biconvex lens, are provided. A configuration including a first lens having a negative refractive power, a second lens having a positive refractive power, and a third lens having a positive refractive power arranged from the object side to the image side is wide-angle. It is easy to correct various aberrations. From the viewpoint of telecentricity, it is advantageous to arrange the diaphragm between the first lens and the second lens. From the viewpoint of miniaturization, wide angle, and high resolution, it is advantageous to arrange the diaphragm between the second lens and the third lens. In the present invention, since miniaturization, wide-angle lensing, and high resolution are more important than telecentricity, the diaphragm is arranged between the second lens and the third lens.

Since the surface of the first lens on the object side is a flat surface or a convex surface, the retention of droplets on the lens surface and the resulting deterioration in resolution are prevented.

The imaging optical system of the embodiment of the present invention includes a fourth lens having a negative refractive power and being a biconcave lens, and a fifth lens having a positive refractive power, and the focal length of the fifth lens is f5. The total focal length is represented by f,
2.6 <f5 / f <7 (1)
Meet.

By making f5 / f smaller than the upper limit of the equation (1), the refractive power of the fifth lens can be maintained relatively large, and an increase in curvature of field and a decrease in the peripheral illumination ratio can be suppressed. By making f5 / f larger than the lower limit value of the equation (1), it is possible to suppress an increase in astigmatism by preventing the refractive power of the fifth lens from becoming excessive.

In addition, the formula f5 / f
4 <f5 / f <7 (2)
By making it larger than the lower limit of, it is easy to set the distance from the exit pupil position of the optical system to the image plane to about 1.5 mm, corresponding to a small and high-resolution sensor with a large incident angle of light rays at the periphery. Become.

A second lens that has a positive refractive power and is a biconvex lens, an aperture, a third lens that has a positive refractive power and is a biconvex lens, and a second lens that has a negative refractive power and is a biconcave lens. Chromatic aberration can be reduced by the configuration in which the four lenses are arranged.

The chromatic aberration of magnification of the second lens having a positive refractive power before the aperture and the chromatic aberration of magnification of the third lens having a positive refractive power after the aperture cancel each other out, and the chromatic aberration of magnification is reduced.

Let the Abbe number of the second lens be ν2, the Abbe number of the third lens be ν3, and the Abbe number of the fourth lens be ν4.
ν4 <ν2 (4)
ν4 <ν3 (5)
When the condition is satisfied, chromatic aberration can be reduced by using only the material of the fourth lens as a highly dispersed resin having a small Abbe number.

The formula TTL / f, where the distance from the apex of the surface of the first lens on the object side to the image plane is TTL.
5.5 <TTL / f <6.5 (6)
By making it larger than the lower limit value of, it becomes easy to make the peripheral illumination ratio equal to or more than a predetermined value, and by making it smaller than the upper limit value, it becomes easy to make the optical system compact.

Let F number be Fno and Fno
4.0 <Fno <6.5 (7)
By making it larger than the lower limit of, the depth of field of the optical system becomes deeper and it is possible to take a wide range of images. By making Fno smaller than the upper limit of the equation (7), it is possible to maintain a resolution compatible with a sensor having a small pixel size, and it is also possible to cope with the miniaturization of the sensor.

Examples of the present invention will be described below.

The material of the first lens, the second lens, the third lens, and the fifth lens of each embodiment is a cycloolefin polymer (grade: E48R). The material of the fourth lens of each embodiment is polycarbonate (grade: EP5000). The material of the sensor cover (optical member 105) is N-BK7.

Each surface of each lens can be expressed by the following formula.

The line connecting the centers of curvature of the two surfaces of each lens is defined as the z-axis. z is a coordinate indicating the position of a point on the lens surface in the z-axis direction with the image side as a positive with respect to the intersection of each lens surface and the z-axis. h indicates the distance from the z-axis to a point on the lens surface. R is the signed radius of curvature at the apex of the lens surface, i.e. the signed central radius of curvature. c is the signed curvature at the apex of the lens surface, that is, the signed central curvature. The absolute value of c is the curvature at the apex of the lens surface, that is, the central curvature, and the sign is positive when the lens surface is convex toward the object side and negative when the lens surface is convex toward the image side. k is a conic constant. Ai is an aspherical coefficient. i and m are integers.

The matching principal axis of each lens is the optical axis.

The aberrations of the imaging optical system of each example are shown for the F line (wavelength 486.1 nm), the d line (wavelength 587.56 nm), and the C line (wavelength 656.27 nm).

The unit of length of "radius of curvature" and "interval" in the table below is millimeter.

Example 1
FIG. 1 is a diagram showing a configuration of an imaging optical system according to the first embodiment. The imaging optical system is a first lens 101 having a negative refractive power and having a plane or a convex surface on the object side arranged from the object side to the image side, and a biconvex lens having a positive refractive power. A second lens 102, an aperture 103, a third lens 104 which has a positive refractive power and is a biconvex lens, a fourth lens 105 which has a negative refractive power and is a biconcave lens, and a positive refraction. A fifth lens 106 having power is provided. The imaging optical system is a five-lens optical system in which each lens is not a junction lens. The luminous flux that has passed through the lens is focused on the image plane 108 after passing through the optical member 107.

Table 1 is a table showing the surface spacing of the optical element, the material properties of the optical element, and the surface shape of the optical element. The surfaces 1 to 4 represent the object side surface of the first lens 101, the image side surface of the first lens 101, the object side surface of the second lens 102, and the image side surface of the second lens 102, respectively. The surface spacing corresponding to the surface 1 represents the distance between the image side surface of the first lens 101 and the object side surface of the second lens 102. The refractive index and Abbe number corresponding to the surface 1 represent the refractive index and Abbe number of the first lens 101.

Table 2 is a table showing the central radius of curvature, the cornic constant, and the aspherical coefficient of the equation (8) of the surfaces 2-4 and 6-11.

FIG. 2 is a diagram showing spherical aberration of the imaging optical system of the first embodiment. The horizontal axis represents the coordinates of the imaging position in the optical axis direction. 0 on the horizontal axis represents the position of the image plane. The vertical axis represents the relative value of the distance from the optical axis of the light beam parallel to the optical axis. 0 represents a ray that coincides with the optical axis, and 1 represents a ray that passes through the edge of the aperture of the diaphragm.

FIG. 3 is a diagram showing the distortion of the imaging optical system of the first embodiment. The horizontal axis represents distortion. The vertical axis represents the angle formed by the optical axis of the main ray.

FIG. 4 is a diagram showing astigmatism of the imaging optical system of the first embodiment. The horizontal axis represents the positions of the tangier image plane and the sagittal image plane of the F line, the d line, and the C line in the optical axis direction. In the figure, Tan represents a tangential image plane and Sag represents a sagittal image plane. The vertical axis represents the angle of the light beam incident on the imaging optical system with respect to the optical axis. The maximum value of the angle on the vertical axis corresponds to the half angle of view.

FIG. 5 is a diagram showing chromatic aberration of magnification of the imaging optical system of Example 1. The horizontal axis represents the chromatic aberration of magnification of the F line and the C line with respect to the d line. The vertical axis represents the angle of the light beam incident on the imaging optical system with respect to the optical axis. The maximum value of the angle on the vertical axis corresponds to the half angle of view.

Focal length of the imaging optical system of Example 1, F number (Fno.), Half angle of view, distance from the apex of the surface of the first lens on the object side to the image plane (TTL), from the image plane to the object side. The distance to the exit pupil (exit pupil position) and the ratio of the peripheral illuminance to the illuminance on the optical axis in the image plane (peripheral illumination ratio) are as follows.

Focal length 0.693mm
Fno. Five
Half angle of view 60 degrees TTL 4.081mm
Exit pupil position 1.491mm
Peripheral illumination ratio 55%

Example 2
FIG. 6 is a diagram showing the configuration of the imaging optical system of the second embodiment. The imaging optical system is a first lens 201 having a negative refractive power and having a plane or a convex surface on the object side, which is arranged from the object side to the image side, and a biconvex lens having a positive refractive power. A second lens 202, an aperture 203, a third lens 204 which has a positive refractive power and is a biconvex lens, a fourth lens 205 which has a negative refractive power and is a biconcave lens, and a positive refraction. It is provided with a fifth lens 206 having power. The imaging optical system is a five-lens optical system in which each lens is not a junction lens. The luminous flux that has passed through the lens is focused on the image plane 208 after passing through the optical member 207.

Table 3 is a table showing the surface spacing of the optical elements, the properties of the materials of the optical elements, and the shape of the surfaces of the optical elements. The surfaces 1 to 4 represent the object side surface of the first lens 201, the image side surface of the first lens 201, the object side surface of the second lens 202, and the image side surface of the second lens 202, respectively. The surface spacing corresponding to the surface 1 represents the distance between the image side surface of the first lens 201 and the object side surface of the second lens 202. The refractive index and Abbe number corresponding to the surface 1 represent the refractive index and Abbe number of the first lens 201.

Table 4 is a table showing the central radius of curvature, the cornic constant, and the aspherical coefficient of the equation (8) of the surfaces 2-4 and 6-11.

FIG. 7 is a diagram showing spherical aberration of the imaging optical system of the second embodiment. The horizontal axis represents the coordinates of the imaging position in the optical axis direction. 0 on the horizontal axis represents the position of the image plane. The vertical axis represents the relative value of the distance from the optical axis of the light beam parallel to the optical axis. 0 represents a ray that coincides with the optical axis, and 1 represents a ray that passes through the edge of the aperture of the diaphragm.

FIG. 8 is a diagram showing the distortion of the imaging optical system of the second embodiment. The horizontal axis represents distortion. The vertical axis represents the angle formed by the optical axis of the main ray.

FIG. 9 is a diagram showing astigmatism of the imaging optical system of the second embodiment. The horizontal axis represents the positions of the tangier image plane and the sagittal image plane of the F line, the d line, and the C line in the optical axis direction. In the figure, Tan represents a tangential image plane and Sag represents a sagittal image plane. The vertical axis represents the angle of the light beam incident on the imaging optical system with respect to the optical axis. The maximum value of the angle on the vertical axis corresponds to the half angle of view.

FIG. 10 is a diagram showing chromatic aberration of magnification of the imaging optical system of the second embodiment. The horizontal axis represents the chromatic aberration of magnification of the F line and the C line with respect to the d line. The vertical axis represents the angle of the light beam incident on the imaging optical system with respect to the optical axis. The maximum value of the angle on the vertical axis corresponds to the half angle of view.

Focal length of the imaging optical system of Example 2, F number (Fno.), Half angle of view, distance from the apex of the surface of the first lens on the object side to the image plane (TTL), from the image plane to the object side. The distance to the exit pupil (exit pupil position) and the ratio of the peripheral illuminance to the illuminance on the optical axis in the image plane (peripheral illumination ratio) are as follows.

Focal length 0.693mm
Fno. Five
Half angle of view 60 degrees TTL 4.096mm
Exit pupil position 1.469mm
Peripheral illumination ratio 53%

Example 3
FIG. 11 is a diagram showing the configuration of the imaging optical system of the third embodiment. The imaging optical system is a first lens 301 having a negative refractive power and having a plane or a convex surface on the object side arranged from the object side to the image side, and a biconvex lens having a positive refractive power. A second lens 302, an aperture 303, a third lens 304 which has a positive refractive power and is a biconvex lens, a fourth lens 305 which has a negative refractive power and is a biconcave lens, and a positive refraction. A fifth lens 306 having power is provided. The imaging optical system is a five-lens optical system in which each lens is not a junction lens. The luminous flux that has passed through the lens is focused on the image plane 308 after passing through the optical member 307.

Table 5 is a table showing the surface spacing of the optical elements, the properties of the materials of the optical elements, and the shape of the surfaces of the optical elements. The surfaces 1 to 4 represent the object side surface of the first lens 301, the image side surface of the first lens 301, the object side surface of the second lens 302, and the image side surface of the second lens 302, respectively. The surface spacing corresponding to the surface 1 represents the distance between the image side surface of the first lens 301 and the object side surface of the second lens 302. The refractive index and Abbe number corresponding to the surface 1 represent the refractive index and Abbe number of the first lens 301.

Table 6 is a table showing the central radius of curvature, the cornic constant, and the aspherical coefficient of the equation (8) of the surfaces 2-4 and 6-11.

FIG. 12 is a diagram showing spherical aberration of the imaging optical system of Example 3. The horizontal axis represents the coordinates of the imaging position in the optical axis direction. 0 on the horizontal axis represents the position of the image plane. The vertical axis represents the relative value of the distance from the optical axis of the light beam parallel to the optical axis. 0 represents a ray that coincides with the optical axis, and 1 represents a ray that passes through the edge of the aperture of the diaphragm.

FIG. 13 is a diagram showing the distortion of the imaging optical system of the third embodiment. The horizontal axis represents distortion. The vertical axis represents the angle formed by the optical axis of the main ray.

FIG. 14 is a diagram showing astigmatism of the imaging optical system of Example 3. The horizontal axis represents the positions of the tangier image plane and the sagittal image plane of the F line, the d line, and the C line in the optical axis direction. In the figure, Tan represents a tangential image plane and Sag represents a sagittal image plane. The vertical axis represents the angle of the light beam incident on the imaging optical system with respect to the optical axis. The maximum value of the angle on the vertical axis corresponds to the half angle of view.

FIG. 15 is a diagram showing chromatic aberration of magnification of the imaging optical system of Example 3. The horizontal axis represents the chromatic aberration of magnification of the F line and the C line with respect to the d line. The vertical axis represents the angle of the light beam incident on the imaging optical system with respect to the optical axis. The maximum value of the angle on the vertical axis corresponds to the half angle of view.

Focal length of the imaging optical system of Example 3, F number (Fno.), Half angle of view, distance from the apex of the surface of the first lens on the object side to the image plane (TTL), from the image plane to the object side. The distance to the exit pupil (exit pupil position) and the ratio of the peripheral illuminance to the illuminance on the optical axis in the image plane (peripheral illumination ratio) are as follows.

Focal length 0.692mm
Fno. Five
Half angle of view 60 degrees TTL 4.077mm
Exit pupil position 1.506mm
Peripheral illumination ratio 56%

Example 4
FIG. 16 is a diagram showing the configuration of the imaging optical system of the fourth embodiment. The imaging optical system is a first lens 401 having a negative refractive power and having a plane or a convex surface on the object side arranged from the object side to the image side, and a biconvex lens having a positive refractive power. A second lens 402, an aperture 403, a third lens 404 which has a positive refractive power and is a biconvex lens, a fourth lens 405 which has a negative refractive power and is a biconcave lens, and a positive refraction. It is equipped with a fifth lens 406 having power. The imaging optical system is a five-lens optical system in which each lens is not a junction lens. The luminous flux that has passed through the lens is focused on the image plane 408 after passing through the optical member 407.

Table 7 is a table showing the surface spacing of the optical elements, the properties of the materials of the optical elements, and the shape of the surfaces of the optical elements. The surfaces 1 to 4 represent the object side surface of the first lens 401, the image side surface of the first lens 401, the object side surface of the second lens 402, and the image side surface of the second lens 402, respectively. The surface spacing corresponding to the surface 1 represents the distance between the image side surface of the first lens 401 and the object side surface of the second lens 402. The refractive index and Abbe number corresponding to the surface 1 represent the refractive index and Abbe number of the first lens 401.

Table 8 is a table showing the central radius of curvature, the cornic constant, and the aspherical coefficient of the equation (8) of the surfaces 2-4 and 6-11.

FIG. 17 is a diagram showing spherical aberration of the imaging optical system of Example 4. The horizontal axis represents the coordinates of the imaging position in the optical axis direction. 0 on the horizontal axis represents the position of the image plane. The vertical axis represents the relative value of the distance from the optical axis of the light beam parallel to the optical axis. 0 represents a ray that coincides with the optical axis, and 1 represents a ray that passes through the edge of the aperture of the diaphragm.

FIG. 18 is a diagram showing distortion of the imaging optical system of the fourth embodiment. The horizontal axis represents distortion. The vertical axis represents the angle formed by the optical axis of the main ray.

FIG. 19 is a diagram showing astigmatism of the imaging optical system of Example 4. The horizontal axis represents the positions of the tangier image plane and the sagittal image plane of the F line, the d line, and the C line in the optical axis direction. In the figure, Tan represents a tangential image plane and Sag represents a sagittal image plane. The vertical axis represents the angle of the light beam incident on the imaging optical system with respect to the optical axis. The maximum value of the angle on the vertical axis corresponds to the half angle of view.

FIG. 20 is a diagram showing chromatic aberration of magnification of the imaging optical system of Example 4. The horizontal axis represents the chromatic aberration of magnification of the F line and the C line with respect to the d line. The vertical axis represents the angle of the light beam incident on the imaging optical system with respect to the optical axis. The maximum value of the angle on the vertical axis corresponds to the half angle of view.

Focal length of the imaging optical system of Example 4, F number (Fno.), Half angle of view, distance from the apex of the surface of the first lens on the object side to the image plane (TTL), from the image plane to the object side. The distance to the exit pupil (exit pupil position) and the ratio of the peripheral illuminance to the illuminance on the optical axis in the image plane (peripheral illumination ratio) are as follows.

Focal length 0.693mm
Fno. Five
Half angle of view 60 degrees TTL 4.051mm
Exit pupil position 1.48mm
Peripheral illumination ratio 56%

Example 5
FIG. 21 is a diagram showing the configuration of the imaging optical system of the fifth embodiment. The imaging optical system is a first lens 501 having a negative refractive power and having a plane or a convex surface on the object side arranged from the object side to the image side, and a biconvex lens having a positive refractive power. A second lens 502, an aperture 503, a third lens 504 which has a positive refractive power and is a biconvex lens, a fourth lens 505 which has a negative refractive power and is a biconcave lens, and a positive refraction. It is provided with a fifth lens 506 having power. The imaging optical system is a five-lens optical system in which each lens is not a junction lens. The luminous flux that has passed through the lens is focused on the image plane 508 after passing through the optical member 507.

Table 9 is a table showing the surface spacing of the optical elements, the properties of the materials of the optical elements, and the shape of the surfaces of the optical elements. The surfaces 1 to 4 represent the object side surface of the first lens 501, the image side surface of the first lens 501, the object side surface of the second lens 502, and the image side surface of the second lens 502, respectively. The surface spacing corresponding to the surface 1 represents the distance between the image side surface of the first lens 501 and the object side surface of the second lens 502. The refractive index and Abbe number corresponding to the surface 1 represent the refractive index and Abbe number of the first lens 501.

Table 10 is a table showing the central radius of curvature, the cornic constant, and the aspherical coefficient of the equation (8) of the surfaces 2-4 and 6-11.

FIG. 22 is a diagram showing spherical aberration of the imaging optical system of Example 5. The horizontal axis represents the coordinates of the imaging position in the optical axis direction. 0 on the horizontal axis represents the position of the image plane. The vertical axis represents the relative value of the distance from the optical axis of the light beam parallel to the optical axis. 0 represents a ray that coincides with the optical axis, and 1 represents a ray that passes through the edge of the aperture of the diaphragm.

FIG. 23 is a diagram showing the distortion of the imaging optical system of the fifth embodiment. The horizontal axis represents distortion. The vertical axis represents the angle formed by the optical axis of the main ray.

FIG. 24 is a diagram showing astigmatism of the imaging optical system of Example 5. The horizontal axis represents the positions of the tangier image plane and the sagittal image plane of the F line, the d line, and the C line in the optical axis direction. In the figure, Tan represents a tangential image plane and Sag represents a sagittal image plane. The vertical axis represents the angle of the light beam incident on the imaging optical system with respect to the optical axis. The maximum value of the angle on the vertical axis corresponds to the half angle of view.

FIG. 25 is a diagram showing chromatic aberration of magnification of the imaging optical system of Example 5. The horizontal axis represents the chromatic aberration of magnification of the F line and the C line with respect to the d line. The vertical axis represents the angle of the light beam incident on the imaging optical system with respect to the optical axis. The maximum value of the angle on the vertical axis corresponds to the half angle of view.

Focal length of the imaging optical system of Example 5, F number (Fno.), Half angle of view, distance from the apex of the surface of the first lens on the object side to the image plane (TTL), from the image plane to the object side. The distance to the exit pupil (exit pupil position) and the ratio of the peripheral illuminance to the illuminance on the optical axis in the image plane (peripheral illumination ratio) are as follows.

Focal length 0.694mm
Fno. Five
Half angle of view 60 degrees TTL 4.1mm
Exit pupil position 1.907mm
Peripheral illumination ratio 58%

Feature Table 11 of the Example is a table showing the feature of the Example.

According to Table 11, Examples 1-5 satisfy the formulas (1) and (6)-(7), and Examples 1-4 satisfy the formula (2). Further, according to Tables 1, 3, 5, 7 and 9, Example 1-5 satisfies the formulas (3)-(5). The exit pupil position (distance from the image plane to the exit pupil in the object side direction) of Example 1-5 is smaller than 2 millimeters, and the peripheral illumination ratio is 53% or more.

Further, according to the aberration diagram of each embodiment, the magnitude of each aberration is as follows. With respect to spherical aberration, the imaging position on the optical axis is in the range of ± 5 micrometers from the image plane. With respect to astigmatism, the positions of the three wavelength tangential and sagittal image planes in the optical axis direction range from the image plane to ± 20 micrometers in all examples and ± 10 in Examples 1, 3 and 4. It is in the range of the micrometer. Distortion is ± 50% or less. The chromatic aberration of magnification of the F line and the C line with respect to the d line is within ± 1 micrometer in all the examples and within ± 0.5 micrometer in the examples 1-4.

As described above, the aberration of Example 1-5 is sufficiently small, and a high-resolution imaging optical system is realized.

Claims (7)

A first lens having a negative refractive force and having a flat or convex surface on the object side, and a second lens having a positive refractive force and being a biconvex lens, arranged in order from the object side to the image side. A third lens having a positive refractive power and being a biconvex lens, a fourth lens having a negative refractive power and being a biconcave lens, and a fifth lens having a positive refractive power are provided. , Each lens is a 5-lens imaging optical system that is not a junction lens,
The focal length of the fifth lens is represented by f5, and the total focal length is represented by f.
2.6 <f5 / f <7
An imaging optical system that meets the requirements. The imaging optical system according to claim 1, wherein all surfaces of the first lens other than the surface on the object side are aspherical surfaces. The imaging optical system according to claim 1 or 2, wherein the fifth lens is a biconvex lens. 4 <f5 / f <7
The imaging optical system according to any one of claims 1 to 3. The Abbe number of the second lens is ν2, the Abbe number of the third lens is ν3, and the Abbe number of the fourth lens is ν4.
｜ ν2-ν3 ｜ <10
ν4 <ν2
ν4 <ν3
The imaging optical system according to any one of claims 1 to 4. Let the distance from the apex of the surface of the first lens on the object side to the image plane be TTL.
5.5 <TTL / f <6.5
The imaging optical system according to any one of claims 1 to 5. With F number as Fno
4.0 <Fno <6.5
The imaging optical system according to any one of claims 1 to 6.

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