JPH0561610B2 - - Google Patents

Description

[Detailed description of the invention]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a focus detection device for a camera that detects a focus state by receiving a beam of light from an object that has passed through an objective lens, such as a photographic lens. Prior Art Detecting the relative position of two images created by the subject light beams that have passed through the first and second parts of the photographing lens that sandwich the optical axis of the photographic lens,
A focus detection device that detects the focus state has already been proposed. The basic configuration of the optical system is shown in FIG.
Furthermore, behind the condenser lens 4, there is an imaging lens 6,
8 are arranged, and line sensors 10 and 12 using, for example, CCD are arranged on their imaging planes. The images 14 and 16 on the line sensors 10 and 12 are formed on the optical axis 18 of each other in the case of so-called front focusing, in which the image of the object to be focused is formed in front of the intended focal plane.
On the other hand, in the case of rear focus, it moves away from the optical axis 18. When in focus, two images 1
The distance between two corresponding points 4 and 16 is a specific length determined from the configuration of the optical system. Therefore, the focus state can be determined by converting the light distribution pattern of the images on the line sensors 10 and 12 into electrical signals and determining their relative positional relationship. The problem to be solved: The relative positional relationship can be found by comparing two image patterns, but when the brightness of the subject is low or the contrast of the subject is low, there is little change in the image pattern, so it is difficult to accurately determine the positional relationship. It becomes difficult to seek. In such a case, even if the amount of defocus is determined based on the above-mentioned positional relationship, the value will not only become invalid, but also cause the lens to be driven in the opposite direction to the in-focus point. An object of the present invention is to provide a focus detection device that can accurately determine whether accurate focus detection is possible or whether the detection result is reliable from the light reception output for focus detection. Means for Solving the Problems The present invention normalizes the best correlation value where the first image signal and the second image signal have the best correlation by the contrast of the subject image, and determines whether focus detection is possible. is characterized in that it is determined based on this standardized value. Effect Even if the best correlation value obtained for a subject with high brightness or high contrast is the same as the best correlation value obtained for a subject with low brightness or low cost contrast, the subject contrast will be different. ,
Correspondingly, the standardized correlation value also changes. Therefore, whether or not focus detection is possible can be determined from the standardized correlation value. Embodiment FIG. 2 is a diagram showing an example of the configuration of an optical system, etc. when the focus detection device according to the present invention is applied to a single-lens reflex camera. In FIG. 2, a photographing lens 22, a reflecting mirror 24, a focus plate 26, a pentaprism 28, etc. are well-known elements constituting a single-lens reflex camera. However, when configuring the camera to automatically perform focusing using the output of the focus detection device, the photographing lens 22 is configured so that the focusing optical system can be driven by a lens driving device 30 including a motor. configured. The reflecting mirror 24 has a semi-transparent central part, and a sub-mirror 3 behind it.
2, through which part of the subject light is guided to a light receiving section 34 of a focus detection device disposed at the bottom of the mirror box. The light receiving section 34 includes a condenser lens 36, a reflecting mirror 38, and an imaging lens group 4.
0, line sensor 42, etc.
The output of the line sensor 42 is processed by the signal processing circuit 44 as will be described later, and a defocus signal indicating the amount of focus shift from the in-focus position and its direction is output. Based on this defocus signal, the display device 46 displays the focus state, and the driving device 30 drives the photographing lens 22 to the in-focus position. FIG. 3 is a diagram showing the optical system of the light receiving section 34, in which a straight line 48 indicates the optical axis of the photographing lens, and a dotted line 50 indicates a surface equivalent to the exposure surface of the film. The condenser lens 52 is not located at the exposure equivalent surface 50, but is placed at a position separated by the focal length f ₁ of the condenser lens 52 from there. Behind the condenser lens 52 are imaging lenses 54 and 5 with the optical axis 48 as the axis of symmetry.
6 are arranged, and field-limiting masks 58 and 60 are provided in front of these imaging lenses. CCD line sensors 62, 64 are arranged on the imaging plane of each imaging lens 54, 56. Here, the reason why the condenser lens 52 is arranged at a position away from the exposure equivalent plane 50 is as follows. The line sensors 62 and 64 are configured with an optical system so that the object image on the exposure equivalent surface 50 is re-imaged, but when the condenser lens 52 is arranged on the exposure equivalent surface 50,
If there are scratches or dust on the surface of this lens, this will appear as an image on the line sensor and become noise in the original image of the object. Therefore, by removing the condenser lens 52 from the equivalent exposure plane, the above noise can be avoided. Furthermore, when it is incorporated into a camera, it can be installed without making any major changes to the camera's optical system. Also, mask 5
8 and 60 are configured in relation to the condenser lens 52 so as to receive only the object light that passes through an aperture area corresponding to a specific aperture value, for example, F5.6, out of the object light that passes through the photographic lens. In this way, when various interchangeable lenses are used as photographic lenses, if the maximum aperture value of the photographic lens is smaller than F56, an image in which some of the light rays are rejected by the pupil mask of the photographic lens itself can be created. This eliminates the need for the line sensors 62 and 64 to receive such damage, and it becomes possible to apply most commonly used interchangeable lenses. Next, points 66, 68, and 70 on the optical axis represent images in the front focus, in-focus, and back focus states for one object point in front of the photographing lens. Each image 66, 68, 70
The incident points on the line sensor 62 are 72, 74, and 76, and the incident points on the line sensor 64 are 78, 80, and 82, respectively. Figure 4 shows images 84, 8 of front focus, focus, and back focus.
6 and 88 are shown in the line sensor area. Reimaging 90, 92 for front focus image 84
are located in front of the light receiving surface 94 of the line sensor and are closer to the optical axis 48 side. Re-images 96 and 98 for the focused image 86 coincide with the light-receiving surface 94 of the line sensor, and re-images 100 and 102 for the rear-focused image 88 are located behind the light-receiving surface 94 of the line sensor and are located from the optical axis 48. is seperated. Therefore, the re-images 90 and 92 for the front-focus image 84 become slightly blurred and enlarged images on the light-receiving surface 94 of the line sensor. Furthermore, the re-images 100 and 102 for the rear focused image 88 are slightly blurred and become reduced images on the light receiving surface 94. Next, with reference to FIG. 5, the relationship between the amount of image movement h in the line sensor 62 and the amount of deviation e of the image from the in-focus position will be described. Among the rays of the image 68 that are formed on the effective axis 48 during focusing, the condenser lens 5
Consider a ray that travels parallel to the optical axis 48 after passing through 2.
When the images 66 and 70 are front-focused or rear-focused by a deviation amount e from the image 68, the aforementioned light rays pass through a point 67 or 71, respectively, which is separated by g from the optical axis 48 at the position of the exposure equivalent surface 50. Here, three points 68, 67, and 71 on the exposure equivalent surface 50 are used as light sources, and an image of the above light sources is formed on the line sensor 62 by an imaging system 55 including a condenser lens 52 and an imaging lens 54. , each statue is 7
Suppose that they are 4, 72, and 76. In addition, the imaging system 55
Let α be the magnification of . If you look at Figure 5 geometrically,
The following formula holds true. g/e=H/f ₁ ……(1) α=h/g ……(2) If g is eliminated from these two equations, e=f ₁ /αHh ……(3), (3) In the equation, f ₁ /αH is a constant determined by the configuration of the imaging system, so if the movement amount h is detected, the shift amount e can be found. However, the fourth
As shown in the figure, only the focused image is normally formed on the exposure equivalent surface 50, and the other images are located before and after it, so strictly speaking, the magnification α
is not constant, but varies depending on the respective positions of the images 66 and 70 serving as light sources with respect to the imaging system 55.
Assuming that the magnification at the time of focusing is α ₀ , as shown in FIG. 13, the magnification is larger than α ₀ in the case of front focus, and smaller than α ₀ in the case of rear focus. Furthermore, the magnification varies depending on the position of the image on the sensor surface due to aberrations such as field curvature of the optical system. Therefore, in calculating a more accurate amount of deviation, a magnification is prepared in advance according to the amount of movement h and used as described later. A circuit for detecting the movement amount h and the deviation amount e will be described below. FIG. 6 is a diagram showing an example of the pixel configuration of the line sensors 62 and 64 shown in FIG.
is called a reference part, and the licensor 64 is called a reference part. pixel
L ₁ to L ₂₆ and R ₁ to _{R 30} are photodiodes and constitute a charge coupled device CCD. A dummy pixel is provided in the blank space between pixels L ₂₆ and R ₁ to connect the two line sensors 62 and 64 to one line.
It may also be configured as a CCD. Furthermore, a charge transfer line 65 may be provided between line sensors 62 and 64 as shown in FIG. Photodiode 6
7 and 69 are for monitoring the intensity of incident light to determine the charge accumulation time of the CCD.
Incidentally, this monitor photodiode may be shaped to fill the gap between the pixels L _i as shown in FIG. 8. This makes it possible to monitor light with an intensity almost close to that of the pixel surface. Next, in the embodiment, the image pattern at the reference portion 62 of the line sensor is divided into three blocks. The first block includes pixels L ₁ to L ₁₀ , the second block includes pixels L ₉ to _{L 18} , and the third block includes pixels L ₁₇ to L 10 .
Each corresponds to the image pattern in L ₂₆ . Each block image pattern consists of 10 pixels, each block having 10 pixels here, but the number of pixels in each block does not necessarily have to be the same. In focus detection, the image of each block and the image of the comparing section 64 are compared. For example, when using the image of the first block, the following comparison operation is performed. First, the image of pixels R ₁ to R ₁₀ in the reference area is symmetrically compared with the image of the first block. The content of the comparison in this case is shown by equation (4), and the sum of the absolute values of the differences in pixel output for each pair of pixels L ₁ and R ₁ , L ₂ and R ₂ , ... L ₁₀ and R ₁₀ is calculated. be done. H ₁ (1)= ₁₀ 〓 ^k=1 |L _k −R _k | ...(4) Next, shift by one pixel from the previous image,
Images of pixels R ₂ to R ₁₁ of the reference section 64 are compared. The processing details are shown in equation (5). H ₁ (2)= ₁₀ 〓 ^k=1 |L _k −R _k+1 | ...(5) Similarly, the comparison process shown by the following equation is performed, and a total of 21 comparison results are obtained. H ₁ (3)= ₁₀ 〓 ^k=1 ｜L _k −R _k+2 ｜ ……(6) H ₁ (21)= ₁₀ 〓 ^k=1 ｜L _k −R _k+20 ｜ ……(7) Now, the image of the first block is, for example, pixel R ₂ ~
If the image matches the image of the R ₁₁ portion, H ₁ (l ₂ ) is the smallest among the 21 comparison results. By finding the pixel area corresponding to this minimum value, the approximate focus position can be detected. A comparison operation similar to that using the first block image is performed using the second and third block images. The content of each comparison is generally expressed by the following formula. H ₁ (l)= ₁₀ 〓 ^k=1 ｜L _k −R _k+l-1 ｜ …(8) H ₁ (l)= ₁₀ 〓 ^k=1 ｜L _k+8 −R _k+l-1 | ...(9) H ₁ (l)= ₁₀ 〓 ^k= 1 | L _k+16 −R _k+l-1 | ...(10) Here l=1, 2, ..., 21. By the above comparison operation, the image of each block is
21 results, for a total of 63 comparison results.
Now, in the case of focusing, the image of the second block is in the comparison section 6.
The optical system is configured to match the image of the second pixel R ₁₁ to R ₂₀ . In this way, in the case of focusing, the image of the first block coincides with the images of the pixels R ₃ to _{R 12} and the image of the third block coincides with the images of the respective portions of the pixels R ₁₉ to _{R 28} . In this case, the focus position can be detected using any block depending on the state of the image. However, for blocks covered with images with low contrast, it may not be possible to identify the minimum value from the comparison results. Therefore, a plurality of blocks having a contrast of a certain value or more are selected and the focus position is detected from the comparison results corresponding to these blocks. In addition, in the case of the front focus state, referring to FIG.
Since they match in the portions closer to the sides, the image of the third block matches the image of a certain portion of the reference portion 64. On the other hand, in the case of rear focus, the two images coincide at a portion remote from the optical axis 48, so that the image of the first block coincides with a certain portion of the reference portion 64. Therefore, in the case of out-of-focus, there is a possibility that the minimum value can be found among the comparison results for the images of the first block or the third block. However, if there is insufficient contrast in the image, focus detection is considered to be impossible, and the minimum value is not detected. In addition, in the first block and the second block, and the second block and the third block, pixels _L9 , _L10 , and _L17 are
L ₁₈ is shared. By sharing pixels in this way, focus detection becomes possible, for example, even if there is image contrast between pixels _L9 and _L10 , but no contrast exists in other pixel areas. If pixels are not shared, if image contrast is located only at the boundary between two blocks, there will be no contrast within each block, and focus detection will become impossible. Now, when the minimum value of the comparison result is found in one of the blocks and the matching area of the images is specified, the focus position of the image or the amount of deviation from the focus position is specified correspondingly. However, the accuracy of the amount of deviation determined through the above process is limited to the resolution of the pixel arrangement pitch. Therefore, interpolation calculation processing as described below is performed, and error factors based on the optical system of the focus detection device are corrected to improve the accuracy of the amount of deviation. FIGS. 9A and 9B are block circuit diagrams showing a circuit configuration for processing the image pattern signal from the line sensor outlined above. This signal processing circuit
It has a control logic 106 that outputs control signals for the operation of the entire system including the CCD 104. Each pixel signal sent out in series from the CCD 104 is sequentially converted into, for example, an 8-bit digital signal by a digitizing circuit 108, and each pixel signal is stored in a random access memory 110 at a predetermined address. When the pixel signal storage is completed, the contrast detection circuit 112 detects the contrasts C 1 , C 2 , and C 3 of each of the first, second, and third blocks from the memory data of the reference section, and detects the contrasts C ₁ , C ₂ , and C ₃ of each of the first, second, and third blocks from the memory data of the reference section. It is determined whether or not there is. The contrasts C ₁ , C ₂ , and C ₃ correspond to the sum of the absolute values of the differences between the outputs of two adjacent pixels, as shown in the following equations. Note that the contrast calculation is performed without extending beyond the block area. Alternatively, the difference between the outputs of every other or every other pixel may be used. C ₁ = ₉ 〓 ^k=1 ｜L _k −L _k+1 ｜ ...(11) C ₂ = ₉ 〓 ^k=1 ｜L _k+8 −L _k+9 ｜ ...(12) C ₂ = ₉ 〓 ^k=1 ｜L _k+16 −L _k+17 ｜ ...(13) The obtained contrasts C ₁ , C ₂ , C ₃ are each stored in the memory 114 at a predetermined address, and are further stored at a predetermined address. Level C ₀ and comparison circuit 11
6, the magnitude relationship is determined. For example, "1" is output if the level C ₀ is exceeded, and "0" is output if it is not, and the contrasts C ₁ , C ₂ ,
The respective determination results d ₁ , d ₂ , d ₃ for C ₃ are stored in the memory 120 . Next, the image comparison circuit 122 compares the image of each block with the image of the reference portion. In this case, the images of blocks whose contrast has not reached the predetermined level C ₀ are not compared, and only the images of the blocks whose contrast exceeds the predetermined level C ₀ are compared with the image of the reference portion. The contents of this comparison are as shown in equations (8), (9), and (10). Twenty-one comparison results are obtained for each block, and these are sequentially stored in the memory 124 at predetermined addresses. Next, the minimum value H ₁ of the comparison results of each block obtained is
(l ₁ ), H ₂ (l ₂ ), H ₃ (l ₃ ) and their respective comparison numbers l ₁ , l ₂ , l ₃ are searched by search circuit 126 and the results are stored in memory 128 . Next, the standard circuit 130 determines the above minimum values H ₁ (l ₁ ), H ₂ (l ₂ ), H ₃ (l ₃ ) and the contrast C ₁ , for blocks whose contrast exceeds a predetermined level.
The ratio between C ₂ and C ₃ is determined. Each is shown by the following formula. NH ₁ = H ₁ (l ₁ ) / C ₁ ... (14) NH ₂ = H ₂ (l ₂ ) / C ₂ ... (15) NH ₃ = H ₃ (l ₃ ) / C ₃ ... (16 ) These ratios mean the following: As mentioned above, for example, when the photographic lens is at or near the in-focus position, focus detection may be possible using any of the three blocks.
In such a case, the problem of selecting a block arises as to which block is best to adopt.
Furthermore, in the case of out-of-focus, there arises the problem of determining which block should be adopted to detect the state of front focus or rear focus. When adopting a specific block, the minimum value H ₁ of each block determined
It may be possible to designate the block that takes the smallest value among (l ₁ ), H ₂ (l ₂ ), and H ₃ (l ₃ ), but this is not appropriate. In general, the contrast state of images is not uniform; for example, an image with high contrast may be located in the area of the first block, and an image with not so high contrast may be located in the other blocks. When detecting coincidence between two image patterns, it is generally advantageous to have a larger contrast. Therefore, contrast is also added to the selection of specific blocks. By the way, for example, the minimum value for the first block
Consider the comparison results H ₁ (l _{1 -1} ) and H ₁ (l ₁ +1) when H ₁ (l 1 ) is shifted forward or backward by one pixel pitch. If this minimum value H ₁ (l ₁ ) is for the in-focus state, H ₁ (l ₁ −l) or H ₁ (l ₁ +1) is the value of the contrast detection circuit 112.
It almost matches the contrast C ₁ calculated by . This is because the contrast C ₁ and the comparison result H(l ₁ −
1) and H ₁ (l ₁ +1) are related to the difference in output between adjacent pixels. The difference is that the contrast C ₁ is for the same image, whereas the comparison results are for different images. Since this is the case, the minimum value H ₁ (l ₁ )
The value NH ₁ divided by the contrast C ₁ is the minimum value H ₁
(l ₁ ) and the comparison result when the pixel is shifted by one bit. Expressing this as a formula, NH _i = H _i (l _i )/C _i ≒ H _i (l _i )/H _i (l _i ±1)...
...(17) However, i=1, 2, 3. Now, if we call NHi a standardized index,
It is assumed that the standardization index corresponding to a block that is in focus or approximately in focus and has a large contrast will be the smallest among the three values, and this is set as the block selection criterion. In reality, the light distribution patterns of the images of the standard part and the reference part do not match completely due to aberrations in the optical system and positional asymmetry of the first and second images with respect to the optical axis. Therefore, the minimum value H _i (l _i ) never takes 0. Also, in the case of out-of-focus state,
Regarding blocks where there is no coincidence of images,
The standardized index takes a relatively large value. Therefore, a reference value NH ₀ is determined in advance for the standardization index, and if this value is exceeded, it is determined that focus detection is impossible. In this way, when the minimum value among the three standardization indexes determined at most is smaller than the reference value NH ₀ , the detected data L _k of the block corresponding to this minimum value is used as information indicating the amount of defocus. adopt. That is, the minimum value detection circuit 132 finds the true minimum value over a plurality of blocks. At the same time, the corresponding block is detected and the minimum value H _k
A comparison number l _k that takes (l _k ) is retrieved from the memory 128 by the selection circuit 134 . Then the minimum value H _k
The standardized minimum value NH _k of the block taking (l _k )
is compared with a predetermined value NH ₀ in a subtraction circuit 136 and NH _k
Proceed to the next step when is smaller than NH ₀ ,
If this is not the case, focus cannot be detected. Now, suppose that l ₁ is obtained for the image of the first block, for example, l ₁ =18. This means that the images on pixels L ₁ to _{L 10} and the images on pixels R ₁₈ to _{R 27} match best. In this case, find the distance D ₁ between the images on the two pixel areas. This interval D ₁ is pixel
is the spacing between L ₁ and R ₁₈ . As shown in Figure 6, the distance between pixels L ₁ and R ₁ is 1.50 mm, and the pixel pitch is P.
If it is 30μ, then D ₁ =1.50+0.03×18=2.04(mm)...(18) can be obtained. Using the comparison number l ₁ for the first block, the image spacing D ₁ is given by: D ₁ = 1.50 + 0.03l ₁ In the same way, the image spacing D ₂ for the second block is found to be 8 from the first block.
Since it is shorter by 8 pixels, D ₂ = 1.50-0.03×8+0.03l ₂ ...(19) Since the third block is 8 pixels shorter than the second block, D ₃ = 1.50-0.03× 8×2+0.03l ₂ ...(20) Further generalizing the above three equations, D _k = 1.50-0.03 {8(k-1)l _k }...(21) The critical accuracy of the interval shown by equation (21) is determined by the pixel pitch P. corresponds to FIG. 10 shows an example of the comparison results for the image of block 2. The comparison number l ₂ that takes the minimum value H ₂ (l ₂ ) is 8. As shown in Figure 10, the comparison result H ₂
If (l ₂ -1) and H ₂ (l ₂ +1) are not equal, the true matching point is not the point with comparison number l ₂ = 8, but the point next to l ₂ = 8 and the minimum value H ₂ (l ₂ ). It exists between the comparison number l ₂ +1=9 which takes a smaller comparison result. When the position of such an intermediate point is determined, the focus detection accuracy is improved more than the pixel pitch. Therefore, a method for determining the position of this intermediate point will be explained. Now, in Figure 10, we extend the line connecting H ₂ (l ₂ -1) and _H 2 (l ₂ ), and on the other hand, the gradient is opposite to this extended line and we get H ₂ (l ₂ +1).
When we draw a line through , we consider the point where the two intersect to be the true coincidence of the two images. In this way,
In the case of H _k (l _k −1)≧H _k (l _k +1) as shown in Fig. 11, the length β between l _k and the true coincidence point q can be calculated as follows from the geometric configuration in the figure. It is shown by the formula. β=1/2・H _k (l _k −1)−H _k (l _k +1)/H _k (l _k −
1) −H _k (l _k )・P ... (22) If Hk (l _k −1) < H _k (l _k +1) as shown in Fig. 12, β=1/2・H _k ( l _k −1)−H _k (l _k −1)/H _k (l _k +
1) −H _k (l _k )・P ...(23). In the circuit shown in FIG. 9, the interpolation calculation circuit 138 calculates equation (22) or equation (23). Furthermore, the equation (21) is corrected by the interpolated value β as shown in the following equation. D' _k = D _k ±β ... (24) Here, the positive sign of the second term β on the right side corresponds to the case where equation (22) is used, and the negative sign corresponds to the case where equation (23) is used. . As described above, the interpolation calculation circuit 138 calculates the distance D' _k between the two images in the reference section 62 and the reference section 64. Next, the deviation amount calculation circuit 140 calculates the deviation amount e of the image of the photographing lens from the in-focus position using the interval D' _k . If the distance between the two images at the time of focus is D ₀ , the amount of movement h of the images in FIG. 5 is given by the following equation. h=1/2(D' _k −D ₀ ) (25) Here, h<0 indicates a front pin, and h>0 indicates a rear pin. In the case of the imaging system shown in Fig. 5, D ₀ =2H, but
In reality, it will differ slightly due to assembly errors, so it is preferable to set an appropriate value as D ₀ during assembly adjustment. Now, when the movement amount h is determined, the deviation amount e is determined based on equation (3), but the magnification α is determined in advance according to h.
For example, by experimentally determining the values shown in Table 1,
It is prepared in the ROM 142 and used to calculate the deviation amount e.

[Table] As described above, the direction and amount of displacement of the photographic lens with respect to the object to be photographed can be determined. FIG. 14 is a circuit diagram showing an embodiment in which a microcomputer is used in the signal processing circuit of the focus detection device of the present invention. The CCD 104 receives three-phase pulses φ ₁ , φ ₂ ,
φ ₃ , the internal transfer line is always in a data transfer state. In the CCD 104, the charge of each pixel is cleared by a clear pulse output from the terminal _P17 of the microcomputer 146. Therefore, the time when the charge is cleared becomes the time when charge accumulation starts. At the start of this charge accumulation, a ramp voltage whose rate of decline changes over time is output from the terminal q ₂ of the CCD 104 in accordance with the brightness of the subject. This voltage is compared with a predetermined constant voltage Vs in a comparator circuit 148,
When the voltage drops to this voltage, the comparator circuit 148 goes "high".
Output voltage. In response to this “high” voltage, the terminal
A shift pulse is output from _P16 , and in response to this, the charge accumulated in each pixel of the CCD 104 is transferred to the transfer line. For CCD104, the terminal
The period from when a clear pulse is applied to _q7 to when a shift pulse is applied to terminal _q6 is the charge accumulation time. In addition to the pixels shown in FIG. 6, the CCD 104 includes a plurality of pixels used as dummy pixels and a plurality of pixels for obtaining a dark output. CCD1
04 first outputs a dummy signal and a dark signal from the output terminal _q1 when a shift pulse is applied, and then outputs a required pixel signal. Note that when the power supply voltage Vcc changes, this change is superimposed on the output of the CCD, so a circuit 150 is used to cancel and remove this change.
is input. This voltage fluctuation removal circuit 150 is
Connect the power supply voltage Vcc to the input 152 through resistors 154 and 156.
It is given a voltage divided by , and outputs a voltage according to the difference between the two inputs. When outputting pixel signals,
One of the initial dark signals of the integral data output of the CCD 104 is sampled and held in the sample and hold circuit 158, and the subsequent pixel signals R _i and _Li are obtained by the subtraction circuit 1.
60, the dark signal of the sample and hold circuit 158 is subtracted. In this way, the pixel signal becomes one in which the voltage fluctuation component and the dark output component are removed. The pixel signal from the subtraction circuit 160 is amplified by the amplifier circuit 162 with an amplification factor depending on the brightness level. The amplification factor is controlled so that it becomes higher as the luminance level becomes lower. The brightness level is detected by the brightness level detection circuit 164 as a change in the slope voltage per fixed time using the slope voltage from the terminal _q2 ,
This change is used as a signal indicating the brightness level. The amplified pixel signal is sent to the multiplexer 16
6 to an input 170 of a voltage comparator circuit 168 that implements a digitization circuit. The digitization circuit converts the 8-bit binary number into D-
The microcomputer 146 is programmed to supply the data to the A-to-A converter circuit 172 and to store the comparison results, and is configured as, for example, a sequential comparison type A-to-D converter circuit. The digitized pixel signal is stored in a memory at a predetermined address according to the pixel addresses R _i and L _i . Thereafter, the aforementioned data processing is performed to detect the amount and direction of deviation of the photographic lens, and these are used for automatic focus adjustment control of the photographic lens and display of the focus state. Now, when power supply to the microcomputer 146 is started, in response, the microcomputer 146 moves to a CCD initialization program. Before focus detection starts,
Charges are accumulated in the transfer lines and pixels of the CCD 104 at a level higher than the normal pixel signal level.
Before extracting the pixel signal, this unnecessary charge is cleared from the transfer line and the pixel. This clearing operation is the initialization of the CCD. In this initialization, a clock pulse with a shorter cycle (for example, 1/16 of the normal pixel signal) is used for normal pixel signal transfer.
The signal is applied to the CCD to repeat the faster-than-normal transfer operation multiple times (for example, 10 times), thus emptying the transfer line. In parallel with this, pixel clearing is also performed. In this case, no pixel signal capture operation is performed. The transfer pulse generation circuit 144 generates transfer pulses φ 1 , φ ₁ , φ ₁ ,
Generate φ ₂ and φ ₃ . A transfer pulse with a shorter period than normal is generated by generating a clock pulse within the transfer pulse generation circuit 144 in response to this "high" voltage when the flip-flop 176 is in the reset state and its output is at a "high" voltage. It is created by changing the frequency division ratio of by a predetermined value.
Flip-flop 176 is reset by a pixel charge clear pulse from microcomputer 146 and set by a shift pulse. Also, from the shift pulse, the transfer pulse generation circuit 14
4 is in a state where a normal transfer pulse is generated.
Note that in the CCD 104, the time from the generation of the charge clear pulse to the generation of the shift pulse is defined as the charge accumulation time, and during this period, the transfer pulse generation circuit 1
44 outputs a transfer pulse with a shorter cycle than normal. However, during the charge accumulation period, CCD10
Since the signal output from 4 through the transfer line is treated as an unnecessary signal, no problem occurs even if the transfer pulse becomes faster. Now, when a predetermined number of transfer cycles are completed as an initialization operation, the microcomputer 1
46, the program moves to the aforementioned focus detection program. First, when a clear pulse is output,
CCD 104 starts accumulating charge. At the same time, the terminal q ₂ of the CCD 104 outputs a ramp voltage that drops from a predetermined voltage at a rate corresponding to the subject brightness, and when this voltage drops to a predetermined level Vs, the output level of the voltage comparison circuit 148 becomes "low". ” to “high” voltage. This “high” voltage is used as an interrupt signal and is
46 outputs a shift pulse from terminal _P16 upon receiving an interrupt. CCD10 by shift pulse
The charges accumulated in the 4 pixels are transferred in parallel to the transfer line, and then serially transferred to the output terminal q ₁
are sequentially output as voltage signals. This voltage signal is digitized as described above and stored in a predetermined memory. When the capture of the pixel signal is completed, a "high" voltage signal, for example, is temporarily output from the terminal _p11 , and in response to this, the multiplexer 166 selects and outputs the constant voltage from the constant voltage circuit 178. Voltage digitization circuit 108
The data is digitized and stored in a predetermined memory. As mentioned above, the interval between similar images formed on the reference part and the reference part during focusing does not match the design value due to assembly errors in the optical system, so this error is corrected. It is used as data for Constant voltage circuit 178 is constant current circuit 18
0 and a semi-fixed resistor 182, and accurate image interval data is set by adjusting the semi-fixed resistor 182 during the adjustment process of the focus detection device. FIG. 15 is a flowchart showing the operation flow of the focus detection device described above. Effects As described above, according to the present invention, the best correlation value obtained by obtaining the best correlation between the first image signal and the second image signal is normalized by the contrast value of the subject image, and focus detection is possible. Since we decided to use this standardized correlation value to determine whether Even if the obtained low-accuracy best correlation value is the same value, it is possible to accurately determine whether focus detection is possible or whether the focus detection result is reliable.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a conventional example of the optical system of a focus detection device, FIG. 2 is a diagram showing an example of the arrangement of the focus detection device of the present invention in a camera, and FIG. 3 is a diagram showing the optical system of the focus detection device of the present invention. FIG. 4 is a diagram showing the configuration of the system, FIG. 4 is a diagram showing the image formation state by the optical system of the focus detection device of the present invention, and FIG. 5 is a diagram showing the amount of focus shift and line in the optical system of the focus detection device of the present invention. 6, 7, and 8 are diagrams showing the relationship between the image on the sensor and the amount of movement, and FIGS. 9A and 9B are diagrams illustrating an example of the pixel configuration of the line sensor of the focus detection device according to the present invention. FIG. 10 is a block circuit diagram showing the configuration of the signal processing circuit of the focus detection device according to the present invention;
11 and 12 are graphs for explaining the operation of the signal processing circuit, and FIG. 13 is a graph showing the magnification of the optical system of the focus detection device according to the present invention.
FIG. 4 is a block circuit diagram when a microcomputer is used in the signal processing circuit of the focus detection device according to the present invention, and FIG. 15 is a flowchart showing the flow of the operation of the signal processing circuit. 2, 22...Photographing lens, 12, 14, 62,
64,104...Line sensor (CCD), 4,3
6,52...Condenser lens, 6,40,5
4, 56...imaging lens, 67, 69...subject brightness monitor photodiode.

Claims (1)

[Claims] 1. In a focus detection device that detects the focus state of the objective lens by detecting the correlation between the first and second images formed by the subject light beams that have passed through different parts of the objective lens, A first line sensor that outputs a first image signal according to the light distribution pattern of the image, and a second line that receives the second image and outputs a second image signal according to the light distribution pattern of this image. Correlation means for determining the correlation between the sensor, the first image signal and the second image signal, and selecting the correlation value from which the best correlation is obtained, and a contrast calculation means for determining the contrast of the subject image; a normalizing means for normalizing the best correlation value with the contrast value; a determining means for determining whether focus detection is possible based on the standardized best correlation value; A focus detection device comprising: means for calculating a focus deviation amount from a predetermined focus position of an objective lens based on a calculation result of the correlation means.