JP4574301B2 - Focus detection device - Google Patents

Description

  The present invention relates to a focus detection device used for focus adjustment of a photographing optical system in a digital camera, a video camera, a silver salt camera, and the like, and an imaging device including the focus detection device.

  Conventional focus detection of a photographing optical system in a digital camera or the like has performed focus detection by a contrast detection method using an image sensor. In general, focus detection by such a contrast detection method is performed on the optical axis of the imaging optical system. Since the extreme value of the contrast is obtained while slightly moving the position, there is a problem that it takes a considerable time to adjust the focus before focusing.

  Therefore, a phase difference type focus detection is used in a single-lens reflex digital camera or the like. In the focus detection of the phase difference detection method, the defocus amount of the photographing optical system can be obtained, so that there is an advantage that the time until focusing is significantly shortened compared with the contrast detection method.

  Phase difference focus detection includes vertical line detection that performs focus detection on a subject having contrast in the horizontal direction and horizontal line detection that performs focus detection on a subject that has contrast in the vertical direction. In order to deal with various subjects, focus detection is performed by mixing these vertical line detection and horizontal line detection in the shooting screen. In addition, cross-type focus detection that performs vertical line detection and horizontal line detection at the same position in the shooting screen is performed.

  In recent years, multipoint focus detection in which a plurality of focus detection areas by vertical line detection and horizontal line detection are provided in a shooting screen, area type focus detection in which focus detection is performed in a continuous area over a wide area, and the like have been proposed. . As a conventional example of such a focus detection method, Patent Document 1 discloses an example of performing cross-type focus detection as well as performing area-type focus detection.

  Such a conventional example is an example in which the focus detection as described above is applied to a single-lens reflex type camera. FIG. 13 shows a central sectional view of the camera. In the figure, L is the optical axis of the photographing optical system, and the photographing optical system is not shown, but is arranged on the left side of the optical axis L in the figure. Reference numeral 2 denotes a primary imaging plane of the photographing optical system, and a main mirror 3 and a sub mirror 4 are arranged on the front surface thereof.

  The main mirror 3 and the sub mirror 4 are retracted out of the photographing light beam during photographing by a known quick return mechanism. On the other hand, at the time of focus detection, it is held at the position shown in the figure, and the light beam used for focus detection passes through a half mirror portion provided near the center of the main mirror 3 and is reflected downward by the sub mirror 3. The In this case, reference numeral 5 denotes a primary imaging plane of a focus detection optical system that is optically equivalent to the primary imaging image plane 2 formed by the sub mirror 4.

  Thereafter, the optical path is turned back by the first plane mirror 6 and passes through the infrared cut glass 7, the diaphragm 8, and the secondary imaging lens 9 in this order. Further, it is folded downward by the second plane mirror 10 and finally guided onto the focus detection sensor 11. The focus detection sensor 11 includes a cover glass and a sensor chip disposed near the secondary imaging plane. An optical axis L ′ is an optical axis after the optical axis L of the photographing optical system passes through the main mirror 3, the sub mirror 4, and the plane mirror 6.

  The sub mirror 4 is constituted by a part of an ellipsoid formed by rotating an ellipse around the central axis A as indicated by a dotted line in the figure. At this time, one of the two focal points of the ellipse is set to the exit pupil of the photographing optical system. Next, the other is set to a point B which is an intersection of the central axis A and the optical axis L turned back by the sub mirror 4.

  Point B is a point where the center point of the focus detection diaphragm 8 is converted into air and then mirror-reversed by the first plane mirror 6. Therefore, the sub mirror 4 keeps the diaphragm 8 and the exit pupil of the photographing optical system in an imaging relationship because of the basic property of an ellipse. That is, in the phase difference type focus detection, it plays the role of a known field lens, and the sub mirror 5 and the diaphragm 8 function as pupil dividing means. When an appropriate aperture is set in the diaphragm 8, a plurality of light beams obtained by dividing the exit pupil of the photographing optical system can be guided to the focus detection optical system.

  FIG. 14 is a plan view of the diaphragm 8 and the secondary imaging lens 9 as viewed from the infrared cut glass 7 side. At this time, since the secondary imaging lens 9 is hidden by the diaphragm 9, it is indicated by a dotted line. The diaphragm 8 includes a pair of openings 8-1 and 8-2, and 8-3 and 8-4. The secondary imaging lens 9 corresponds to each opening, and a pair of lens portions 9-1 and 9-2. , 9-3 and 9-4. Therefore, out of the light beams passing through the exit pupil of the photographing optical system, the light beam divided in the vertical direction by the openings 8-1 and 8-2 and the light beam divided in the horizontal direction by the openings 8-3 and 8-4. An image is formed by a secondary imaging lens 9 having four lens portions. Four optical images are formed on the secondary imaging surface, and phase difference type focus detection is performed by detecting a phase shift of the four optical images accompanying defocusing of the photographing optical system by the focus detection sensor 11.

  FIG. 15 is a plan view of the sensor chip of the focus detection sensor 11 as viewed from the second plane mirror 10 side. In the sensor chip, four sensor areas corresponding to the four lens portions of the secondary imaging lens 9 are formed. The lens portions 9-1 and 9-2 are formed in the sensor regions 11-1 and 11-2. -3 and 9-4 correspond to the sensor regions 11-3 and 11-4, respectively. The optical images projected on the sensor areas 11-1 and 11-2 are optical images of light beams that have passed through the openings 8-1 and 8-2, that is, light beams obtained by dividing the exit pupil of the photographing optical system in the vertical direction. Therefore, the optical image also moves in the vertical direction with the defocusing of the photographing optical system. Therefore, in the sensor regions 11-1 and 11-2, it is possible to detect the phase difference of the optical image by setting the pixel row arrangement direction to the vertical direction. Similarly, in the sensor areas 11-3 and 11-4, the arrangement direction of the pixel rows is the horizontal direction. In the sensor areas 11-1 and 11-2, since a subject having a contrast component in the vertical direction can be detected best, horizontal lines are detected, and in the sensor areas 11-3 and 11-4, vertical lines are detected. Will be.

  By the way, since the diaphragm 8 is projected onto the exit pupil of the photographing optical system by the sub mirror 5, the opening when FIG. 14 is enlarged at a predetermined magnification indicates a light flux passage region on the exit pupil. Therefore, if FIG. 14 itself is multiplied by the projection magnification and is considerably enlarged on the exit pupil, the distance between the centers of gravity of the openings 8-1 and 8-2 is the base line length of the horizontal line detection, and the openings 8-4 and The center-of-gravity interval of 8-5 is the baseline length for vertical line detection. The circumscribed circle including the openings 8-1 and 8-2 indicates the exit pupil diameter capable of focus detection in the horizontal line detection, while the circumscribed circle including the openings 8-3 and 8-4 is in the vertical line detection. The exit pupil diameter allows focus detection. With the latter exit pupil diameter, focus detection is possible for both vertical line detection and horizontal line detection. That is, only horizontal line detection functions at a small exit pupil diameter, and both vertical and horizontal line detection functions at a large exit pupil diameter. For example, only horizontal line detection is performed when the F number is F5.6, and both vertical and horizontal line detection is performed at F2.8.

  FIG. 16 is a diagram in which each sensor region is back-projected within the imaging range on the primary imaging plane 2. In this figure, distortion caused by distortion of the focus detection optical system is ignored. There are focus detection areas 13 and 14 in the imaging range 12, and the focus detection area 13 is a back projection of the sensor areas 11-1 and 11-2, and is represented as the focus detection area 13 because they almost coincide. . Similarly, the focus detection area 14 corresponds to the sensor areas 11-3 and 11-4. Therefore, the focus detection area 13 represents a range in which horizontal line detection is performed, and the focus detection area 14 represents a range in which vertical line detection is performed, and a hatched portion in the drawing where the two overlap is a range in which cross-type focus detection is performed.

  As described above, in the conventional example, phase difference type focus detection is performed in a wide range, and cross-type focus detection is realized in a part of the region.

  In general, in order to define a wider range as a focus detection region within the imaging range, it is necessary to guide a wider range of light flux to the focus detection optical system. Therefore, for that purpose, it is necessary to move the sub-mirror 4 to the primary imaging plane 2 side as much as possible to make the light reflection area of the sub-mirror 4 larger. Then, as shown in FIG. 18, the primary imaging plane 5 of the focus detection optical system moves to the sub mirror 5 side.

  In a focus detection optical system using a known field lens, it is necessary to set a field lens and a field mask near the primary imaging plane 5 of the focus detection optical system. Therefore, since these members enter the photographing light beam, a retracting mechanism is necessary. However, in the conventional example, the sub-mirror 4 is formed of a spheroid and has the function of the pupil dividing means, so there is no need to provide a field lens. In addition, the sub mirror 4 also serves as a field mask by being configured so as not to reflect light except in a region necessary for focus detection.

For this reason, in the conventional example, it is possible to easily increase the size of the submirror 4, and as a result, phase difference focus detection in a wide range is realized.
JP-A-9-184965

  However, the conventional example has the following problems.

  In the shooting screen, the horizontal direction, which is the optical movement direction, becomes smaller due to the influence of the curvature of field, and the range in which vertical line detection is performed is limited to the vicinity of the center compared to the range in which horizontal line detection is performed.

  An object of the present invention is made in view of the problems of the prior art, and an object of the present invention is to provide a focus detection apparatus that realizes phase difference type focus detection in a wide range within an imaging range.

  The present invention that achieves this object includes a pupil splitting unit that splits the exit pupil of the photographing optical system to form a pair of light beams, and a pair of two that re-images the pair of light beams to form a pair of optical images. And a focus detection device that detects a distance between the pair of optical images accompanying defocusing of the imaging optical system and adjusts the focus of the imaging optical system, and includes at least one of the secondary imaging lenses of the pair The optical surface has a radius of curvature at an effective portion through which a light beam passes in order to form the optical image in the detection direction of the pair of optical image intervals in the detection direction, and decreases in the direction of enlargement of the pair of optical image intervals in the detection direction. To a curved surface that changes greatly without extreme values.

  Further, pupil dividing means for dividing the exit pupil of the photographing optical system in at least two directions to form a pair of light beams respectively, and a pair of secondary images for re-imaging the pair of light beams to form a pair of optical images In the focus detection apparatus that detects the distance between the pair of optical images accompanying defocusing of the lens and the photographing optical system and performs focus adjustment of the photographing optical system, the two directions of the pair of light beams divided by the pupil dividing unit The at least one optical surface of the pair of secondary imaging lenses corresponding to the pair of light beams having the longer focus detection baseline length is different in the detection direction of the pair of optical image intervals. A curved surface in which the radius of curvature at an effective portion through which light passes to form the optical image changes without any extreme value from small to large in the expanding direction of the pair of optical image intervals in the detection direction.

  Further, pupil dividing means for dividing the exit pupil of the photographing optical system to form a pair of light beams, a pair of secondary imaging lenses for re-imaging the pair of light beams to form a pair of optical images, In the focus detection device that detects the optical image interval of the pair accompanying the defocusing of the photographing optical system and adjusts the focus of the photographing optical system, the optical image of the pair has a shape whose longitudinal direction is one direction, At least one optical surface of the secondary imaging lens has a radius of curvature at an effective portion through which a light beam passes in order to form the optical image in the detection direction of the pair of optical image intervals in the detection direction. In the curved surface that changes from small to large without an extreme value in the direction of enlargement of the image interval, the longitudinal direction coincides with the detection direction.

  In the present invention, it is possible to satisfactorily correct the curvature of field of the secondary imaging optical image in the pupil division direction as compared with the conventional case, so that vertical line detection and horizontal line detection are greatly improved over a wide range within the photographing range. It becomes possible to carry out with accuracy.

  Examples of the present invention will be described below.

  The first embodiment is an example in which the focus detection apparatus of the present invention is applied to a single-lens reflex digital camera as in the conventional example. FIG. 1 is a central sectional view of the camera, and only the part related to focus detection is displayed.

  In the figure, 101 is a photographing optical system, L is an optical axis of the photographing optical system, and 101 a is an exit pupil of the photographing optical system 101. Reference numeral 102 denotes a primary imaging plane on which a solid-state image sensor for receiving an image formed by the photographing optical system 101 is disposed.

  A main mirror 103 and a sub mirror 104 are disposed between the photographing optical system 101 and the primary imaging plane 102. Near the center of the main mirror 103 is a half mirror, and a part of the light beam that has passed through the photographing optical 101 is transmitted, and the other light beam is reflected upward and guided to a finder optical system (not shown). The light beam transmitted through the main mirror 103 is reflected downward by the sub mirror 104. Reference numeral 105 denotes a primary imaging plane of a focus detection optical system that is optically equivalent to the primary imaging plane 102, and L ′ denotes an optical axis of the focus detection optical system in which the optical axis L is refracted and reflected by each member. It is.

  The main mirror 103 and the sub mirror 104 are configured by a known quick return mechanism, and are retracted outside the photographing light beam at the time of photographing, and are held at the positions shown in the figure otherwise.

  The light beam reflected by the sub mirror 104 is reflected again by the plane mirror 106, passes through the infrared cut glass 108 and the diaphragm 109, forms an image again by the secondary imaging lens 110, passes through the cover glass 111, and then passes through the sensor chip 112. To.

  The sub mirror 104 is constituted by a part of an ellipsoid formed by rotating an ellipse indicated by a dotted line around the A axis as in the conventional example. At this time, one of the two focal points of the ellipse is a point D in the vicinity of the exit pupil 101a of the photographing optical system. The other focal point is set at a point B where the center point of the diaphragm 109 is converted into air by the thickness of the infrared cut glass 108 and mirror-reversed by the plane mirror 106.

  Therefore, the exit pupil 101a and the stop 109 of the photographic optical system are maintained in a substantially imaging relationship by the sub-mirror 104 due to the property of the elliptical focus, and the exit pupil of the photographic optical system 101 is set by setting an appropriate aperture in the stop 109. It becomes possible to guide the luminous flux obtained by dividing 101a. In the claims, the sub mirror 104 and the stop 109 correspond to pupil dividing means.

  FIG. 2 is a plan view of the diaphragm 109 as viewed from the infrared cut glass 108 side. The aperture 109 is formed with a pair of openings 109a and 109b, 109c and 109d. These openings are projected onto the exit pupil 101a by the sub mirror 104.

  FIG. 3 is a plan view of the secondary imaging lens 110 viewed from the diaphragm 109 side. Therefore, the lens shape on the exit side is indicated by a dotted line. In the figure, a pair of lens portions 110a and 110b, 110c and 110d are provided on the exit side, and each correspond to four openings. On the incident side, a lens unit 110e including one optical surface that is commonly used by the four lens units is provided. The light beam that has passed through each opening forms an image in the vicinity of the sensor chip 112 through the incident side and the emission side, and forms four optical images. In the cross-sectional view of FIG. 1, only two of the lens portions 110 and 110b among the four lenses on the exit side are displayed.

  FIG. 4 is a plan view of the sensor chip 112 as viewed from the cover glass 111 side. A pair of sensor areas 112a and 112b, 112c and 112d are provided on the sensor chip, and line sensors having the vertical direction as the pixel arrangement direction are densely arranged in the sensor areas 112a and 112b. In 112d, line sensors whose horizontal direction is the pixel arrangement direction are densely arranged. Each sensor area corresponds to four lens portions. That is, the light beam that has passed through the opening 109a forms one optical image through the lens portions 110e and 110a, and this optical image is detected by the sensor region 112a. The same applies to the sensor regions 112b, 112c, and 112d.

  FIG. 5 shows an optical image formed on the sensor chip 112. The optical image 115a corresponds to the lens 110a, the optical image 115b corresponds to the lens 110b, the optical image 115c corresponds to the lens 110c, and the optical image 115d corresponds to the lens 110d. In each optical image, the sub-mirror 104 made of an ellipsoid is inclined and arranged in the optical path of the focus detection optical system, so that fan-shaped distortion occurs. The four optical images are divided in the exit pupil 101a in accordance with the defocusing of the photographing optical system 101. The optical images 115a and 115b are in the vertical direction indicated by the arrow O, and the optical images 115c and 115d are the arrow P. The change in the optical image interval is detected as a phase difference. That is, the arrow O is the detection direction of the optical image interval between the optical images 115a and 115b, and the arrow P is the detection direction of the optical image interval between the optical images 115c and 115d.

  As a means for detecting the phase difference, a one-dimensional image signal corresponding to the optical image is formed from the output signal of each sensor region line sensor, and the phase difference is detected using correlation calculation. Since the relationship between the phase difference and the defocus amount of the photographing optical system 101 can be approximated by a predetermined function, focus adjustment can be performed by detecting this phase difference.

  By the way, in the secondary imaging lens 110, the lens portion 110e on the incident side is a substantially spherical concave surface, and the center of the sphere is in the vicinity of the primary imaging surface 105 of the focus detection optical system. The exit side lens portions 110a and 110b are convex spherical surfaces, and the centers of the spheres are near the centers of the openings 109a and 109b, respectively. Therefore, the light beam that is emitted from the center of the primary imaging surface 105 of the focus detection optical system and passes through the centers of the aperture openings 109a and 109b is hardly refracted by the secondary imaging lens 110.

  On the other hand, the exit-side lenses 110c and 110d are convex curved surfaces and function to correct field curvature symmetrically. Also at this time, the light beam that is emitted from the center of the primary imaging surface 105 of the focus detection optical system and passes through the centers of the aperture openings 109c and 109d is configured not to be refracted by the secondary imaging lens 110.

  In FIG. 1, light rays La and Lb are the light rays corresponding to the lens portions 110a and 110b, and are hardly refracted by the secondary imaging lens 110. Although only the lens portions 110a and 110b are illustrated in FIG. 1, the same applies to the lens portions 110c and 110d. Accordingly, even if the wavelength changes, the light is not refracted in the same manner, so that highly accurate focus detection without wavelength dependency can be performed.

  As described above, since a larger field curvature occurs as the focus detection baseline length is longer as described in the conventional example, curved surfaces for correcting the field curvature are provided in the corresponding lens units 110c and 110d, and the focus detection baseline length is increased. The lens portions 110a and 110b corresponding to the shorter one are a secondary imaging optical system based on a spherical system as in the conventional example.

  FIG. 6 is a diagram in which each sensor area is back-projected onto the primary imaging plane 102 and shows a focus detection area on the primary imaging plane 102. Although the sub-mirror 104 is actually a curved surface, fan-shaped distortion occurs. In the figure, this distortion is ignored and represented by a rectangle.

  In the figure, a region 113 indicated by a diagonal line rising to the right is a focus detection region corresponding to the sensor regions 112a and 112b, and a horizontal line can be detected in this region. On the other hand, a region 114 indicated by a diagonal line rising to the left performs vertical line detection in a focus detection region corresponding to the sensor regions 112c and 112d. The common area of the focus detection areas 113 and 114 can simultaneously perform vertical line detection and horizontal line detection, and becomes a cross detection area.

  The above is the configuration of the focus detection apparatus in the present invention. As described above, the lens portions 110c and 110d are formed of curved surfaces that correct the curvature of field. The luminous flux that passes through the openings 109c and 109d of the stop 8 corresponding to the lens portions 110c and 110d has a larger inclination angle with respect to the optical axis L than the luminous flux that passes through the apertures 109a and 109b. A large curvature of field occurs.

  The horizontal direction, which is the optical movement direction, becomes smaller due to the influence of the curvature of field, and the vertical line detection range is limited to the vicinity of the central portion compared to the horizontal line detection range. The reason for this will be described below. .

  The optical images formed on the sensor regions 11-3 and 11-4 in FIG. 15 are secondary images of the light beams that have passed through the pair of openings 8-3 and 8-4 having a longer base length in FIG. It is obtained by re-imaging with lenses 9-3 and 9-4. For example, when considering an optical image formed by the aperture 8-3 and the secondary imaging lens 9-3, this secondary imaging optical system looks at the primary imaging surface 5 of the focus detection optical system from an oblique direction. An optical image is formed. Therefore, this optical image has a large curvature of field, particularly in the horizontal direction, which is the optical image moving direction accompanying the defocusing of the taking lens. In addition, this curvature of field has asymmetry in the horizontal direction with respect to the center of the optical image.

  FIG. 17A is a diagram for explaining this. For example, a schematic diagram in a cross section perpendicular to the paper surface including the optical axis L ′ of the paper surface, in which the primary image forming surface 5 and subsequent surfaces of the focus detection system in FIG. FIG. Note that. Except for the main parts necessary for the description, for the sake of simplicity, the description will be made ignoring the light refraction at the secondary imaging lens.

  In the figure, optical axes L3 and L4 are optical axes of the secondary imaging lenses 9-3 and 9-4 parallel to the optical axis L '. Similarly, paying attention to the optical image formed by the aperture 8-3 and the secondary imaging lens 9-3, first, the size is centered on the optical axis L3 on the primary imaging surface 5 of the focus detection optical system. When there is an image of Oa, an optical image having a size of Ia is formed on the focus detection sensor. Then, when the image forming points of the optical image are connected, a parabolic curve symmetric about the optical axis L3 is formed as shown by the curve 12, and this is a field curvature.

  In general, if spherical aberration, coma, and astigmatism are completely removed from an optical system, points are imaged as points both on-axis and off-axis, but each point is not a plane but a curved surface. It's above. This curved surface is called a Petzval surface, and a curve 12 in FIG. 17 is obtained by observing the Petzval surface on a certain cross section.

  Next, in practice, an image of size 0b exists on the primary imaging surface 5 of the focus detection optical system with the optical axis L ′ as the center. Since the secondary imaging lens 9-3 is viewed obliquely with respect to the 0b image, the formed optical image is Ib, and the field curvature is asymmetric with respect to the center of the optical image.

  FIG. 17B shows the field curvature of the pair of optical images formed by the pair of secondary imaging lenses 9-3 and 9-4. A curved line 13 shows the curvature of field of the optical image formed by the secondary imaging lens 9-4. The curved line 12 has a shape obtained by inverting the mirror about the optical axis L '.

  In phase difference focus detection, even if field curvature occurs, if the field curvature is symmetric about the center of the optical image in the optical image movement direction, the field curvature of the pair of optical images at an arbitrary image height Since they are almost the same, focus detection can be performed. However, in the state shown in FIG. 17B, for example, the point images of the paired secondary imaging system corresponding to the point image C on the primary imaging surface 5 are C-3 and C-4, respectively. The curvature of field is significantly different. That is, the field curvature of the pair of optical images at an arbitrary image height is different except at the center of the optical image.

  Therefore, in consideration of the influence of the curvature of field, the focus detection area cannot be expanded in the horizontal direction, which is the moving direction of the pair of optical images. Then, the focus detection area becomes a small area in the horizontal direction, which is the optical image moving direction, like the focus detection area 14 in FIG.

  In the cross section orthogonal to FIG. 17, the field curvature of the pair of optical images is approximately the same, so that the focus detection area can be enlarged in the vertical direction compared to the horizontal direction.

  Further, such asymmetric field curvature is remarkable when the primary imaging plane 5 is viewed more obliquely, that is, in the case of a paired secondary imaging optical system having a large parallax, and the secondary that does not have a large parallax. In the horizontal line detection, which is an imaging optical system, the focus detection area can be enlarged to some extent. However, in the vertical line detection, which is a secondary imaging optical system having a large parallax, the focus detection area in the horizontal direction can be enlarged. Realization is difficult.

  Therefore, in this embodiment, the lenses 110c and 110d are provided with curved surfaces for correcting the field curvature, while the lens portions 110a and 110b are not provided with the curved surfaces for correcting the field curvature. Details of the lens portions 110a and 110b and the lens portions 110c and 110d will be described below.

  First, the lens units 110a and 110b that are not corrected for field curvature will be described. FIG. 7 is a straight developed view of the optical path of the focus detection optical system in the cross-sectional view of FIG. It is.

  In the figure, a convex lens 201 is obtained by replacing the sub mirror 104 in FIG. 1 as an optical member during straight development. Since the sub mirror 104 is formed of an ellipsoid having a condensing power, it can be replaced with a convex lens. The primary imaging surface 105 of the focus detection optical system is slightly smaller than the primary imaging surface 102, but depends on the imaging optical system 101 side. This is because the sub mirror 104, that is, the convex lens 201 has a positive optical power. When the light beam that forms an image on the primary imaging surface 102 actually passes through the convex lens 201, the primary imaging surface of the focus detection optical system. An image is formed on 105.

  The light beam 202 emitted from the stop center point E forms an image at a point B on the exit pupil 101a. This is apparent when the focal points of the ellipsoidal submirror 104 in FIG. Accordingly, since the openings 109a and 109b of the stop 109 are projected onto the exit pupil 101a, a pair of light beams obtained by dividing the exit pupil 101a are guided to the secondary imaging optical system.

  The light beams 204 and 205 form an image at the center of the primary image forming surface 105, pass through the center points and both end points of the apertures 109 a and 109 b of the stop 109, and form an image on the sensor chip 112. Indicates. Actually, optical images having the sizes shown in the optical images 115a and 115b in FIG. 5 are formed, and field curvature occurs in the optical images for the reason described in the conventional example. At this time, the pair of optical images moves in the direction of arrow V in accordance with the defocusing of the photographing optical system 101. This arrow V corresponds to the arrow O in FIG. 5, and the phase difference type focus detection is performed by setting the direction of the arrow V as the detection direction of the optical image interval.

  FIG. 8A is a diagram showing the field curvature of a pair of optical images, that is, a curve connecting the imaging points near the sensor chip 112 at each image height of the cross section. Except for the aperture 109 and the secondary imaging lens 110, FIG. Omitted. In the drawing, a curved line 206a indicated by a dotted line indicates the field curvature of an optical image formed by the lens unit 110a, and a curved line 206b indicates the lens part 110b. Since the secondary imaging optical system using the lens units 110a and 110b has a parallax of the primary imaging surface 105 of the focus detection optical system with an inclination angle θ, the field curvatures 206a and 206b are the center of each optical image. The shape is asymmetric with respect to 207a and 207b. Here, the inclination angle θ is a representative inclination angle formed between the optical axis L and a principal ray passing through the center of the aperture opening at an image height of 0 on the primary imaging surface 105 of the focus detection optical system.

  However, since the inclination angle θ is small, that is, the distance between the aperture openings 109a and 109b is small, the asymmetry of the field curvature is slight. Further, in this cross section, the size of the optical image necessary for focus detection is perpendicular to the focus detection region 113 in FIG. 6 and the short direction, so that the curvature of field is better than the longitudinal direction.

  On the other hand, the field curvature of the optical image corresponding to the long side is shown in FIG. FIG. 8B is a cross section orthogonal to FIG. 8A and including the optical image center 207a and the center of the aperture opening 109a. A curved line 208a indicated by a dotted line is a field curvature of an optical image formed by the lens unit 110a. The lens unit 110b side is the same as the lens unit 110a in this cross section, and a description thereof will be omitted.

  In this cross section, the secondary imaging optical system is symmetrical with respect to the field curvature 208a and the optical image center 207a because the primary imaging plane 105 of the focus detection optical system is viewed at an inclination angle of 0. Therefore, as shown in FIG. 6, the horizontal direction of the focus detection area 113 can be made larger than the vertical direction. In addition, since the photographing screen is generally a horizontally long rectangle, it is preferable in terms of convenience to set the focus detection area having the horizontal direction as the longitudinal direction. That is, since the asymmetric field curvature of the secondary imaging lens in the phase difference type focus detection occurs in the detection direction V that is the optical image interval movement direction, the longitudinal direction of the optical image corresponding to the longitudinal direction of the focus detection region is changed. By configuring so as to be orthogonal to the detection direction V, it is possible to eliminate the influence of field curvature as much as possible even on an optical surface without correction.

  Next, the lens units 110c and 110d having curved surfaces for correcting field curvature will be described. FIG. 9 is a diagram in which the focus detection optical system optical path is straightly developed in a cross section orthogonal to the cross section of FIG. 1, and is a diagram for mainly explaining the light beams passing through the lens portions 110c and 110d, and the lens portions 110a and 110b. Is omitted.

  In FIG. 9, the light beam 203 emitted from the stop center point E is imaged at a point B on the exit pupil 101a as in FIG. 7, and the exit pupil 101a is divided by the openings 109c and 109d of the stop 109. The light beam is guided to the secondary imaging optical system.

  The light beams 208 and 209 form an image on the center of the primary imaging surface 102 and pass through the center points and both end points of the pair of openings 109c and 109d of the stop 109 and form an image on the sensor chip 112. Indicates. Actually, optical images having the sizes indicated by the optical images 115c and 115d in FIG. 5 are formed, and field curvature occurs in the optical images. At this time, the pair of optical images moves in the direction of arrow W in accordance with the defocusing of the photographing optical system 101. This arrow W corresponds to the arrow P in FIG. 5, and the phase difference type focus detection is performed by setting the arrow W direction as the detection direction of the optical image interval. The lenses 110c and 110d are provided with curved surfaces that make the field curvature of these optical images substantially symmetric. This curved surface will be described below.

  FIG. 10A is a diagram showing the field curvature of a pair of optical images, that is, a curve connecting the imaging points near the sensor chip 112 at each image height of the cross section. Except for the aperture 109 and the secondary imaging lens 110, FIG. Omitted. In the drawing, a curved line 206c indicated by a dotted line indicates the field curvature of an optical image formed by the lens unit 110c, and a curved line 206d indicates the lens unit 110d. The secondary imaging optical system by the lens portions 110a and 110b has a parallax due to an inclination angle larger than the inclination angle θ in FIG. 8A, which is the inclination angle θ ′ of the primary imaging surface 105 of the focus detection optical system. Yes. The field curvatures 206c and 206c are substantially symmetrical with respect to the optical image centers 207c and 207d.

  A curved line 211c indicates a curvature of field when a normal spherical system that does not perform a field curvature correction is used for the lens unit 110c. It can be seen that the two field curvatures 206c and 211c are corrected so as to be symmetrical.

  At this time, the lens portion 110c is a part of an ellipse indicated by a dotted line 210 in this cross section. This ellipse 210 is a so-called off-axis ellipse arranged with an axis F as a major axis and an axis G as a minor axis, and is inclined with respect to the optical axis L so that the light beam passes through the off-axis region. If the intersection of the major axis F and the ellipse is H, and the intersection of the minor axis G and the ellipse is I, the radius of curvature is the smallest at the point H due to the properties of the ellipse, and gradually increases from the point H to the point I. It becomes the largest at point I.

  That is, in the effective portion through which the light beam of the lens unit 101a passes, the radius of curvature changes in one direction from small to large in the direction of enlargement of the optical image interval in the detection direction W, and the radius of curvature changes without an extreme value. It has become. For non-off-axis shapes other than the circles (spheres) used in normal imaging optics, ellipses, parabolas, hyperbolic curves, and other non-circular (sphere) shapes, the extreme value of the radius of curvature change is always in the effective part where the light passes. Exists. For example, in FIG. 10A, assuming that the portion including the point H of the ellipse 210 is an effective portion, the radius of curvature changes from large to small to large, and the point H becomes an extreme value of the amount of change in radius of curvature ( Check the meaning of the description).

  Therefore, if the lower end point is J and the upper end point is K on the field curvature 206c, the light beam focused on the point J passes through the point H side between the points H and I of the ellipse 210, while the point K The light beam that forms an image passes through the point I side. Then, the light beam that forms an image at the point J has a small radius of curvature, so that the light beam is refracted more. On the other hand, the light beam that forms an image at the point K has a large curvature radius, so the light beam is not refracted much. Then, if the imaging position at the optical image center 207c is made to coincide with the field curvatures 206c and 211c, the point J is imaged more forward and the point K is imaged further rearward as shown in FIG. Therefore, it is possible to correct the field curvature tilted as a whole to be symmetrical with respect to the optical image center 207c.

  In this cross section, the size of the optical image necessary for focus detection is the horizontal direction of the focus detection region 114 in FIG. 6 and becomes the longitudinal direction. However, since the field curvature is corrected well, the focus detection region can be arbitrarily set. Can be enlarged. In addition, the lens unit 101d has a shape that is mirror-reversed with respect to the lens unit 101c with the optical axis L as the center line, and the effects relating to the field curvature correction also coincide with each other in the mirror-reversed state.

  Next, a cross section orthogonal to FIG. FIG. 10B is a cross section orthogonal to FIG. 10A and including the optical image center 207c and the center of the aperture opening 109c.

  The lens portion 110c is a spheroid formed by rotating the ellipse 210 in FIG. 10A around the axis F, and the range through which the light passes is a so-called off-axis spheroid. A curved line 208c indicated by a dotted line is a field curvature of the optical image formed by the lens unit 110c. The lens unit 110d side is the same as the lens unit 110c in this cross section, and a description thereof will be omitted.

  Even in this cross section, the secondary imaging optical system sees the primary imaging surface 105 of the focus detection optical system at an inclination angle of zero. The cross-sectional shape of the lens portion 110c is a shape obtained by obliquely cutting the spheroidal surface, and becomes a part of an ellipse having a vertex at the intersection with the optical axis L as indicated by 212. Accordingly, the field curvature 208c is symmetrical with respect to the optical image center 209c.

  Since the lens unit 110c is a spheroidal surface having the axis F as a rotation axis, when the lens unit 110c is cut along a plane including the axis F, the cross-sectional shape at that time becomes an ellipse 213 having an apex at the intersection with the optical axis L. And the same shape. On the other hand, when cut along the plane including the axis I, the cross-sectional shape becomes a circle 214. And the shape cut | disconnected by the intermediate surface turns into the ellipse 212, and the shape cut | disconnected by the surface which passes the center of the ellipse 210 between the points I and H changes gradually from the circle 214 to the ellipse 213.

  At this time, when the shape near the vertex of the ellipse through which the light beam passes is approximated by a circle and the curvature radius at that time is considered, also in the cross section of FIG. It can be seen that the radius is large. Since this characteristic coincides with the cross section of FIG. 10A, even if the curvature of field over the three-dimensional space of the optical image formed by the lens portion 110c is considered, it is symmetrically centered on the optical image center 207c and is corrected well. ing.

  The center of this elliptical surface is almost in the vicinity of the center of the opening 109c as in the conventional example, and is configured to prevent the occurrence of chromatic aberration by minimizing the refraction of the chief ray. Although known in spherical systems, the chromatic aberration at the center of the optical image is almost completely removed by finally decentering the opening 109c and the ellipsoid in the vertical direction in FIG.

  By the way, because the off-axis elliptical surface is actually used, the shape changes in various cross-sections, and each point image of the optical image has a different spot shape depending on the position and is not axially symmetric. Considering only these two cross sections, the approximate state can be known. In particular, in the phase difference type focus detection, it is only necessary that the field curvature in the cross section of FIG. 10A, which is the detection direction W of the optical image interval, is symmetrical, so that a sufficiently wide range of focus detection can be realized.

  As described above, in the present embodiment, simultaneous detection of vertical line and horizontal line detection of phase difference focus detection can be realized with high accuracy in a wide range within the imaging range. For this purpose, first, the lens units 110a and 110b, which are secondary imaging optical systems corresponding to the shorter focus detection baseline length, are known optical imaging systems using spherical surfaces, and the lens units corresponding to the longer focus detection baseline length. For 110c and 110d, curved surfaces for correcting curvature of field in the optical image moving direction were used.

  Next, the lens units 110a and 110b that do not correct the field curvature are orthogonal to the detection direction of the optical image interval and the longitudinal direction of the optical image, and the lens units 110c and 110d that have a curved surface that corrects the field curvature are optical images. The detection direction of the interval was matched with the longitudinal direction of the optical image. Therefore, by providing the minimum necessary curved surface, the secondary imaging optical system has a configuration in which the curvature of field does not occur as much as possible, and it is possible to realize a highly accurate phase-difference focus detection over a wide range. Although it is possible to correct the curvature of field better if curved surfaces are provided on all of the exit side lenses 110a to 110d, since this curved surface is an off-axis optical system compared to a spherical system, it is relatively difficult to manufacture. Therefore, the manufacturing yield can be improved by using the minimum necessary configuration.

  In this embodiment, an elliptical surface is used as a curved surface, but the present invention is not limited to this, and a paraboloid, a hyperboloid, or the like may be considered as a surface having the same function, or an aspherical surface may be used. These surfaces are all represented by a well-known aspherical expression with a conic coefficient, and conversely, in this embodiment, one aspherical surface with a conic coefficient is used. Then, the radius of curvature of the effective portion through which the light beam passes at least in the detection direction of the optical image interval may be off-axis (tilted) so as to change from small to large in the direction of expansion of the optical image interval. Further, with respect to the inclination angle of the ellipse surface with respect to the optical axis L and the major / minor axis ratio of the ellipse, the configurations shown in FIGS. 10A and 10B are merely examples, and the baseline length of the focus detection optical system and the required one It depends on the optical image performance, lens material, etc., and is optimized under each condition.

  The second embodiment is an example in which the incident-side surface of the secondary imaging lens 110 is divided corresponding to each lens portion on the exit side in a modification of the first embodiment. By doing so, not only the field curvature but also the spot shape of the point image can be improved.

  FIG. 11 is a plan view of the secondary imaging lens 110 viewed from the stop 109 side, as in FIG. In the drawing, the incident surface is divided into three regions 301ab, 301c, and 301d, and the lens portion 301ab corresponds to the lens portions 110a and 110b on the exit side, and is a single spherical surface made of a concave surface. The lens portion 301c corresponds to the lens portion 110c, and the lens portion 301d corresponds to the lens 110d, and is also a spherical surface made of a concave surface.

  In the first embodiment, since the incident surface is only one optical surface, the shape of the incident surface has to be determined by balancing the lens portion 110a (110b) and the lens portion 110c (110d). However, since the incident surface of the lens unit 110 is close to the stop 109, the four light beams corresponding to the four lens units are sufficiently separated from each other, and the optical surface on the incident side for each light beam. Can be set. Therefore, in this embodiment, the incident surface is divided into three, and different optical surfaces on the incident side are provided in the lens portion 110a (110b) and the lens portion 110c (110d). Each optical image not only has a curvature of field, but also increases the degree of freedom of the optical design of the entrance surface, so that the spot image spot performance can be improved, and the phase difference between the vertical and horizontal lines can be improved with higher accuracy. The system focus detection can be realized in a wide range within the imaging region.

  The third embodiment is an example in which free curved surfaces are used for the lens portions 110a and 110b as a modification of the first embodiment.

  FIG. 12 is a diagram for explaining the free-form surface, and is an enlarged view of the vicinity of the lens portion 110c in FIG. The shape of the lens portion 110c in this cross section is an ellipse 210 as in the first embodiment. The free-form surface is created based on this cross section.

  First, in the part where the light beam of the ellipse 210 passes, the light beam emitted from the center of the aperture opening 109c, that is, the center of the ellipse 210 is drawn at an appropriate pitch angle. Light rays 401 to 405 in the figure correspond to this, and since this is a principal ray, it is hardly refracted on the exit surface. The light beam 403 is a principal light beam having an image height of zero.

  Next, the imaging points of the light rays including these principal rays are obtained. In the figure, points Q, R, S, T, and U correspond to it.

  Next, considering a plane that is orthogonal to the cross section of FIG. 12 and that includes the light ray 401, a circle in which a light ray passing through the plane forms an image at a point Q and a circle that intersects the ellipse 210 are obtained. The arrow R1 in the figure corresponds to the radius of this circle. Such a circle is obtained by a cross section including each principal ray. R2, R3, R4, and R5 are the calculated circle radii. Since the distance from the lens unit 110c to each imaging point is different, the respective radii also increase from R1 to R5.

  Finally, the desired free-form surface can be obtained by smoothly connecting these circles. When actually calculating the free-form surface, the free-form surface can be made sufficiently smooth as an optical surface by finely setting the pitch angle.

  Therefore, according to the present embodiment, the spot image can be corrected to an ideal state because the imaging point is corrected in both the cross section that is the detection direction of the optical image interval and the cross section that is orthogonal thereto. Therefore, a wide range of highly accurate phase difference type focus detection can be realized.

  In this embodiment, the cross-sectional shape orthogonal to FIG. 12 is based on a circle. However, the present invention is not limited to this, and other aspherical surfaces may be used. That's fine.

  As described above in Examples 1, 2, and 3, the secondary imaging optical system is provided with a spheroid and a free-form surface, so that the field curvature of the secondary imaging optical image in the pupil division direction is excellent. It can be corrected to. Therefore, it is possible to detect the vertical line and the horizontal line with high accuracy over a wide range within the imaging range.

It is a center sectional view of a camera. It is a top view of an aperture stop. It is a top view of a secondary image formation lens. It is a top view of a sensor chip. 2 shows an optical image formed on a sensor chip. It is the figure which back-projected the sensor area | region on the primary image formation surface. FIG. 2 is a straight development of the focus detection optical system optical path of FIG. 1. (A) is a figure which shows the field curvature of a pair of optical image in FIG. FIG. 9B is a diagram illustrating field curvature of a pair of optical images in a cross section orthogonal to FIG. FIG. 2 is a straight development of a focus detection optical system optical path in a cross section orthogonal to FIG. 1. (A) is a figure which shows the field curvature of a pair of optical image in FIG. FIG. 10B is a diagram illustrating field curvature of a pair of optical images in a cross section orthogonal to FIG. It is a top view of the secondary image formation lens in a 2nd Example. It is the figure which expanded the lens part 110c vicinity. It is the center cross section of the camera in a prior art example. It is a top view of the aperture_diaphragm | restriction and a secondary image formation lens in a prior art example. It is a top view of the sensor chip in a prior art example. It is the figure which back-projected the sensor area | region in a prior art example to a primary image formation surface. (A) is a figure which shows the curvature of field of the optical image of a pair in a prior art example. FIG. 18B is a diagram showing field curvature of a pair of optical images in a cross section orthogonal to FIG.

Explanation of symbols

101 imaging optical system 101a exit pupil 102 primary imaging surface 104 submirror 105 primary imaging surface of focus detection optical system 109 stop 110 secondary imaging lens 112 sensor chip 210 ellipse

Claims (7)

Pupil splitting means for splitting an exit pupil of a photographing optical system to form a pair of light beams; a pair of secondary imaging lenses for re-imaging the pair of light beams to form a pair of optical images; and the photographing optics In a focus detection apparatus for adjusting the focus of the photographing optical system by detecting the pair of optical image intervals associated with system defocusing,
In at least one optical surface of the pair of secondary imaging lenses, the radius of curvature in the detection direction of the optical image interval of the pair is extremely small from the small to the large in the increasing direction of the optical image interval of the pair in the detection direction. A focus detection device characterized by a curved surface that changes without value.   In the optical surface, a curved surface in which the radius of curvature changes from small to large in an extreme direction toward the enlargement direction of the pair of optical image intervals in the detection direction in an effective portion through which light passes to form the optical image. The focus detection apparatus according to claim 1, wherein Pupil dividing means for dividing the exit pupil of the photographing optical system in at least two directions to form a pair of luminous fluxes; a pair of secondary imaging lenses for re-imaging the paired luminous fluxes to form a pair of optical images; In the focus detection device for adjusting the focus of the photographing optical system by detecting the optical image interval of the pair accompanying the defocusing of the photographing optical system,
The focus detection baseline lengths in the two directions of the pair of light beams divided by the pupil splitting unit are different, and at least one of the pair of secondary imaging lenses corresponding to the pair of light beams having the longer focus detection baseline length. The two optical surfaces have a radius of curvature at an effective portion through which a light beam passes in order to form the optical image in the detection direction of the pair of optical image intervals in a direction in which the optical image interval of the pair in the detection direction increases. A focus detection device characterized by a curved surface that changes from small to large without an extreme value. Pupil splitting means for splitting an exit pupil of a photographing optical system to form a pair of light beams; a pair of secondary imaging lenses for re-imaging the pair of light beams to form a pair of optical images; and the photographing optics In a focus detection apparatus that detects the optical image interval of the pair accompanying system defocusing and performs focus adjustment of the imaging optical system,
The pair of optical images has a shape in which one direction is a longitudinal direction, and at least one optical surface of the pair of secondary imaging lenses forms the optical image in the detection direction of the pair of optical image intervals. The longitudinal direction and the detection direction coincide with each other in a curved surface in which the radius of curvature at the effective portion through which the light passes changes from the small to the large in the detection direction in the direction of enlargement of the pair of optical image intervals. A focus detection device. In the claims 1-4, the focus detection device, characterized in that the curved surface is provided on the optical surface of the exit side of the secondary imaging lenses of the said pair. In the claims 1-5, the focus detection device, wherein the curved surface is part of a spheroid. The focus detection device according to any one of claims 1 to 6 , comprising:
An imaging apparatus comprising imaging means for receiving subject light through a lens whose position is adjusted according to a result of the focus detection apparatus.

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