Focus detection device and image pick-up device

A focus detection device includes a micro lens array having a plurality of micro lenses, a light receiving element array having a plurality of light receiving elements for each micro lens and that receives light rays from a plurality of partial areas in which pupils of an imaging optical system are different from each other, in a plurality of light receiving elements respectively through each micro lens and a focus detection calculation circuit. The device generates at least three signal strings respectively corresponding to images of light rays which have been transmitted through at least three of the partial areas, based on signals output from the plurality of light receiving elements of the light receiving element array. The device obtains, from the at least three signal strings, shift amounts of two signal strings corresponding to two partial areas, and detects a focus adjustment state of an imaging optical system based on the obtained plurality of shifts amounts.

BACKGROUND

This invention relates to a focus detection device and an image pick-up device.

A focus detection device is known in which a pupil of a shooting optical system is divided by a plurality of pairs of re-imaging lenses whose intervals (between-axis distance) of apertures of pairs of re-imaging lenses are different, relative image shift amounts are respectively calculated for a plurality of pairs of images formed by light beams whose pupils have been divided, and a true defocus amount is obtained by eliminating a false focus from a plurality of defocus amounts. See, for example, Japanese Patent No. 2910102.

SUMMARY

However, focus detection accuracy is not sufficient in the above-mentioned conventional focus detection device.

A focus detection device according to one aspect of the invention includes a micro lens array in which a plurality of micro lenses are arranged. The device also includes a light receiving element array which has a plurality of light receiving elements for each of the micro lenses and receives, by the respective plurality of light receiving elements via the respective micro lenses, a light beam from a plurality of partial areas, in which pupils of an imaging optical system are different from each other. The device also includes a signal string generator which generates at least three signal strings respectively corresponding to images of light beams which have been transmitted through at least three of the partial areas, based on signals output from the plurality of light receiving elements of the light receiving element array. The device also includes a focus detector which obtains, within the at least three signal strings, shift amounts of two of the signal strings corresponding to two of the partial areas, for a plurality of groups of the partial areas, and detects a focus adjustment state of the imaging optical system based on the obtained plurality of shift amounts.

In accordance with some embodiments, the two signal strings are signal strings corresponding to partial areas adjacent to each other within the plurality of partial areas.

In accordance with some embodiments, the plurality of shift amounts include a shift amount between a first signal string corresponding to a first partial area within the plurality of partial areas and a second signal string corresponding to a second partial area different from the first partial area, and a shift amount between the first signal string and a third signal string corresponding to a third partial area different from the first and second partial areas.

In accordance with some embodiments, the focus detector calculates the focus adjustment state by weighted addition of the respective ones of the plurality of shift amounts.

In accordance with some embodiments, the focus detector multiplies and adds separate conversion coefficients of the shift amounts for two of the respective signal strings corresponding to the two partial areas, and calculates a defocus amount of the imaging optical system.

In accordance with some embodiments, the conversion coefficients are set according to the position of the light receiving elements of the light receiving element array.

In accordance with some embodiments, the signal string generator generates a plurality of signal strings based on light beams which have been transmitted through at least three partial areas included in an area corresponding to an aperture stop in which the imaging optical system is restricted.

In accordance with some embodiments, the signal string generator generates the plurality of signal strings by adding outputs of a plurality of light receiving elements within the plurality of light receiving elements of the light receiving element array.

In accordance with some embodiments, the plurality of light receiving elements are one-dimensionally arranged.

In accordance with some embodiments, the plurality of light receiving elements are two-dimensionally arranged.

According to some embodiments, an image pick-up device can include the focus detection device.

According to aspects of the invention, an accurate defocus amount can be detected by eliminating a false focus with respect an object having a cyclic pattern.

DETAILED DESCRIPTION OF EMBODIMENTS

An embodiment is explained, in which the invention is applied to a single lens reflex digital camera. The invention is not limited to a single lens reflex digital camera, but also can be applied to all types of optical devices (for example, a field scope, etc.) which perform focus adjustment of an imaging optical system.

FIG. 1is a cross-sectional schematic view showing a structure of a digital single lens reflex camera provided with a focus detection device of an embodiment of the invention. Drawings and descriptions of general equipment and devices of a camera, other than equipment and devices related to a focus detection device and an image pick-up device of this invention, are omitted from the drawings to simplify the description. With respect to a camera of an embodiment, a lens barrel20is mounted to a camera body1, and the lens barrel20can be replaced with all types of shooting lenses. Furthermore, in this embodiment, an explanation is given by using an example of replaceable lens type camera; however, the invention is not limited to a replaceable lens type camera, but also can be applied to a fixed lens type camera.

The camera body1is provided with an image pick-up element2, a shutter3, a focus detection optical system4, a focus detection sensor5, a focus detection calculation circuit6, a camera control circuit7, a drive circuit8, a quick return mirror9, a sub mirror10, a viewfinder screen11, a transparent type liquid crystal display12, a pentaprism13, a light measurement lens14, a light measurement sensor15, an eyepiece16, an operating member17, etc.

The image pick-up element2can include a CCD, a CMOS, etc. and converts an object image, imaged onto a surface of the pick-up element2by an image pick-up lens23within the lens barrel20, into an electrical signal and outputs the electrical signal. The shutter3is released only for a shutter time that is based on an exposure calculation result or manually set by an operator, when a shutter button (undepicted) is completely pressed (when a shutter is released), and the image pick-up element2is exposed. The focus detection optical system4, the focus detection sensor5, and the focus detection calculation circuit6constitute a phase difference detection type focus detection device that detects a defocus amount showing a focus adjustment state of the shooting lens (imaging optical system)23. Details of the focus detection device will be described hereafter.

The camera control circuit7includes undepicted peripheral parts such as a microcomputer and a memory and performs sequential control of light measurement, focus detection, shooting and the like, and calculation control such as exposure calculation. The drive circuit8drivingly controls a lens and stop driving actuator25arranged within the lens barrel20. The light measurement sensor15outputs a light measurement signal corresponding to brightness of the respective areas by dividing a shooting screen into a plurality of areas.

The lens barrel20is provided with a zooming lens21, a focusing lens22, a stop (or diaphragm)24, the lens and stop driving actuator25, a lens memory26, etc. InFIG. 1, the zooming lens21and the focusing lens22are collectively shown as one shooting lens23; however, separate optical elements can be provided. The zooming lens21is driven by the actuator25in an optical axis direction and is a lens which changes a focal length of the shooting lens23. Furthermore, the focusing lens22is driven by the actuator25in an optical axis direction and is a lens which performs focus adjustment of the shooting lens23. The stop24is driven by the actuator25and changes an aperture stop diameter. In the lens memory26, information is stored concerning the lens barrel20and the shooting lens23, such as an aperture F value of the shooting lens23, a pupil length PO, and a focal length.

The operating member17, which is operated by an operator, can include a plurality of user-operatable buttons, switches, etc, arranged on the camera body1and the lens barrel20. The operating member17includes a release half-press switch which is turned on when the shutter button is halfway pressed, a release full-press switch which is turned on when the shutter button is completely pressed, etc.

Other than at the shooting time, the quick return mirror9and the sub mirror10are placed within a shooting optical path as shown inFIG. 1. At this time, part of the light from the object which has been transmitted through the shooting lens23is reflected by the quick return mirror9and is guided to the viewfinder screen11. An object image is imaged onto the screen11. The transparent type liquid crystal display12overlaps and displays a focus detection area mark on an object image of the screen11and also displays, outside of the object, image information concerning shooting, such as a shutter speed, a stop value, number of shooting frames, and the like.

An object image on the screen11is guided to the eyes of an operator through the pentaprism13and the eyepiece lens16and also is guided to the light measurement sensor15via the pentaprism13and the light measurement lens14. The camera control circuit7performs exposure calculation based on a light measurement signal for each light measurement area to be output from the light measurement sensor15and calculates a shutter speed and a stop value corresponding to brightness of the subject field. Additionally, when a manual exposure shooting mode is set, a set shutter speed and stop value are used that are set by an operator by operating the operating member17.

Meanwhile, another part of the light from the object which has been transmitted through the shooting lens23is transmitted through the quick return mirror9, is reflected by the sub mirror10, and is guided to the focus detection sensor5through the focus detection optical system4. In this embodiment, focus detection areas are set in a plurality of positions within a shooting screen. The focus detection sensor5outputs a focus detection signal representing a focus adjustment state of the shooting lens23for each focus detection area. The focus detection calculation circuit6calculates a defocus amount representing a focus adjustment state of the shooting lens23, based on the focus detection signal for each focus detection area. The camera control circuit7calculates a lens drive amount based on the defocus amount, drives the actuator25by the drive circuit8, and drives the focusing lens22to a focused position.

During shooting, the quick return mirror9and the sub mirror10are shielded from the shooting optical path (the mirrors move up), light beams from the object which have been transmitted through the shooting lens23are guided while the shutter3is open, and an object image that is imaged on an image pick-up surface of the image pick-up element2is captured.

FIGS. 2 and 3are diagrams showing details of the focus detection optical system4and the focus detection sensor5. In the diagrams, the focus detection optical system4is a micro lens array in which a plurality of micro lenses41are two-dimensionally arranged. The micro lens array is arranged in a vicinity of a plane that matches a focal plane of the shooting lens23, that is, a plane conjugate to an image pick-up surface of the image pick-up element2.FIGS. 2 and 3show only a few (25) micro lenses; however, it is preferable that many more micro lenses are arranged at a pitch of 100 microns or less. Thus, if the micro lens array provides a width within a range of, for example, 5 mm square, the number of micro lenses becomes extremely large.

The focus detection sensor5is a light receiving element array51in which a plurality of light receiving elements (photoelectric conversion elements) are two-dimensionally arranged. The array51is arranged in the back of the focus detection optical system (micro lens array)4. Additionally, inFIG. 3, in order to simplify the explanation, the light receiving element array51includes, for each micro lens41in the micro lens array, an array of light receiving elements disposed in six rows and six columns, for a total of 36 light receiving elements for each of the micro lenses41, arranged in a square shape. However, the number of light receiving elements for each micro lens is not limited to the number of this embodiment. Additionally, instead of arranging a distinct group of light receiving elements for each micro lens, a single large group of light receiving elements, arranged as a two-dimensional light receiving element array, can span across all of the lenses41in the array.

Light rays (beams) from the object are transmitted through the quick return mirror9, are reflected by the sub mirror10, and are guided to the focus detection sensor (light receiving element array)5via the focus detection optical system (micro lens array)4.

FIG. 4is a diagram showing a relationship between the plurality of light receiving element arrays51of the focus detection sensor5and partial pupils A-F on an exit pupil23aof the shooting lens23. The respective light receiving element arrays51under each micro lens41are shot onto the partial pupils A-F on the exit pupil23aby each micro lens41. The light rays which have been transmitted through the respective partial pupils A-F on the exit pupil23aare guided to each light receiving element array51through the respective micro lenses41, and each light receiving element array51photoelectrically converts its received light.

InFIG. 4, in order to reduce a frequency of generating a false focus by a cyclic pattern, an image shift amount should be reduced. Therefore, a detection aperture angle needs to be made small, but as the detection aperture angle is made smaller, detection accuracy of a defocus amount deteriorates. Thus, in this embodiment, an entire detection aperture angle θTot is divided according to the partial pupils A-F, an image shift amount is obtained and totaled for each detection aperture angle θdiv, which results from the entire detection aperture angle θTot having been divided into small angles. The total image shift amount corresponding to the entire detection aperture angle θTot, that is, partial pupils A and F at both ends, is converted to a defocus amount. As described above, in a pupil division type phase difference detection type focus detection which performs pupil division by using a re-imaging lens, a detection aperture angle cannot be made sufficiently small. However, according to the pupil division type phase difference detection type focus detection of this embodiment, which performs pupil division by using a micro lens array, by reducing a detection aperture angle of adjacent division pupils, many divided pupils can be aligned. By so doing, a false focus cannot be easily generated with respect to an object having a cyclic pattern, and an accurate defocus amount can be detected. Detection accuracy of the defocus amount can be improved. Additionally, the respective image shifts in many divided-pupil pairs are obtained, a statistical average such as weighted addition is performed over many image shifts, and a defocus amount is obtained. Thus, various error elements are canceled, and detection accuracy is improved.

FIG. 5is a diagram showing a detailed structure of the focus detection calculation circuit6. The focus detection calculation circuit6is provided with an AID converter61, a memory62, a microcomputer63, etc. and includes a plural signal string generation portion64, an image shift calculation portion65, and a defocus amount calculation portion66, which are implemented by software of the microcomputer63. After the output of the plurality of light receiving element arrays51of the focus detection sensor5is sequentially read and converted to a digital signal by the A/D converter61, it is stored in the memory62.

The plural signal string generation portion64obtains pupil information (aperture F value, pupil distance PO, etc.) of the shooting lens23and the lens barrel20from the lens memory26via the camera control circuit7, obtains a stop control F value at the time of shooting from the camera control circuit7, specifies a range of partial pupils which can be used for calculation of an image shift amount based on the information, and generates signal strings with respect to the partial pupils.

FIG. 6is an enlarged view of the light receiving element array51under each micro lens41. According to this embodiment, in order to simplify the explanation, an example of the light receiving element array51(six rows by six columns) is used. As shown inFIG. 6, each light receiving element has an address for classification. Here, a signal string is generated by using only the third line showing light receiving elements (3.1)-(3.6) of each light receiving element array51. For example, output signals a(1), b(1), . . . , f(1) of the light receiving element array51which is at the bottom ofFIG. 4correspond to output signals of (3.1), (3.2), . . . , (3.6) of the third line shown inFIG. 6. In the same manner, the output signals of the other light receiving element arrays51correspond to output signals of light receiving elements (3.1), (3.2), . . . , (3.6) of the third line of the light receiving element array51.

Within the light receiving elements of the third line of each light receiving element array51, a signal string {a(i)} can be shown as follows in which only output signals of light receiving elements receiving light rays which have been transmitted through the partial pupil are aligned:
{a(i)}=a(1),a(2),a(3), . . . ,a(n)  (1).
This signal string {a(i)} shows that the output signals of the light receiving element (3.1) of each light receiving element array51are aligned in the order of the light receiving element array51. In the same manner, within the light receiving element of the third line of each light receiving element array51, a signal string which aligns only the output signals of the light receiving elements receiving light rays that have been transmitted through the respective partial pupils B, C, D, E, and F is defined as {b(i)}, {c(i)}, {d(i)}, {e(i)}, {f(i)}. According to the above-mentioned (1), these are shown. The signal string {b(i)} shows that the output signals of the light receiving element (3.2) of each light receiving element array51are sequentially aligned in the order of the light receiving element array51. The signal string {c(i)} shows that the output signals of the light receiving element (3.3) of each light receiving element array51are sequentially aligned in the order of the light receiving element array51. The signal string {d(i)} shows that the output signals of the light receiving element (3.4) of each light receiving element array51are sequentially aligned in the order of the light receiving element array51. Furthermore, the signal string {e(i)} shows that the output signals of the light receiving element (3.5) of each light receiving element array51are sequentially aligned in the order of the light receiving element array51. The signal string {f(i)} shows that the output signals of the light receiving element (3.6) of each light receiving element array51are sequentially aligned in the order of the light receiving element array51.

Furthermore, if a signal string is thus generated by using only the output signal of the light receiving elements of the third line of each light receiving element array51, it is not necessary to two-dimensionally arrange a plurality of light receiving elements under each micro lens. For example, as shown inFIG. 9, a one-dimensional light receiving element array51A can be used, which aligns six light receiving elements under each micro lens41a.

With respect to the image shift calculation portion65, within the signal string generated by the plural signal string generation portion64, the signal string in which the partial pupils are adjacent to each other is considered as one pair, and a correlation amount C (M) of each pair of signal strings is calculated by the following equation. For example, a correlation amount C (N)_ab of a pair of signal strings {a(i)} and {b(i)} corresponding to the adjacent partial pupils A and B is
C(N)—ab=Σ|a(i)−b(j)|  (2).
According to equation (2), N represents a shift amount which is j−i=N, and is a total sum of Σ, in which an upper limit is qL, and a lower limit is pL. An accurate shift amount can be obtained as follows based on the correlation amount C (N) which has been thus discretely obtained. Within the correlation amount C (N), CO is a correlation amount giving a minimum value when the shift amount is N, Cr is a correlation amount when a shift amount is (N−1), and Cf is a correlation amount when a shift amount is (N+1). An accurate shift amount based on the aligned three points, that is, a shift amount N x_ab can be determined by the following equation:
DL=0.5X(Cr−Cf),
E=MAX {Cf−C0,Cr−C0},
Nx—ab=N+DL/E(3).

In the same manner, based on the signal strings {b(i)} and {c(i)], {c(i)} and {d(i)}, {d(i)} and {e(i)}, {e(i)} and {f(i)}of adjacent partial pupils within the partial pupils B-F, shift amounts N x_bc, N x_cd, N x_de, N x_ef are calculated for each detection aperture angle θdiv divided by the partial pupils B-F (seeFIG. 4). InFIG. 4, the entire detection aperture angle θTot is a total sum of the divided detection aperture angles θdiv, so the total shift amount N x_Tot with respect to the entire detection aperture angle θTot (partial pupils A and F at both ends) is a total of the image shift amounts of the divided detection aperture angles θdiv. This can be obtained by the following equation:
N x_Tot=N x—ab+N x—bc+N x—cd+N x—de+N x—ef(4).
A correction amount (constant cst) corresponding to a position of the focus detection surface is added to the calculated total shift amount N x_Tot, and the total shift amount Δ n on the focus detection surface is obtained.
Δn=N x_Tot+cst  (5).

The defocus amount calculation portion66converts a total shift amount Δn into a defocus amount Df by using a constant Kf depending on the entire detection aperture angle θTot.
Df=Kf X Δn  (6).

Thus, according to this embodiment, based on the signal output from a plurality of light receiving elements of the light receiving element array51, a plurality of signal strings corresponding to the respective light fluxes which have been transmitted through at least three partial pupils (A-F) of the shooting lens23are generated. The shift amount of two signal strings within these signal strings is calculated, and based on the shift amount formed of these shift amounts, a defocus amount of the shooting lens23is detected. Thus, by eliminating a false focus with respect to an object having a cyclic pattern, an accurate defocus amount can be detected. In addition, a total shift amount is obtained by adding a plurality of shift amounts based on three signal strings or more corresponding to the light fluxes which have been transmitted through three partial pupils or more, so irregularity of the defocus amount decreases due to a statistical effect, and detection accuracy can be improved.

A modified example now will be described. In the above-mentioned embodiment, based on the signal output from a plurality of light receiving elements of the light receiving element array51, a plurality of signal strings corresponding to the respective light rays which have been transmitted through at least three partial pupils (A-F) of the shooting lens23are generated, a shift amount of two of the signal strings is calculated, and a defocus amount of the shooting lens23is detected based on the total of these shift amounts. However, the defocus amount of the shooting lens23also can be detected by summing up (averaging) these defocus amounts, after the respective shift amounts are converted to the defocus amounts by using a conversion coefficient.

For example, if the shift amounts of two signal strings are N x_ab, N x_bc, N x_cd, N x_de, N x_ef, and coefficients by which the shift amounts of the two signal strings are converted to the defocus amount are k ab, k bc, k cd, k de, k ef, the defocus amount D is obtained by the following equation.
D=(k ab×N x—ab+k bc×N x—bc+k cd×N x—cd+k de×N x—de+k ef×N x—ef)/5  (7).
In this case, there are five defocus amounts, so an average defocus amount is obtained by dividing a sum of the five defocus amounts by five. In general, the sum of the defocus amounts is divided by the number of the defocus amounts that were added together. Furthermore, if the shift amounts of the two signal strings are converted to the defocus amount, the defocus amount D also can be obtained by adding weights q ab, q bc, q cd, q de, q ef according to the positions of the light receiving elements which output the respective signal strings.
D=(q ab×k ab×N x—ab+q bc×k bc×N x—bc+q cd×k cd×N x—cd+q de×k de×N x—de+q ef×k ef×N x—ef)/{(q ab+q bc+q cd+q de+q ef)×5}  (8).

Another modified example will be described. In the above-mentioned embodiment, an example is shown in which the divided detection aperture angles θdiv are equal. However, even when the detection aperture angles θdiv are different from each other, by using an appropriate conversion coefficient for each detection aperture angle θdiv, an accurate defocus amount can be obtained according to the above-mentioned method. In other words, the invention is not limited to the case in which a plurality of light receiving elements of the light receiving elements51are arranged at the same pitch, but part of the light receiving elements also can be arranged at a different pitch.

Another modified example will be described. When an actual stop is stopped during shooting, a defocus amount Df is calculated by using only the light rays passing through the stop. For example, according to the example shown inFIG. 4, when partial pupils A and F are shaded by a stop and substantially match the light rays of the portion in which the partial pupils B-E are stopped, a defocus amount Df is obtained by the following equation.
Df=(k bc×N x—bc+k cd×N x—cd+k de×N x—de)/3+kcst  (9).
Here, there are three defocus amounts, so their sum is divided by three to obtain average. The defocus amount obtained by equation (9) is a defocus amount corresponding to the partial pupils B and E, excluding the shaded partial pupils A and F at both ends. By so doing, an accurate defocus amount can be detected without being affected by shading of the focus detection light rays by an aperture stop during shooting.

Another modified example will be described. According to the above-mentioned embodiment, an example is shown in which a signal string is generated by aligning output signals of one light receiving element from each light receiving element array51. However, a signal string also can be generated in which a plurality of light receiving element outputs of each light receiving element array51are added in advance and the calculation result is considered as one light receiving element.

For example, if output signals of the light receiving elements of the third and the fourth lines of each light receiving element array51are added and output signals a(1), b(1), . . . , f(1) of the light receiving element array51of the lowest end shown inFIG. 4are generated, as shown inFIG. 7, output signals of light receiving elements (3.1) and (4.1) are added and considered as an output signal a(1), output signals of light receiving elements (3.2) and (4.2) are added and considered as an output signal b(1), output signals of light receiving elements (3.3) and (4.3) are added and considered as an output signal c(1). Furthermore, output signals of light receiving elements (3.4) and (4.4) are added and considered as an output signal d(1), output signals of light receiving elements (3.5) and (4.5) are added and considered as an output signal e(1), and output signals of light receiving elements (3.6) and (4.6) are added and considered as an output signal f(1). In the same manner, output signals a(2)-f(2), a(3)-f(3), a(4)-f(4), a(5)-f(5), and a(6)-f(6) of another light receiving element array51are generated.

Furthermore, when output signals of eight light receiving elements within each light receiving element array51are added together, and output signals a(1), b(1), c(1), d(1), e(1) of the light receiving element array51of the lowest end shown inFIG. 4are generated, as shown inFIG. 8, output signals of eight light receiving elements are added together. In this case, the number of light receiving elements to be added is large, so the number of output signals decreases. Furthermore, in the same manner, output signals a(2)-e(2), a(3)-e(3), a(4)-e(4), a(5)-e(5), and a(6)-e(6) of another light receiving element array51are generated.

Thus, as a signal string is generated by adding an output signal of at least two light receiving elements within a plurality of light receiving elements of the light receiving element array51, an effect of noise included in the light receiving element output is minimized, and focus detection accuracy can be improved. Furthermore, if dark locations exist, a signal amount can be increased (because more locations are used), so adverse effects of dark locations can be reduced.

Furthermore, in the above-mentioned embodiment and the modified examples, in order to simplify the explanation, an example of the micro lens array4was used in which a plurality of micro lenses41are two-dimensionally arranged. However, a micro lens array also can be used in which a plurality of micro lenses are arranged only in positions corresponding to a focus detection area on a shooting screen.