Patent Publication Number: US-2022221688-A1

Title: Stereoscopic optical system and image pickup apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a stereoscopic optical system and an image pickup apparatus suitable to capture a stereoscopic (three-dimensional) image. 
     Description of the Related Art 
     As an image pickup apparatus that can acquire a stereoscopic image by imaging, an image pickup apparatus is used in which two optical systems are disposed with a parallax with each other. Japanese Patent Laid-Open No. (“JP”) 2020-8629 discloses an image pickup apparatus in which image circles of two fisheye lenses fall within a single image sensor. 
     It is said that a baseline length between the two optical systems should be set to or within the human eye width (about 60 to 65 mm) in order to acquire a realistic stereoscopic image. Since the image sensor is generally smaller than the human eye width, the image circles partially shift from the image sensor if the fisheye lenses are simply arranged. Accordingly, the image pickup apparatus disclosed in JP 2020-8629 houses the image circles of the two fisheye lenses within a single image sensor by arranging two reflective elements in each fisheye lens so as to bend the optical path. 
     JP 2020-8629 discloses examples with angles of view from 158° to 183°, but a wider angle of view is demanded for more realistic stereoscopic images. 
     However, the fisheye lens including two reflective elements needs a longer overall length to secure a space for the reflective elements. Therefore, a lens closest to an object and a lens closest to an image plane are separated from a diaphragm (aperture stop) and an outer diameter of the lens on the object side tends to be larger as the angle of view is increased. In order to reduce the outer diameter of the lens closest to the object, if a refractive power of a negative lens unit closest to the object including that lens is increased, an on-axis light ray is significantly diverged and enters the diaphragm and thus the diaphragm diameter becomes large. 
     SUMMARY OF THE INVENTION 
     The present invention provides a small stereoscopic optical system that can capture a stereoscopic image at a wide angle of view, and an image pickup apparatus having the same. 
     A stereoscopic optical system according to one aspect of the present invention includes two optical systems arranged in parallel. Each of the two optical systems includes, in order from an object side to an image side, a front unit having a negative refractive power, an intermediate unit having two reflective surfaces, and a rear unit. A distance between optical axes of rear units is smaller than a distance between optical axes of front units in the two optical systems due to bending of an optical path by the two reflective surfaces. At least one of the intermediate unit and the rear unit has a positive refractive power. The front unit includes, in order from the object side to the image side, a first subunit consisting of one or more negative lenses disposed on the object side of a positive lens closest to an object in the front unit, and a second subunit having a positive power. The following inequalities are satisfied: 
       0.20≤ f 1 A/f 1≤0.42
 
       0.61≤| f 3/ f 2|≤0.90
 
     where f1, f2, f3, and f1A are focal lengths of the front unit, the intermediate unit, the rear unit, and the first subunit for d-line, respectively. An image pickup apparatus according to another aspect of the present invention includes the above stereoscopic optical system comprising two optical systems arranged in parallel, and an image sensor configured to image optical images formed by the two optical systems. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a vertical sectional view of a stereoscopic optical system according to Example 1. 
         FIG. 2  is an aberration diagram of the stereoscopic optical system according to Example 1. 
         FIG. 3  is a vertical sectional view of a stereoscopic optical system according to Example 2. 
         FIG. 4  is an aberration diagram of the stereoscopic optical system according to Example 2. 
         FIG. 5  is a vertical sectional view of a stereoscopic optical system according to Example 3. 
         FIG. 6  is an aberration diagram of the stereoscopic optical system according to Example 3. 
         FIG. 7  is a vertical sectional view of a stereoscopic optical system according to Example 4. 
         FIG. 8  is an aberration diagram of the stereoscopic optical system according to Example 4. 
         FIG. 9  is a vertical sectional view of a stereoscopic optical system according to Example 5. 
         FIG. 10  is an aberration diagram of the stereoscopic optical system according to Example 5. 
         FIG. 11  is a horizontal sectional view of the stereoscopic optical system according to Example 1. 
         FIG. 12  is an external view of an image pickup apparatus including the stereoscopic optical system according to any one of the examples. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Referring now to the accompanying drawings, a description will be given of examples according to the present invention. Each of stereoscopic optical systems according to Examples 1 to S includes two optical systems arranged in parallel with a single image sensor (image pickup element) in order to acquire a stereoscopic image by imaging. The stereoscopic optical system according to each example is used for an image pickup apparatus such as a digital camera, a video camera, a broadcasting camera, a surveillance camera, and a film-based camera. 
       FIGS. 1, 3, 5, 7, and 9  illustrate vertical sections of the stereoscopic optical systems according to Examples (numerical examples)  1 ,  2 ,  3 ,  4 , and  5 , respectively.  FIG. 11  illustrates a horizontal section of the stereoscopic optical system according to Example 1. 
     In each figure, a left side is an object side (front side) and a right side is an image side (rear side). L 1  (i=1, 2, 3) denotes the order of lens units counted from the object side to the image side. SP denotes a diaphragm (aperture stop). IP denotes an image plane. Disposed on the image plane IP is an imaging plane of a solid-state image sensor (photoelectric conversion element) such as a CCD sensor and a CMOS sensor, and a film plane of a film-based camera. Although not illustrated, an optical block such as an optical filter, a face plate, a low-pass filter, and an infrared cut filter can be disposed on the object side of the image plane IP. 
     As illustrated in  FIG. 11 , the stereoscopic optical system includes two optical systems arranged in parallel in the horizontal direction. In Example 1, a baseline length that is a distance (Din) between surface apexes of the lens surface closest to the object in the two optical systems, in other words, a distance between the optical axes of the first lens units L 1  in the two optical systems (a distance between the optical axes of the front units) is set to 60 mm. The baseline length may be about 60 to 65 mm, which corresponds to the human eye width. 
     Each optical system includes two reflective elements PR 1  and PR 2  each having a reflective surface, and as illustrated in  FIG. 11 , a light ray is reflected twice by the reflective surfaces to bend the optical path.  FIGS. 1, 3, 5, 7, and 9  illustrate an enlarged optical path. Each example uses prisms for the first and second reflective elements PR 1  and PR 2 , but may use mirrors.  FIG. 11  also illustrates the image sensor (IP) viewed from the front side at the right part. The image circles of the two optical systems are formed within a single image sensor, and the two optical images formed by the two optical systems are imaged by the image sensor. The two optical images formed by the two optical systems may be imaged by different image sensors. 
       FIGS. 2, 4, 6, 8, and 10  illustrate various aberrations (such as the spherical aberration, the astigmatism, the distortion, and the chromatic aberration) in the in-focus state at infinity of the optical systems of the numerical examples 1, 2, 3, 4, and 5, respectively. In the spherical aberration diagram, Fno denotes an F-number, a solid line denotes the spherical aberration for the d-line (wavelength 587.6 nm), and a broken line denotes the spherical aberration for the g-line (wavelength 435.8 nm). In the astigmatism diagram, a solid line ΔS denotes a sagittal image plane, and a broken line ΔM denotes a meridional image plane. The distortion diagram is illustrated for the d-line. The chromatic aberration diagram illustrates the lateral chromatic aberration for the g-line, ω is the half angle of view (°). 
     Numerical example 1 is an example in which an image sensor has a short side length of 24 mm and a long side length of 36 mm. Since the baseline length of 60 mm is longer than the long side length of 36 mm of the image sensor, two reflective elements PR 1  and PR 2  are provided to each optical system so that both optical systems become closer to each other at part close to the image sensor. Thereby, the image circles of both optical systems are housed within the single image sensor. 
     It is necessary to house the image circle of one optical system within a range of 18 mm, which is half of the long side length of the image sensor. In numerical example 1, the image circle diameter of each optical system is set to 17.5 mm, and a distance of 1 mm is set between the image circles of the two optical systems. A distance (Lout) between the surface apexes of the lens surface closest to the image plane of the two optical systems, in other words, a distance between the optical axes of the third lens units L 3  (the rear units) in the two optical systems becomes 18.5 mm. An on-axis distance between the two reflective elements (reflective surfaces) in the optical systems becomes 20.75 mm. The image circle of each optical system is formed so as to be inscribed in the short side of the image sensor. 
     The optical system according to each example employs a conformal projection method. The conformal projection method is defined as y=fω, where y is a half image height, w is a half angle of view, and f is a focal length. In numerical example 1, since the image circle diameter is 17.5 mm, the half image height y=8.75 mm, the focal length f=4.78 mm, and the half angle of view ω=104.88°. 
     Numerical example 2 is also an example in which an image sensor has a baseline length of 60 mm, a short side length of 24 mm, and a long side length of 36 mm. The image circle diameter of each optical system is set to 17.1 mm, and a distance between the image circles of the two optical systems is set to 1.4 mm. A surface apex distance (Dout) of the lens surfaces closest to the image plane of the two optical systems becomes 18.5 mm. The image circle of each optical system is separated from the short side of the image sensor by 0.2 mm so that the image circle does not come off from the image sensor even if the position of the optical axis shifts due to a manufacturing error. In numerical example 2, a half image height y is 8.55 mm, a focal length f is 5.13 mm, and a half angle of view w is 95.49°. 
     The baseline length, the sire of the image sensor, and the diameter and interval of the image circle in numerical example 3 are the same as those in numerical example 1. A focal length f is 4.74 mm, and a half angle of view w is 105.75°. 
     Numerical example 4 is an example in which an image sensor has a baseline length (Din) of 65 mm, a short side length of 24 mm, and a long side length of 36 mm. A surface apex distance (Dout) of the lenses closest to the image plane of the two optical systems is set to 7.8.5 mm, and an on-axis distance between the two reflective elements of each optical system is set to 23.25 mm. In this numerical example, an image circle diameter is 17.5 mm, a half image height y is 8.75 mm, a focal length f is 4.89 mm, and a half angle of view w is 102.44′. 
     Numerical example 5 is an example in which an image sensor has a baseline length (Din) of 60.5 mm, a short side length of 15 mm, and a long side length of 22.5 mm. An image circle diameter of each optical system is set to 10.5 mm, and a distance between the image circles of the two optical systems is set to 1 mm. A surface apex spacing (Dout) of the lenses closest to the image plane of the two optical systems is 11.5 mm. An image circle of each optical system is separated from the short side of the image sensor by 0.25 mm. In this numerical example, a half image height y is 5.25 mm, a focal length f is 3.02 mm, and a half angle of view w is 99.67°. 
     The optical system according to each example adopts the conformal projection method, but may use various projection methods such as the equisolid angle projection method and the stereographic projection method. 
     A description will now be given of the specific configuration of the optical system according to each example. The optical system according to each example includes, in order from the object side to the image side, a first lens unit (front unit) L 1  having a negative refractive power, a first reflective element PR 1 , an intermediate unit that includes a second lens unit L 2  that includes a diaphragm SP and has a positive refractive power, and a second reflective element PR 2 , and a third lens unit (rear unit) L 3  having a positive refractive power. The retrofocus type refractive power arrangement in which the front unit having the negative refractive power and the rear unit having the positive refractive power arranged via the diaphragm SP can make wholly small the optical system with a wide angle of view. 
     In each example, each of the second lens unit L 2  and the third lens unit L 3  has a positive refractive power, but at least one of them may have a positive refractive power. 
     In each example, the second lens unit L 2  is disposed between the first and second reflective elements PR 1  and PR 2  in the intermediate unit, but the incident surfaces and exit surfaces of the first reflective element PR 1  and the second reflective element PR 2  may be formed as refractive surfaces instead of the second lens unit L 2 . 
     In the optical system according to each example, the diaphragm SP is disposed between the two reflective elements PR 1  and PR 2 . As illustrated in  FIG. 11 , in order to form the image circles of the two optical systems within the single image sensor, it is necessary to make the lenses of the two optical systems after the second reflective element PR 2  close to each other. That is, due to bending of the optical path by the first and second reflective elements PR 1  and PR 2 , the distance between the optical axes of the third lens units L 3  (the rear units) of the two optical systems needs to be smaller than the distance between the optical axes of the first lens units L 1  (the front units). 
     If the diaphragm SP is closer to the object than the first reflective element PR 1 , a diameter of a lens closest to the image plane increases and may interfere with the lens closest to the image plane of the other optical system. On the other hand, if the diaphragm SP is disposed on the image side of the second reflective element PR 2 , a diameter of a lens closest to the object increases, may cause the optical system to be heavier or may interfere with the lens closest to the object of the other optical system. 
     If two reflective elements are included in each optical system as in the optical system according to each example, the overall optical length becomes long. The first lens unit L 1  and the third lens unit L 3  are located at positions away from the diaphragm SP, respectively. If the angle of view is simply increased in this configuration, the diameters of the lenses closest to the object and the image plane, whose effective diameters are determined by the off-axis light rays, increase, causing interference between the lenses and an increase in weight. 
     In order to suppress the increase in effective diameter of the lens closest to the object as the angle of view is increased, the refractive power of the first lens unit L 1  may be increased. Since the distance between the first lens unit L 1  and the diaphragm SP is long, when the refractive power of the first lens unit L 1  is increased, the on-axis light ray is significantly diverged in the first lens unit L 1 , and as a result, the diaphragm SP becomes larger. In addition, the first reflective element PR 1  becomes larger and the weight increases. This tendency becomes more remarkable as the diameter of the optical system is increased. Accordingly, in order to suppress an increase in effective diameter of the lens closest to the object and an increase in diaphragm diameter, it is necessary to properly set the refractive power arrangement of the lenses included in the first lens unit L 1 . 
     In the optical system according to each example, the first lens unit L includes, from the object side to the image side, a  1 A lens unit (first subunit) L 1 A and a  1 B lens unit (second subunit) LIB. The  1 A lens unit L 1 A has a negative refractive power as a whole, and includes one or more lenses from a negative lens closest to the object to a negative lens on the object side of a positive lens on the image side of the negative lens closest to the object in the first lens unit L 1 . That is, the  1 A lens unit L 1 A includes one or more negative lenses disposed on the object side of the positive lens closest to the object among one or more positive lenses included in the first lens unit L 1 . The  1 B lens unit L 1 B has a positive refractive power as a whole, and includes at least one lens after the above positive lens. 
     The optical system according to each example satisfies the following inequality (conditional expression) (1). 
       0.20≤ f 1 A/f 1≤0.42  (1)
 
     where f1 A is a focal length of the  1 A lens unit L 1 A, and f1 is a focal length of the first lens unit L 1 . 
     The inequality (1) defines a condition to reconcile a reduced diameter of the lens closest to the object and the miniaturization of the intermediate unit including the first reflective element PR 1 , the second lens unit, and the second reflective element PR 2 . In each example, the first lens unit L 1  includes the  1 A lens unit L 1 A having a strong negative refractive power and the  1 B lens unit L 1 B having a strong positive refractive power so that the first lens unit L 1  as a whole has a weak negative refractive power. The  1 A lens unit L 1 A having the strong negative refractive power reduces the diameter of the lens closest to the object, and the  1 B lens unit L 1 B having the strong positive refractive power reduces the divergence of the on-axis light ray passing through the first lens unit L 1 , thereby making small the intermediate unit. 
     If f1A/f1 is lower than the lower limit in the inequality (1), the refractive power of the  1 A lens unit L 1 A becomes too large, the intermediate unit may become large, or the off-axis aberration such as the curvature of field and the distortion may increase. If f1A/f1 is higher than the upper limit in the inequality (1), the refractive power of the  1 A lens unit L 1 A becomes too small, and the diameter of the lens closest to the object may become large. 
     The numerical range of the inequality (1) may be set as follows: 
       0.22≤ f 1 A/f 1≤0.40  (1a)
 
     The numerical range of the inequality (1) may be set as follows: 
       0.23≤ f 1 A/f 1≤0.35  (1b)
 
     In the optical system according to each example, the second lens unit L 2  having a positive refractive power is disposed between the first and second reflective elements (reflective surfaces), and the off-axis light ray that has passed through the diaphragm SP is converged on the optical axis. This configuration can suppress an increase in effective diameter of the third lens unit L 3 . However, the second lens unit L 2  is disposed near the diaphragm SP, and has a large influence on the on-axis aberration such as the spherical aberration and the coma. In order to reconcile the high performance of the optical system and the small diameter of the third lens unit L 3 , it is necessary to properly set the refractive powers of the second lens unit L 2  and the third lens unit L 3 . 
     Accordingly, the optical system according to each example satisfies the following inequality (2): 
       0.61≤| f 3/ f 2|≤0.90  (2)
 
     where f2 is a focal length of the second lens unit L 2  for the d-line, and f3 is a focal length of the third lens unit L 3  for the d-line. 
     The inequality (2) defines a condition relating to the optical performance of the optical system and the miniaturization of the third lens unit L 3 . If |f3/f2| is lower than the lower limit in the inequality (2), the refractive power of the second lens unit L 2  becomes too small, the convergence of the off-axis light ray deteriorates, and the third lens unit L 3  becomes large and may interfere with the other lens. If |f3/f2| is higher than the upper limit in the inequality (2), the refractive power of the second lens unit L 2  becomes too large, the on-axis aberration such as the spherical aberration and the coma increases, and the optical performance deteriorates. 
     The numerical range of the inequality (2) may be set as follows: 
       0.63≤| f 3/ f 2|≤0.88  (2a)
 
     The numerical range of the inequality (2) may be set as follows: 
       0.64≤| f 3/ f 2|≤0.85  (2b)
 
     The above configuration and the above inequality can provide a wider angle of view and a larger diameter in a small stereoscopic optical system that horizontally arranges two optical systems to form each image circle within the single image sensor. 
     The following inequality may be satisfied: 
       −1.95≤ f 1 B/f 1≤−1.10  (3)
 
     where f1B is a focal length of the  1 B lens unit L 1 B for the d-line. 
     The inequality (3) defines a condition relating to the miniaturization and the optical performance of the first reflective element PR 1  and the diaphragm SP. If f1B/f1 is lower than the lower limit in the inequality (3), the refractive power of the  1 B lens unit L 1 B becomes too small, and the on-axis light beam diverged from the  1 A lens unit L 1 A cannot be converged, causing the first reflective element PR 1  and the diaphragm SP to become large. If f1B/f1 is higher than the upper limit in the inequality (3), the off-axis aberration such as the curvature of field and the distortion generated in the  1 B lens unit L 1 B becomes significant and deteriorates the optical performance. 
     The numerical range of the inequality (3) may be set as follows: 
       −1.80≤ f 1 B/f 1≤−1.15  (3a)
 
     The numerical range of the inequality (3) may be set as follows: 
       −1.66≤ f 1 B/f 1≤−1.20  (3b)
 
     The following inequality (4) may be satisfied: 
       6.10≤ f 2/ f≤ 10.90  (4)
 
     where f is a focal length of the optical system for the d-line, and f2 is a focal length of the second lens unit L 2  for the d-line. 
     The inequality (4) defines a conditions relating to the optical performance of the optical system and the reduced diameter of the third lens unit L 3 . If f2/f is lower than the lower limit in the inequality (4), the refractive power of the second lens unit L 2  becomes too large, and the on-axis aberration such as the spherical aberration and the coma increases. If f2/f is higher than the upper limit in the inequality (4), the refractive power of the second lens unit L 2  becomes too weak, and the convergence of the off-axis light ray that has passed through the diaphragm SP on the optical axis deteriorates. As a result, the effective diameter of the third lens unit L 3  increases and may interfere with the third lens unit L 3  of the other optical system. 
     The numerical range of the inequality (4) may be set as follows: 
       6.60≤ f 2/ f≤ 10.40  (4a)
 
     The numerical range of the inequality (4) may be set as follows: 
       7.10≤ f 2/ f≤ 9.90  (4b)
 
     The following inequality (5) may be satisfied: 
       4.40≤ f 3/ f≤ 9.60  (5)
 
     where f3 is a focal length of the third lens unit for the d-line. 
     The inequality (5) defines a condition relating to the optical performance of the optical system and the reduced diameter of the third lens unit L 3 . If f3/f is lower than the lower limit in the inequality (5), the refractive power of the third lens unit L 3  becomes too large, and the off-axis aberration such as the curvature of field and the distortion increases. If f3/f is higher than the upper limit in the inequality (5), the refractive power of the third lens unit L 3  becomes too weak, and the convergence of the off-axis light ray that has passed through the diaphragm SP on the optical axis deteriorates. As a result, the effective diameter of the third lens unit L 3  increases, and may interfere with the third lens unit L 3  of the other optical system. 
     The numerical range of the inequality (5) may be set as follows: 
       4.70≤ f 3/ f≤ 9.20  (5a)
 
     The numerical range of the inequality (5) may be set as follows: 
       5.00≤ f 3/ f≤ 8.80  (5b)
 
     The following inequality (6) may be satisfied: 
       0.05 ≤D out/ D in ≤0.50  (6)
 
     where Din is a distance (baseline length) between surface apexes of the lenses closest to the object of the two optical systems, and Dout is a distance between surface apexes of the lenses closest to the image plane. 
     The inequality (6) is a conditional expression relating to a stereoscopic effect and discomfort when the viewer views a stereoscopic image. If Dout/Din is lower than the lower limit in the inequality (6), the baseline length becomes much wider than the human eye width, so that the parallax between the images obtained through the two optical systems becomes too large, and the viewer may feel discomfortable or tired. If Dout/Din is higher than the upper limit in the inequality (6), the parallax becomes too small to obtain a stereoscopic effect. 
     The numerical range of the inequality (6) may be set as follows: 
       0.10 ≤D out/ D in ≤0.45  (6a)
 
     The numerical range of the inequality (6) may be set as follows: 
       0.15 ≤D out/ D in ≤0.40  (6b)
 
     The following inequality (7) may be satisfied: 
       1.80≤ N 1 Ave≤ 2.20  (7)
 
     where N1Aave is an average of the refractive indexes among the plurality of negative lenses included in the  1 A lens unit L 1 A for the d-line. 
     The inequality (7) defines a condition relating to the optical performance of the optical system. As described above, the first lens unit L 1 A has a strong negative refractive power in order to reduce the diameter of the first lens unit L 1 . Therefore, the off-axis aberration such as the curvature of field and the distortion is likely to occur, and a glass material having a high refractive index may be used for the high image quality. If N1Aave is lower than the lower limit in the inequality (7), the refractive index of the negative lens in the  1 A lens unit L 1 A is too low and the off-axis aberration increases. If N1Aave is higher than the upper limit in the inequality (7), the workability of the lens is lowered. 
     The numerical range of the inequality (7) may be set as follows: 
       1.85≤ N 1 Ave≤ 2.10  (7a)
 
     The numerical range of the inequality (7) may be set as follows: 
       1.88≤ N 1 Ave≤ 2.05  (7b)
 
     The following inequality (8) may be satisfied: 
       1.78≤ N 1 Bp≤ 2.20  (8)
 
     where N1Bp is a refractive index of the positive lens having the strongest refractive power among at least one positive lens included in the  1 B lens unit L 1 B for the d-line. 
     The inequality (8) defines a condition relating to the optical performance of the optical system and the miniaturization of the first reflective element PR 1  and the diaphragm SP. As described above, the  1 B lens unit L 1 B converges the on-axis light beam strongly diverged by the  1 A lens unit L 1 A, and miniaturizes the subsequent first reflective element PR 1  and aperture SP. If N1Bp is lower than the lower limit in the inequality (8), the refractive power of the  1 B lens unit becomes too strong, the aberration generated in the  1 B lens unit becomes significant, and the optical performance deteriorates. If N1Bp is higher than the upper limit in the inequality (8), the workability of the lens is deteriorated. 
     The numerical range of the inequality (8) may be set as follows: 
       1.80≤ N 1 Bp≤ 2.10  (8a)
 
     The numerical range of the inequality (8) may be set as follows: 
       1.83≤ N 1 Bp= 2.05  (8b)
 
     The following inequality (9) may be satisfied: 
       85,0°≤ω≤120.0°  (9)
 
     where ω is a half angle of view by ray tracing of the optical system. 
     If w is lower than the lower limit in the inequality (9), the angle of view is insufficient when the viewer views the stereoscopic image, and sufficient reality cannot be gained. If w is higher than the upper limit in the inequality (9), the number of pixels per incident angle on the image sensor decreases, and the resolution of the stereoscopic image becomes insufficient. 
     The numerical range of the inequality (9) may be set as follows: 
       87.5′≤ω≤115.0°  (9a)
 
     The numerical range of the inequality (9) may be set as follows: 
       90.0°≤ω≤110.0°  (9b)
 
     The following inequality (10) may be satisfied: 
       2.00≤ Fno≤ 4.50  (10)
 
     where Fno is an F-number of the optical system. 
     If Fno is lower than the lower limit in the inequality (10), the first and second reflective elements PR 1  and PR 2  and the diaphragm SP become large. If Fno is higher than the upper limit in the inequality (10), the image quality deteriorates due to increased noises during imaging. 
     The numerical range of the inequality (10) may be set as follows: 
       2.20≤ Fno≤ 4.10  (10a)
 
     The numerical range of the inequality (be set as follows: 
       2.40≤ Fno≤ 3.60  (10b)
 
     Next follows a description of a configuration of each lens unit according to each example. In Examples 1, 2, 4, and 5, the  1 A lens unit L 1 A includes, in order from the object side to the image side, a negative meniscus lens with a convex surface facing the object side, a negative meniscus lens with a convex surface facing the object side, and a biconcave lens. In Example 4, an aspherical surface on the image side of the biconcave lens corrects the off-axis aberration such as the curvature of field and the distortion. In Example 3, the  1 A lens unit L 1 A includes, in order from the object side to the image side, a negative meniscus lens with a convex surface facing the object side, a biconcave lens, and a negative meniscus lens with a convex surface facing the image side. 
     In Example 1, the  1 B lens unit L 1 B includes, in order from the object side to the image side, a positive meniscus lens with a convex surface facing the image side, and a negative meniscus lens with a convex surface facing the image side. An aspherical surface on the image side of the negative meniscus lens corrects the off-axis aberration such as the curvature of field and the distortion. In Examples 2 and 4, the  1 B lens unit L 1 B includes a positive meniscus lens with a convex surface facing the image side. In Example 3, the  1 B lens unit L 1 B includes, in order from the object side to the image side, a cemented lens of a positive meniscus lens with a convex surface facing the image side and a negative meniscus lens with a convex surface facing the image side. An aspherical surface on the image side of the negative meniscus lens corrects the off-axis aberration such as the curvature of field and the distortion. In Example 5, the  1 B lens unit L 1 B includes a biconvex lens. 
     Examples 1 to 5 use prisms for the first and second reflective elements PR 1  and PR 2 . 
     In Examples 1 to 4, the second lens unit L 2  includes, in order from the object side to the image side, the diaphragm SP and a cemented lens of a negative meniscus lens with a convex surface facing the object side and a biconvex lens. In Example 5, the second lens unit L 2  includes, in order from the object side to the image side, the diaphragm SP, a negative meniscus lens with a convex surface facing the object side, and a biconvex lens. 
     In Examples 1 and 3, the third lens unit L 3  includes, in order from the object side to the image side, a biconvex lens, a cemented lens of a biconcave lens and a biconvex lens, and a cemented lens of a negative meniscus lens with a convex surface facing the object side and a biconvex lens, in Example 2, the third lens unit L 3  includes, in order from the object side to the image side, a biconvex lens, a biconvex lens, and a cemented lens of a biconvex lens and a biconvex lens. In Example 4, the third lens unit L 3  includes, in order from the object side to the image side, a cemented lens of a biconvex lens and a negative meniscus lens with a convex surface facing the image side, a biconvex lens, and a cemented lens of a negative meniscus lens with a convex surface facing the object side and a biconvex lens. In Example 5, the third lens unit L 3  includes, in order from the object side to the image side, a biconvex lens, a cemented lens of a negative meniscus lens with a convex surface facing the object side and a biconvex lens, and a cemented lens of a biconcave lens and a biconvex lens. 
     NM Specific numerical data of numerical examples 1 to 5 will be shown below. In each numerical example, i denotes the order of a surface or an optical element counted from the object side, r denotes a radius of curvature of an i-th optical surface (i-th surface), and d denotes an on-axis distance between an i-th surface and an (i+1)-th surface. nd denotes a refractive index of an optical material between an i-th surface and an (i+1)-th surface for the d-line, and vdi denotes an Abbe number based on the d-line of an optical material between an i-th surface and an (i+1)-th surface. 
     The Abbe number vd is expressed as follows: 
         vd =( Nd− 1)/( NF - NC ) 
     where Nd, NF, and NC are refractive indexes for the d-line (587.6 nm), the F-line (486.1 nm), and the C-line (656.3 nm) in the Fraunhofer lines. 
     Numerical data also shows an effective diameter (mm) of an i-th surface. BF denotes a backfocus (mm). The backfocus is described by converting an on-axis distance from the final surface (lens surface closest to the image plane) of the optical system to the paraxial image plane to an air equivalent length. The overall lens length is a length obtained by adding the backfocus to an on-axis distance from the frontmost surface (lens surface closest to the object) to the final surface of the optical system. 
     The first and second reflective elements in each numerical example are described as one block material. In reality, each reflective element is disposed so as to bend a light ray by 45° with respect to the center of the block material. 
     Each numerical example adopts a method in which the entire optical system is drawn in the optical axis direction during focusing from an infinity object to a short-distance object. For the weight reduction of the driving unit that drives the optical system, the optical system may be partially driven during focusing. 
     An asterisk “*” attached to a surface number means that the surface is aspherical surface. The aspheric shape is expressed as follows: 
     
       
         
           
             X 
             = 
             
               
                 
                   
                     H 
                     2 
                   
                   ⁢ 
                   
                     / 
                   
                   ⁢ 
                   R 
                 
                 
                   1 
                   + 
                   
                     
                       1 
                       - 
                       
                         
                           ( 
                           
                             1 
                             + 
                             K 
                           
                           ) 
                         
                         ⁢ 
                         
                           
                             ( 
                             
                               H 
                               ⁢ 
                               
                                 / 
                               
                               ⁢ 
                               R 
                             
                             ) 
                           
                           2 
                         
                       
                     
                   
                 
               
               + 
               
                 A4H 
                 4 
               
               + 
               
                 A6H 
                 6 
               
               + 
               
                 A8H 
                 8 
               
               + 
               
                 
                   A10H 
                   10 
                 
                 ⁡ 
                 
                   ( 
                   
                     + 
                     
                       A12H 
                       12 
                     
                   
                   ) 
                 
               
             
           
         
       
     
     where X-axis is set to the optical axis direction, H-axis is set to a direction orthogonal to the optical axis, a light traveling direction is set positive, R is a paraxial radius of curvature, K is a conical constant, A4, A6, A8, A10 and (A12) are aspherical coefficients, and “e-x” of the conical constant and the aspherical coefficient means × 10   −x . 
     Table 1 summarizes values of the inequalities (1) to (10) in Examples (numerical examples) 1 to 5. 
     NUMERICAL EXAMPLE 1 
       
     
       
         
           
               
             
               
                   
               
             
            
               
                 UNIT: mm 
               
               
                 Surface Data 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Effective  
               
               
                 Surface No: 
                 r 
                 d 
                 nd 
                 vd 
                 Diameter 
               
               
                   
               
               
                  1 
                 34.245 
                 1.75 
                 2.00100 
                 29.1 
                 38.55 
               
               
                  2 
                 10.727 
                 8.36 
                   
                   
                 21.09 
               
               
                  3 
                 40.942 
                 0.95 
                 1.95375 
                 32.3 
                 19.57 
               
               
                  4 
                 17.998 
                 5.62 
                   
                   
                 17.39 
               
               
                  5 
                 −14.473 
                 0.95 
                 1.88300 
                 40.8 
                 16.55 
               
               
                  6 
                 485.400 
                 2.55 
                   
                   
                 17.39 
               
               
                  7 
                 −30.711 
                 4.23 
                 2.00069 
                 25.5 
                 17.94 
               
               
                  8 
                 −13.665 
                 0.59 
                   
                   
                 18.90 
               
               
                  9 
                 −12.726 
                 1.55 
                 1.58313 
                 59.4 
                 18.41 
               
               
                  10* 
                 −13.253 
                 6.90 
                   
                   
                 18.95 
               
               
                 11 
                 ∞ 
                 13.00 
                 1.51633 
                 64.1 
                 12.30 
               
               
                 12 
                 ∞ 
                 3.00 
                   
                   
                 8.43 
               
               
                 13 
                 ∞ 
                 0.50 
                   
                   
                 8.81 
               
               
                 (diaphragm) 
                   
                   
                   
                   
                   
               
               
                 14 
                 34.485 
                 0.70 
                 1.90043 
                 37.4 
                 8.91 
               
               
                 15 
                 12.617 
                 3.80 
                 1.54072 
                 47.2 
                 8.84 
               
               
                 16 
                 −19.342 
                 0.25 
                   
                   
                 9.09 
               
               
                 17 
                 ∞ 
                 12.00 
                 1.51633 
                 64.1 
                 9.01 
               
               
                 18 
                 ∞ 
                 1.63 
                   
                   
                 10.41 
               
               
                 19 
                 13.588 
                 3.89 
                 1.43875 
                 94.7 
                 12.17 
               
               
                 20 
                 −19.839 
                 0.20 
                   
                   
                 12.25 
               
               
                 21 
                 −41.411 
                 0.85 
                 1.83481 
                 42.7 
                 12.09 
               
               
                 22 
                 9.542 
                 5.12 
                 1.49700 
                 81.5 
                 12.15 
               
               
                 23 
                 −25.605 
                 0.30 
                   
                   
                 13.08 
               
               
                 24 
                 23.211 
                 0.75 
                 1.90043 
                 37.4 
                 13.98 
               
               
                 25 
                 13.141 
                 4.61 
                 1.49700 
                 81.5 
                 13.83 
               
               
                 26 
                 −73.713 
                 13.51 
                   
                   
                 14.32 
               
               
                 Image plane 
                 ∞ 
                   
                   
                   
                   
               
               
                   
               
            
           
           
               
            
               
                 ASPHERIC DATA 
               
               
                   
               
               
                 10th Surface 
               
               
                 K = 0.00000e+000 A 4 = 3.30939e−005 A 6 = −1.46934e−008  
               
               
                 A 8 = 8.65575e−010 A 10 = −1.57365e−012 
               
               
                   
               
               
                 VARIOUS DATA 
               
               
                   
               
            
           
           
               
               
            
               
                 Focal Length 
                 4.78 
               
               
                 Fno 
                 2.80 
               
               
                 Half Angle of View (°) 
                 104.88 
               
               
                 Image Height 
                 8.75 
               
               
                 Overall Lens Length 
                 97.55 
               
               
                 BF 
                 13.51 
               
               
                 Entrance pupil position 
                 10.45 
               
               
                 Exit pupil position 
                 −50.46 
               
               
                 Front Principal Position 
                 14.87 
               
               
                 Rear Principal Position 
                 8.73 
               
               
                   
               
            
           
           
               
            
               
                 SINGLE LENS DATA 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                 Structural 
                 Front  
                 Rear  
               
               
                   
                 Starting 
                 Focal 
                 Lens  
                 Principal 
                 Principal 
               
               
                 Unit 
                 Surface 
                 Length 
                 length 
                 Position 
                 Position 
               
               
                   
               
               
                 L1 
                 1 
                 −14.49 
                 26.54 
                 −1.19 
                 −40.65 
               
               
                 L2 
                 13 
                 38.90 
                 5.00 
                 3.20 
                 −0.17 
               
               
                 L3 
                 19 
                 30.52 
                 15.71 
                 4.44 
                 −7.19 
               
               
                   
               
            
           
           
               
            
               
                 Single Lens Data 
               
            
           
           
               
               
               
            
               
                 Lens 
                 Starting Surface 
                 Focal Length 
               
               
                   
               
               
                 1 
                 1 
                 −16.21 
               
               
                 2 
                 3 
                 −34.37 
               
               
                 3 
                 5 
                 −15.90 
               
               
                 4 
                 7 
                 21.89 
               
               
                 5 
                 9 
                 6615.95 
               
               
                 6 
                 11 
                 0.00 
               
               
                 7 
                 14 
                 −22.44 
               
               
                 8 
                 15 
                 14.74 
               
               
                 9 
                 17 
                 0.00 
               
               
                 10 
                 19 
                 19.06 
               
               
                 11 
                 21 
                 −9.22 
               
               
                 12 
                 22 
                 14.70 
               
               
                 13 
                 24 
                 −34.87 
               
               
                 14 
                 25 
                 22.84 
               
               
                   
               
            
           
         
       
     
     NUMERICAL EXAMPLE 2 
       
     
       
         
           
               
             
               
                   
               
             
            
               
                 UNIT: mm 
               
               
                 Surface Data 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Effective  
               
               
                 Surface No: 
                 r 
                 d 
                 nd 
                 vd 
                 Diameter 
               
               
                   
               
               
                  1 
                 37.835 
                 1.90 
                 1.85150 
                 40.8 
                 38.26 
               
               
                  2 
                 11.044 
                 5.89 
                   
                   
                 21.36 
               
               
                  3 
                 16.936 
                 1.05 
                 2.00100 
                 29.1 
                 19.71 
               
               
                  4 
                 10.396 
                 7.26 
                   
                   
                 16.44 
               
               
                  5 
                 −14.348 
                 0.85 
                 1.85150 
                 40.8 
                 15.40 
               
               
                  6 
                 78.512 
                 1.83 
                   
                   
                 15.94 
               
               
                  7 
                 −171.522 
                 3.90 
                 1.91650 
                 31.6 
                 16.52 
               
               
                  8 
                 −16.120 
                 6.99 
                   
                   
                 17.01 
               
               
                  9 
                 ∞ 
                 13.40 
                 1.51633 
                 64.1 
                 10.84 
               
               
                 10 
                 ∞ 
                 2.05 
                   
                   
                 7.72 
               
               
                 11  
                 ∞ 
                 1.65 
                   
                   
                 8.01 
               
               
                 (diaphragm) 
                   
                   
                   
                   
                   
               
               
                 12 
                 32.437 
                 0.50 
                 1.90043 
                 37.4 
                 8.29 
               
               
                 13 
                 11.736 
                 3.00 
                 1.54072 
                 47.2 
                 8.25 
               
               
                 14 
                 −19.189 
                 0.15 
                   
                   
                 8.46 
               
               
                 15 
                 ∞ 
                 13.40 
                 1.51633 
                 64.1 
                 8.42 
               
               
                 16 
                 ∞ 
                 2.17 
                   
                   
                 12.34 
               
               
                 17 
                 40.449 
                 2.00 
                 1.80809 
                 22.8 
                 14.04 
               
               
                 18 
                 −112.028 
                 0.15 
                   
                   
                 14.23 
               
               
                 19 
                 19.706 
                 4.25 
                 1.49700 
                 81.6 
                 14.55 
               
               
                 20 
                 −26.991 
                 0.32 
                   
                   
                 14.29 
               
               
                 21 
                 −125.299 
                 0.75 
                 2.00069 
                 25.5 
                 13.72 
               
               
                 22 
                 9.886 
                 6.90 
                 1.49700 
                 81.6 
                 13.06 
               
               
                 23 
                 −22.112 
                 13.21 
                   
                   
                 14.22 
               
               
                 Image plane 
                 ∞ 
                   
                   
                   
                   
               
               
                   
               
            
           
           
               
            
               
                 VARIOUS DATA 
               
               
                   
               
            
           
           
               
               
            
               
                 Focal Length 
                 5.13 
               
               
                 Fno 
                 2.91 
               
               
                 Half Angle of View (°) 
                 95.49 
               
               
                 Image Height 
                 8.55 
               
               
                 Overall Lens Length 
                 93.57 
               
               
                 BF 
                 13.21 
               
               
                 Entrance pupil position 
                 10.91 
               
               
                 Exit pupil position 
                 −54.55 
               
               
                 Front Principal Position 
                 15.66 
               
               
                 Rear Principal Position 
                 8.08 
               
               
                   
               
            
           
           
               
            
               
                 SINGLE LENS DATA 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                 Structural 
                 Front  
                 Rear  
               
               
                   
                 Starting 
                 Focal 
                 Lens  
                 Principal 
                 Principal 
               
               
                 Unit 
                 Surface 
                 Length 
                 length 
                 Position 
                 Position 
               
               
                   
               
               
                 L1 
                 1 
                 −12.41 
                 22.68 
                 1.27 
                 −25.53 
               
               
                 L2 
                 11 
                 39.29 
                 5.15 
                 3.80 
                 −0.08 
               
               
                 L3 
                 17 
                 27.80 
                 14.37 
                 1.87 
                 −8.65 
               
               
                   
               
            
           
           
               
            
               
                 Single Lens Data 
               
            
           
           
               
               
               
            
               
                 Lens 
                 Starting Surface 
                 Focal Length 
               
               
                   
               
               
                 1 
                 1 
                 −18.93 
               
               
                 2 
                 3 
                 −29.24 
               
               
                 3 
                 5 
                 −14.19 
               
               
                 4 
                 7 
                 19.18 
               
               
                 5 
                 9 
                 0.00 
               
               
                 6 
                 12 
                 −20.66 
               
               
                 7 
                 13 
                 13.94 
               
               
                 8 
                 15 
                 0.00 
               
               
                 9 
                 17 
                 36.99 
               
               
                 10 
                 19 
                 23.63 
               
               
                 11 
                 21 
                 −9.13 
               
               
                 12 
                 22 
                 14.81 
               
               
                   
               
            
           
         
       
     
     NUMERICAL EXAMPLE 3 
       
     
       
         
           
               
             
               
                   
               
             
            
               
                 UNIT: mm 
               
               
                 Surface Data 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Effective  
               
               
                 Surface No: 
                 r 
                 d 
                 nd 
                 vd 
                 Diameter 
               
               
                   
               
               
                  1 
                 32.204 
                 1.75 
                 2.00100 
                 29.1 
                 38.50 
               
               
                  2 
                 10.699 
                 10.58 
                   
                   
                 21.07 
               
               
                  3 
                 −78.044 
                 0.95 
                 2.00100 
                 29.1 
                 18.41 
               
               
                  4 
                 24.653 
                 4.81 
                   
                   
                 16.60 
               
               
                  5 
                 −14.061 
                 0.95 
                 1.91082 
                 35.3 
                 16.18 
               
               
                  6 
                 −60.746 
                 1.47 
                   
                   
                 17.36 
               
               
                  7 
                 −43.270 
                 4.57 
                 2.00069 
                 25.5 
                 17.96 
               
               
                  8 
                 −13.755 
                 1.50 
                 1.58313 
                 59.4 
                 18.81 
               
               
                  9* 
                 −14.442 
                 7.07 
                   
                   
                 19.10 
               
               
                 10 
                 ∞ 
                 13.00 
                 1.51633 
                 64.1 
                 12.30 
               
               
                 11 
                 ∞ 
                 3.00 
                   
                   
                 8.29 
               
               
                 12 
                 ∞ 
                 0.50 
                   
                   
                 8.64 
               
               
                 (diaphragm) 
                   
                   
                   
                   
                   
               
               
                 13 
                 40.250 
                 0.70 
                 1.87070 
                 40.7 
                 8.72 
               
               
                 14 
                 12.989 
                 3.80 
                 1.54072 
                 47.2 
                 8.67 
               
               
                 15 
                 −20.730 
                 0.25 
                   
                   
                 8.94 
               
               
                 16 
                 ∞ 
                 12.00 
                 1.51633 
                 64.1 
                 8.88 
               
               
                 17 
                 ∞ 
                 1.50 
                   
                   
                 10.86 
               
               
                 18 
                 18.363 
                 3.55 
                 1.43875 
                 94.7 
                 12.39 
               
               
                 19 
                 −20.988 
                 0.20 
                   
                   
                 12.62 
               
               
                 20 
                 −78.396 
                 0.85 
                 1.83481 
                 42.7 
                 12.58 
               
               
                 21 
                 13.807 
                 3.95 
                 1.49700 
                 81.5 
                 12.71 
               
               
                 22 
                 −74.957 
                 0.30 
                   
                   
                 13.35 
               
               
                 23 
                 15.313 
                 0.75 
                 2.00100 
                 29.1 
                 14.22 
               
               
                 24 
                 9.426 
                 6.50 
                 1.49700 
                 81.5 
                 13.58 
               
               
                 25 
                 −65.349 
                 13.51 
                   
                   
                 14.31 
               
               
                 Image plane 
                 ∞ 
                   
                   
                   
                   
               
               
                   
               
            
           
           
               
            
               
                 ASPHERIC DATA 
               
               
                   
               
               
                 9th Surface 
               
               
                 K = 0.00000e+000 A 4 = 3.82080e−005 A 6 = 1.00161e−007  
               
               
                 A 8 = −1.70767e−010 A 10 = 4.27075e−012 
               
               
                   
               
               
                 VARIOUS DATA 
               
               
                   
               
            
           
           
               
               
            
               
                 Focal Length 
                 4.74 
               
               
                 Fno 
                 2.80 
               
               
                 Half Angle of View (°) 
                 105.75 
               
               
                 Image Height 
                 8.75 
               
               
                 Overall Lens Length 
                 98.00 
               
               
                 BF 
                 13.51 
               
               
                 Entrance pupil position 
                 10.62 
               
               
                 Exit pupil position 
                 −49.18 
               
               
                 Front Principal Position 
                 15.01 
               
               
                 Rear Principal Position 
                 8.77 
               
               
                   
               
            
           
           
               
            
               
                 SINGLE LENS DATA 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                 Structural 
                 Front  
                 Rear  
               
               
                   
                 Starting 
                 Focal 
                 Lens  
                 Principal 
                 Principal 
               
               
                 Unit 
                 Surface 
                 Length 
                 length 
                 Position 
                 Position 
               
               
                   
               
               
                 L1 
                 1 
                 −15.82 
                 26.58 
                 −2.05 
                 −45.04 
               
               
                 L2 
                 12 
                 44.37 
                 5.00 
                 3.39 
                 0.03 
               
               
                 L3 
                 18 
                 28.89 
                 16.09 
                 3.69 
                 −7.86 
               
               
                   
               
            
           
           
               
            
               
                 Single Lens Data 
               
            
           
           
               
               
               
            
               
                 Lens 
                 Starting Surface 
                 Focal Length 
               
               
                   
               
               
                 1 
                 1 
                 −16.69 
               
               
                 2 
                 3 
                 −18.63 
               
               
                 3 
                 5 
                 −20.28 
               
               
                 4 
                 7 
                 18.70 
               
               
                 5 
                 8 
                 −2533.25 
               
               
                 6 
                 10 
                 0.00 
               
               
                 7 
                 13 
                 −22.29 
               
               
                 8 
                 14 
                 15.38 
               
               
                 9 
                 16 
                 0.00 
               
               
                 10 
                 18 
                 22.95 
               
               
                 11 
                 20 
                 −14.00 
               
               
                 12 
                 21 
                 23.81 
               
               
                 13 
                 23 
                 −26.16 
               
               
                 14 
                 24 
                 17.07 
               
               
                   
               
            
           
         
       
     
     NUMERICAL EXAMPLE 4 
       
     
       
         
           
               
             
               
                   
               
             
            
               
                 UNIT: mm 
               
               
                 Surface Data 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Effective  
               
               
                 Surface No: 
                 r 
                 d 
                 nd 
                 vd 
                 Diameter 
               
               
                   
               
               
                  1 
                 32.204 
                 1.75 
                 2.00100 
                 29.1 
                 38.51 
               
               
                  2 
                 10.624 
                 7.72 
                   
                   
                 20.97 
               
               
                  3 
                 24.232 
                 0.95 
                 2.00100 
                 29.1 
                 19.34 
               
               
                  4 
                 12.748 
                 6.79 
                   
                   
                 16.60 
               
               
                  5 
                 −12.957 
                 0.95 
                 1.85400 
                 40.4 
                 15.34 
               
               
                  6* 
                 278.285 
                 2.15 
                   
                   
                 16.29 
               
               
                  7 
                 −48.376 
                 4.10 
                 2.00100 
                 29.1 
                 17.03 
               
               
                  8 
                 −14.676 
                 6.90 
                   
                   
                 17.92 
               
               
                  9 
                 ∞ 
                 13.00 
                 1.51633 
                 64.1 
                 12.30 
               
               
                 10 
                 ∞ 
                 1.50 
                   
                   
                 8.64 
               
               
                 11 
                 ∞ 
                 1.50 
                   
                   
                 8.87 
               
               
                 (diaphragm) 
                   
                   
                   
                   
                   
               
               
                 12 
                 44.079 
                 0.70 
                 1.83481 
                 42.7 
                 9.13 
               
               
                 13 
                 13.067 
                 3.80 
                 1.54072 
                 47.2 
                 9.11 
               
               
                 14 
                 −17.531 
                 3.25 
                   
                   
                 9.41 
               
               
                 15 
                 ∞ 
                 12.00 
                 1.51633 
                 64.1 
                 9.00 
               
               
                 16 
                 ∞ 
                 1.50 
                   
                   
                 11.20 
               
               
                 17 
                 33.445 
                 3.77 
                 1.49700 
                 81.5 
                 12.40 
               
               
                 18 
                 −12.867 
                 0.85 
                 1.80400 
                 46.6 
                 12.76 
               
               
                 19 
                 −34.227 
                 0.00 
                   
                   
                 13.48 
               
               
                 20 
                 27.869 
                 4.13 
                 1.49700 
                 81.5 
                 14.06 
               
               
                 21 
                 −22.810 
                 0.30 
                   
                   
                 14.20 
               
               
                 22 
                 148.722 
                 0.75 
                 1.95375 
                 32.3 
                 13.82 
               
               
                 23 
                 10.283 
                 6.47 
                 1.49700 
                 81.5 
                 13.32 
               
               
                 24 
                 −27.769 
                 13.51 
                   
                   
                 14.32 
               
               
                 Image plane 
                 ∞ 
                   
                   
                   
                   
               
               
                   
               
            
           
           
               
            
               
                 ASPHERIC DATA 
               
               
                   
               
               
                 6th Surface 
               
               
                 K = 0.00000e+000 A 4 = 3.25358e−005 A 6 = 2.20598e−007  
               
               
                 A 8 = −6.10712e−009 A 10 = 3.74834e−011 A 12 = −7.69846e−014 
               
               
                   
               
               
                 VARIOUS DATA 
               
               
                   
               
            
           
           
               
               
            
               
                 Focal Length 
                 4.89 
               
               
                 Fno 
                 2.80 
               
               
                 Half Angle of View (°) 
                 102.44 
               
               
                 Image Height 
                 8.75 
               
               
                 Overall Lens Length 
                 98.35 
               
               
                 BF 
                 13.51 
               
               
                 Entrance pupil position 
                 10.31 
               
               
                 Exit pupil position 
                 −85.68 
               
               
                 Front Principal Position 
                 14.97 
               
               
                 Rear Principal Position 
                 8.67 
               
               
                   
               
            
           
           
               
            
               
                 SINGLE LENS DATA 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                 Structural 
                 Front  
                 Rear  
               
               
                   
                 Starting 
                 Focal 
                 Lens  
                 Principal 
                 Principal 
               
               
                 Unit 
                 Surface 
                 Length 
                 length 
                 Position 
                 Position 
               
               
                   
               
               
                 L1 
                 1 
                 −17.10 
                 24.42 
                 0.63 
                 −31.44 
               
               
                 L2 
                 11 
                 36.40 
                 6.00 
                 4.41 
                 0.04 
               
               
                 L3 
                 17 
                 30.57 
                 16.27 
                 4.35 
                 −7.37 
               
               
                   
               
            
           
           
               
            
               
                 Single Lens Data 
               
            
           
           
               
               
               
            
               
                 Lens 
                 Starting Surface 
                 Focal Length 
               
               
                   
               
               
                 1 
                 1 
                 −16.51 
               
               
                 2 
                 3 
                 −28.03 
               
               
                 3 
                 5 
                 −14.48 
               
               
                 4 
                 7 
                 19.84 
               
               
                 5 
                 9 
                 0.00 
               
               
                 6 
                 12 
                 −22.48 
               
               
                 7 
                 13 
                 14.48 
               
               
                 8 
                 15 
                 0.00 
               
               
                 9 
                 17 
                 19.22 
               
               
                 10 
                 18 
                 −26.11 
               
               
                 11 
                 20 
                 25.94 
               
               
                 12 
                 22 
                 −11.61 
               
               
                 13 
                 23 
                 16.00 
               
               
                   
               
            
           
         
       
     
     NUMERICAL EXAMPLE 5 
       
     
       
         
           
               
             
               
                   
               
             
            
               
                 UNIT: mm 
               
               
                 Surface Data 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Effective  
               
               
                 Surface No: 
                 r 
                 d 
                 nd 
                 vd 
                 Diameter 
               
               
                   
               
               
                  1 
                 21.335 
                 1.10 
                 2.00100 
                 29.1 
                 26.01 
               
               
                  2 
                 7.424 
                 6.76 
                   
                   
                 14.61 
               
               
                  3 
                 286.662 
                 0.60 
                 2.00100 
                 29.1 
                 13.07 
               
               
                  4 
                 12.363 
                 4.48 
                   
                   
                 11.49 
               
               
                  5 
                 −12.962 
                 0.60 
                 1.88300 
                 40.8 
                 10.31 
               
               
                  6 
                 18.337 
                 1.15 
                   
                   
                 10.60 
               
               
                  7 
                 31.801 
                 3.11 
                 1.84666 
                 23.9 
                 11.39 
               
               
                  8 
                 −12.417 
                 3.47 
                   
                   
                 11.69 
               
               
                  9 
                 ∞ 
                 10.00 
                 1.51633 
                 64.1 
                 9.50 
               
               
                 10 
                 ∞ 
                 6.85 
                   
                   
                 6.62 
               
               
                 11 
                 ∞ 
                 0.15 
                   
                   
                 7.03 
               
               
                 (diaphragm) 
                   
                   
                   
                   
                   
               
               
                 12 
                 26.581 
                 0.80 
                 1.88300 
                 40.8 
                 7.10 
               
               
                 13 
                 8.264 
                 0.15 
                   
                   
                 7.04 
               
               
                 14 
                 8.470 
                 4.38 
                 1.54072 
                 47.2 
                 7.20 
               
               
                 15 
                 −13.128 
                 2.17 
                   
                   
                 7.70 
               
               
                 16 
                 ∞ 
                 10.00 
                 1.51633 
                 64.1 
                 7.50 
               
               
                 17 
                 ∞ 
                 1.03 
                   
                   
                 8.13 
               
               
                 18 
                 9.163 
                 2.19 
                 1.43875 
                 94.7 
                 9.07 
               
               
                 19 
                 −230.699 
                 0.13 
                   
                   
                 9.00 
               
               
                 20 
                 23.408 
                 0.50 
                 2.00100 
                 29.1 
                 8.93 
               
               
                 21 
                 6.789 
                 3.52 
                 1.49700 
                 81.5 
                 8.52 
               
               
                 22 
                 −14.303 
                 0.15 
                   
                   
                 8.78 
               
               
                 23 
                 −50.245 
                 0.50 
                 2.00100 
                 29.1 
                 8.76 
               
               
                 24 
                 11.044 
                 2.70 
                 1.80810 
                 22.8 
                 8.88 
               
               
                 25 
                 −33.265 
                 13.50 
                   
                   
                 9.14 
               
               
                 Image plane 
                 ∞ 
                   
                   
                   
                   
               
               
                   
               
            
           
           
               
            
               
                 VARIOUS DATA 
               
               
                   
               
            
           
           
               
               
            
               
                 Focal Length 
                 3.02 
               
               
                 Fno 
                 2.80 
               
               
                 Half Angle of View (°) 
                 99.67 
               
               
                 Image Height 
                 5.25 
               
               
                 Overall Lens Length 
                 80.00 
               
               
                 BF 
                 13.50 
               
               
                 Entrance pupil position 
                 7.33 
               
               
                 Exit pupil position 
                 −46.97 
               
               
                 Front Principal Position 
                 10.19 
               
               
                 Rear Principal Position 
                 10.48 
               
               
                   
               
            
           
           
               
            
               
                 SINGLE LENS DATA 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                 Structural 
                 Front  
                 Rear  
               
               
                   
                 Starting 
                 Focal 
                 Lens  
                 Principal 
                 Principal 
               
               
                 Unit 
                 Surface 
                 Length 
                 length 
                 Position 
                 Position 
               
               
                   
               
               
                 L1 
                 1 
                 −6.61 
                 17.81 
                 1.75 
                 −19.10 
               
               
                 L2 
                 11 
                 29.80 
                 5.48 
                 4.14 
                 0.67 
               
               
                 L3 
                 18 
                 25.05 
                 9.69 
                 1.71 
                 −5.04 
               
               
                   
               
            
           
           
               
            
               
                 Single Lens Data 
               
            
           
           
               
               
               
            
               
                 Lens 
                 Starting Surface 
                 Focal Length 
               
               
                   
               
               
                 1 
                 1 
                 −11.85 
               
               
                 2 
                 3 
                 −12.92 
               
               
                 3 
                 5 
                 −8.52 
               
               
                 4 
                 7 
                 10.90 
               
               
                 5 
                 9 
                 0.00 
               
               
                 6 
                 12 
                 −13.86 
               
               
                 7 
                 14 
                 10.25 
               
               
                 8 
                 16 
                 0.00 
               
               
                 9 
                 18 
                 20.14 
               
               
                 10 
                 20 
                 −9.70 
               
               
                 11 
                 21 
                 9.81 
               
               
                 12 
                 23 
                 −9.01 
               
               
                 13 
                 24 
                 10.55 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Ex. 1 
                 Ex. 2 
                 Ex. 3 
                 Ex. 4 
                 Ex. 5 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Inequality 1 
                 f1A/f1 
                 0.29 
                 0.33 
                 0.24 
                 0.31 
                 0.30 
               
               
                 Inequality 2 
                 f3/f2 
                 0.78 
                 0.71 
                 0.65 
                 0.84 
                 0.84 
               
               
                 Inequality 3 
                 f1B/f1 
                 −1.57 
                 −1.55 
                 −1.24 
                 −1.64 
                 −1.65 
               
               
                 Inequality 4 
                 f2/f 
                 8.14 
                 7.65 
                 9.36 
                 7.44 
                 9.87 
               
               
                 Inequality 5 
                 f3/f 
                 6.38 
                 5.42 
                 6.09 
                 6.25 
                 8.30 
               
               
                 Inequality 6 
                 Dout/Din 
                 0.31 
                 0.31 
                 0.31 
                 0.28 
                 0.19 
               
               
                 Inequality 7 
                 N1Aave 
                 1.95 
                 1.90 
                 1.97 
                 1.95 
                 1.96 
               
               
                 Inequality 8 
                 N1Bp 
                 2.00 
                 1.92 
                 2.00 
                 2.00 
                 1.85 
               
               
                 Inequality 9 
                 ω 
                 104.9 
                 95.4 
                 105.8 
                 102.4 
                 99.7 
               
               
                 Inequality 10 
                 Fno 
                 2.80 
                 2.91 
                 2.80 
                 2.80 
                 2.80 
               
               
                   
               
            
           
         
       
     
       FIG. 12  illustrates a digital still camera as an image pickup apparatus that uses the stereoscopic optical system according to any one of the above examples for an imaging optical system. Reference numeral  20  denotes a camera body, and reference numeral  21  denotes the imaging optical system that is the stereoscopic optical system according to any one of Examples 1 to 5. Reference numeral  22  denotes a solid-state image sensor such as a CCD sensor and a CMOS sensor, which is built in the camera body  20  and configured to image an optical image formed by the imaging optical system  21 . Reference numeral  23  denotes a recorder configured to record image data generated by processing an imaging signal from the image sensor  22 , and reference numeral  24  denotes a rear display unit that displays the image data. 
     The stereoscopic optical system according to each example can provide a small stereoscopic camera having a high optical performance. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2021-004230, filed on Jan. 14, 2021, which is hereby incorporated by reference herein in its entirety.