JPH0333708A - Focus detector - Google Patents

Abstract

PURPOSE:To accurately discriminate the possibility of accurate focus detection or the reliability of its detection result by standardizing the best correlation value that a 1st image signal and a 2nd image signal have the best correlation with the contrast of a subject image, and discriminating whether or not the focus detection is possible according to the standardized value. CONSTITUTION:Even if the best correlation value obtained for a high-brightness, high-contrast subject and the best correlation value obtained for a low- brightness, low-contrast subject are equal, the subjects are different in contrast, so the standardized correlation value varies correspondingly. Here, the best correlation value that the 1st image signal and 2nd image signal have the best correlation is standardized with the contrast value of the subject image and whether or not the focus detection is possible is discriminated according to the standardized correlation value. Consequently, whether or not the focus detection is possible or whether the focus detection result is reliable can be accurately be discriminated.

Description

DETAILED DESCRIPTION OF THE INVENTION 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.

The first part and the second part of the conventional photographic lens sandwich the optical axis of the photographic lens.
A focus detection device has already been proposed in which the focus state is determined by detecting the correlated position of two images created by the subject light beams that have passed through the respective areas. The basic configuration of the optical system is shown in Figure 1, with a condenser lens (
4), further behind the condenser lens (4), imaging lenses (6) and (8) are arranged, and on their imaging planes, for example, line sensors (10) and (12) by COD are arranged. ing. Image on line sensor (10), (12) (
14) and (16) are in the case of the so-called front bin, where the image of the object to be focused is formed in front of the planned focal plane.
They move toward each other toward the optical axis (18), and conversely, in the case of the rear bin, they move away from the optical axis (18). When the image is in focus, the distance between two corresponding points of the two images (4) and (16) becomes 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!
It becomes difficult to obtain the positional relationship with high accuracy. In such a case, even if the amount of defocus is determined based on the above-mentioned positional relationship, the value will not only be invalid, but also the lens will 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 contrast, the subject contrast is different. , the standardized correlation value also changes accordingly.

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 camera with 7 lenses. In Fig. 2, the photographic lens (22), the reflector (
24), focusing plate (26), pentaprism (28), etc. are well-known elements constituting a single-lens camera. However, if the camera is configured to automatically focus using the output of the focus detection device, the shooting lens (2
2) is configured such that the focusing optical system can be driven by a lens driving device (30) including a motor. The reflecting mirror (24) has a semi-transparent central part, and a sub-mirror (32) is provided behind it, through which a part of the subject light is placed at the lower part of the mirror box and brought into focus. The light is guided to the light receiving section (34) of the detection device. Light receiving part (34)
is composed of a condenser lens (36), a reflecting mirror (38), an imaging lens group (40), a 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 and direction of focus deviation from the in-focus position is output. The focus state is displayed on the display device (46) based on this defocus signal, and the photographing lens (22) is driven to the in-focus position by the drive device (30).

FIG. 3 is a diagram showing the optical system of the light receiving section (34), and shows the straight line (
48) indicates the optical axis of the photographing lens, and the dotted line (50) indicates a plane equivalent to the film exposure plane. Condenser lens (52
) is arranged not at the position of the exposure equivalent plane (50) but at a position away from it by the focal length f of the condenser lens (52).

Imaging lenses (54) and (56) are arranged behind the condenser lens (52) with the optical axis (48) as the axis of symmetry, and in front of these imaging lenses is a field-limiting mask (58).
, (60) are provided. Each imaging lens (54), (5
6) On the imaging plane, there is a COD line sensor (62),
(64) is the arrangement. Here, condenser lens (5
2) is arranged at a position away from the exposure equivalent plane (50) for the following reason. Line sensor (62), (64)
The optical system is configured so that the object image on the exposure equivalent surface (50) is re-imaged, but when a condenser lens (52) is placed on this exposure equivalent surface (50), the surface of this lens If there is a flaw or a lump attached to the object, 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-mentioned 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, masks (58), (
60) is a condenser lens (52) so as to receive only the subject light passing through an aperture area corresponding to a specific aperture value, for example F5.6, out of the subject light passing through the photographic lens.
constituted in relation to In this way, when various interchangeable lenses are used as photographic lenses, if the photographic lens has an open aperture value smaller than F56, an image in which some of the light rays are kicked out by the pupil mask part of the photographic lens itself can be created. This eliminates the need for the line sensors (62) and (64) to receive this, 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. Multi-image (66), (68), (7
The incident points of 0) on the line sensor (62) are (72), (74), and (76), respectively, and on the line sensor (64), they are (78), (80), and (82).

Figure 4 shows images (84), (86) of front focus, focus, and back focus.
), (88) are shown reimaging in the line sensor area. Re-imaging (90), (92) for the 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. Focused image (8
Re-imaging (96) and (98) for 6) coincide with the light-receiving surface (94) of the line sensor, and re-imaging (100) and (102) for the rear focused image (88) coincide with the light-receiving surface (94) of the line sensor. 94) and away from the optical axis (48). Therefore, re-imaging (90
), (92) are on the light receiving surface (94) of the line sensor,
The image will be slightly blurred and stretched. In addition, the rear focus image (8
Re-imaging (100) and (102) for 8) is the light receiving surface (
94) The upper image is slightly blurred and becomes a reduced image.

Next, with reference to FIG. 5, the relationship between the amount of movement of the image in the line sensor (62) and the amount of deviation e of the image from the in-focus position will be described. An image (6) that is formed on the optical axis (48) during focusing
Among the rays in 8), consider the ray that travels parallel to the optical axis (48) after passing through the condenser lens (52). Image (66) of front focus or back focus by deviation amount e from image (68)
, (70), the aforementioned rays are at points (6
7) or (71). Here, the exposure equivalent surface (50
) using the three points (68), (67), and (71) as light sources, and an imaging system (55) consisting of a condenser lens (52) and an imaging lens (54) to form a line image for the above light source. Images are formed on the sensor (62), and each image is (7
4), (72), and (76). Let the magnification of the imaging system (55) be a. If we look at Figure 5 geometrically, the following equation holds.

e f yσ −−・・・・・・ (2) If g is eliminated from these two equations, 1 e−h・・・・・・ (3) α H , and in equation (3), f1/ Since αH is a constant determined by the configuration of the imaging system, if the movement Rh is detected, the amount of deviation e can be determined. However, as shown in Figure 4, only the in-focus image is properly formed on the exposure equivalent plane (50), and the other images are located before and after it, so strictly speaking, the magnification α is not constant, and the imaging system (5
5) differs depending on the respective positions of the images (66) and (70) that serve as light sources. The magnification when focusing is α. Then, in the case of the front pin as shown in Figure 13, σ. larger, α for posterior pins. It becomes smaller. 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 and used as described later. A circuit for detecting the amount of movement and the amount of deviation e will be described below.

FIG. 6 is a diagram showing an example of the pixel configuration of the line sensors (62) and (64) in FIG. 3, where the line sensor (62) is called a reference part and the line sensor (64) is called a reference part. Pixel (
Ll) to (L26) and (R+) to (R3°) are photodiodes that form the bottom of a charge coupled device (CCD).

In addition, a dummy pixel is provided in the blank space between the pixel (Lza) and (R1), and the two line sensors (62)
, (64) may be used as the COD of one line at the bottom of the groove. Furthermore, as shown in Fig. 7, the line sensor (62) and (
64), a charge transfer line (65) may be laid between them. Photodiodes (67) and (69) are for monitoring the intensity of incident light to determine the charge accumulation time of the COD. In addition, this monitor photodiode is the 8th photodiode.
As shown in the figure, the shape may be such that it fills the gap between the pixels (L,). 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 part (62) of the line sensor is divided into three blocks. The first block has pixels (L,) to (Ll.), the second block has pixels (L,) to (Lea), and the third block has pixels (
They correspond to the image patterns in Lla) to (Lis), respectively. The image pattern of each block consists of 10 pixels. Although each block has 10 pixels here, the number of pixels does not necessarily have to be the same. In focus detection, the image of each block and the comparison section (6
4) is compared. For example, when using the image of the first block, the following comparison operation is performed.

First, the image of the portion of pixels (R1) to (R1,) 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 in each pair of pixels L, R, L, R2, ... LIO and RI6 is calculated. .

Next, the reference part (
Images of pixels (R2) to (R1□) of 64) are compared. The processing details are shown in equation (5).

below, In the same way, the comparison process shown in the following formula is performed, and a total of 2 One comparison result is obtained.

For example, if the image of the first block matches the image of pixels R3 to R0, H+(
QX) becomes the minimum. By finding the pixel area corresponding to this minimum value, the approximate focus position can be detected.

An operation similar to the comparison operation using the image of the first block is
This is done using images of the second and third blocks.

The content of each comparison is generally expressed by the following formula: shown.

Here, Q-1, 2, . . . , 21.

By the above comparison operation, 21 images are obtained for each block image.
In total, 63 comparison results are obtained.

Now, in the case of focusing, the image of the second block is the comparison part (62)
The optical system is adjusted so as to match the image of the pixels (Ro) to (R2°). In this way, in the case of focusing, the image of the first block matches the images of pixels (R3) to (R1), and the image of the third block matches the images of the respective parts of pixels (Ro) to (R2M). In this case, the focus position can be detected using any block depending on the state of the image.

However, in a block 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 bin state, refer to Fig. 4 and refer to the reference part (
62) and the reference portion (64) coincide in the portion closer to the optical axis (48), so the image of the third block coincides with the image of the portion comprising the reference portion (64). On the other hand, in the case of the rear bin, the two images coincide in parts away from the optical axis (48), so the image of the first block is the reference part (64).
Matches the part consisting of. 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. Incidentally, in the first block and the second block, and the second block and the third block, respectively, the pixels L and Ll. Andori, Atori,
is shared. When pixels are shared in this way, focus detection becomes possible even when, for example, image contrast exists in the pixel areas L, L, and L, and contrast does not exist 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 any block 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, an interpolation calculation process 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.

FIG. 9 (AXB) is a block circuit diagram showing a circuit configuration for processing the image pattern signal from the line sensor outlined above. This signal processing circuit is a ccD (104)
It has a control logic (106) that outputs control signals for the operation of the entire system including. C0D(104
Each pixel signal sent out in series from ) 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 (l l 2) calculates the contrasts C3, C2, and C1 of each of the 11th second and third blocks from the memory data of the reference section.
is detected, and it is determined whether or not it is equal to or higher than a predetermined level. The contrast C3°C,,C, corresponds to the sum of the absolute values of the differences between the outputs of two adjacent pixels, as shown by the following equation. 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.

The desired contrast C,,C! , C, are each stored in a memory (114) at a predetermined address, and the magnitude relationship is determined by a comparison circuit (116) with a predetermined level C0. For example, "l" is output if the level C0 is exceeded, and "O" is output if the level is not exceeded.
Judgment results d1 for contrasts C0Cm and Cs. d2. d3 is stored in 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 the blocks whose contrast has not reached the predetermined level C are not compared, and only the images of the blocks whose contrast exceeds the predetermined level C0 are compared with the image of the reference portion. The contents of this comparison are as shown in equations (8), (9), and (to).

21m comparison results are obtained for each block, and these are stored in memory (124) at predetermined child addresses in sequence. Next, the minimum values Hlca, ), Hz (Qt), Hi (a
s) and their respective comparison numbers a,, C,, a, are searched by the search circuit (-126), and the results are stored in the memory (1
28).

Next, the standardization circuit (130) sets the minimum value Hl(
+21), H2 (Qh), H3 (+23) and contrast C1, c2. The ratio between c and Each is shown by the following formula.

1 - 3 These ratios mean the following: As described 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 block selection 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 the front bin or the rear bin. When adopting a specific block, the minimum value 1(+(+2+)
.

It may be thought that it would be sufficient to specify the block that takes the smallest value among Hi(12) and Hi(123), but this is not appropriate.In general, the contrast state of the image is -
Rather, 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, consider the comparison results H1(12+-1) and H+(c,+1) when the minimum value Hl(L) for the first block is shifted forward or backward by l pixel pitch, for example.

If this minimum value Hl (L) is for the in-focus state, then H, (L-1") or H, (ff, +
1) substantially matches the contrast C1 determined by the contrast detection circuit (112). This is because the contrast C1, comparison result H+(12+ 1), Hrc(
This is because each of h+1) is related to the difference in output between adjacent pixels. The difference is that the contrast C1 is for the same image, whereas the comparison results are for different images.

Therefore, the value NH, which is obtained by dividing the minimum value Hl(L) by the contrast C1, is the minimum value Hl((2+) and the pixel l
This approximately corresponds to the comparison result when the pitch is shifted. This can be expressed as a formula: C+ Hl (L:I=1), where i=1.2.3.

Now, if we call NHi a standardization index, we consider that the standardization index corresponding to a block that is in focus or almost in focus and has a large contrast is the smallest among the three values, and then we call this a block. stipulated in the selection criteria.

In reality, the light distribution pattern of the images of the standard part and the reference part is
Because they cannot be perfectly matched due to aberrations in the optical system and positional asymmetry of the first and second images with respect to the optical axis,
The minimum values H, CQ→ never take O. Furthermore, in the case of an out-of-focus state, the standardization index takes a relatively large value for blocks where images do not match at all. Therefore, a reference value NH, 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, the detected data Lh of the block corresponding to this minimum value is adopted as information indicating the amount of defocus. do. 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 comparison number Qk that takes the minimum value H20, is stored in the memory (128).
is extracted by a selection circuit (134). Thereafter, the standardized minimum value NHk of the block that takes the minimum value Hh (Qh) is compared with a predetermined value NH, in a subtraction circuit (136), and when NHk is smaller than NH, the process proceeds to the next step;
If this is not the case, focus cannot be detected. Now, 12. for the image of the first block. For example, 12
Suppose that it is +-18. This is the pixel (L+) ~ (L I
This means that the image on pixel R1+) and the image on pixels (R18) to (Rit) match best.

In this case, the distance Dl between the images on the two pixel regions is determined.

This interval Dl is the interval between pixels (L, ) and (Ro). As shown in Fig. 6, if the distance between pixels (L,) and (R1) is 1.50 mm and the pixel pitch P is 30 μ, then D = t, so + 0.03X 1g = 2.0
4 (mm) --(18). 1st
Using the comparison number Ql for the block, calculate the image spacing,
is expressed by the following equation.

D I = 1.50 + 0.03121 Similarly, the second
When determining the image spacing D2 for the block, the first
Since it is 8 pixels shorter than the block, -1,50-0,03x8+0.03p, -
-------(19) Since the third block is 8 pixels shorter than the second block, D3-1.50-0゜03X 8 X 2 + 0.0
3α,...(20). D h-1,50-0,03 (80 cm
1)+Qk)...(21). The critical accuracy of the interval expressed by equation (21) corresponds to the pixel pitch P.

FIG. 1O shows an example of the comparison results for the image of block 2. The comparison number Ql that takes the minimum value Hz (Qx) is 8. As shown in Figure 10, the comparison result H2C (h l
) and Hl (12□+1) are not equal, the true matching point is not the point of comparison number ax-8, but the comparison number a* that takes the next smallest comparison result from 122-8 and the minimum value H2 (Qx)
+ Exists between 1 and 9. 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, Hz(L-1) and )I
When we extend a line connecting z(Qx) and draw a line that has the opposite slope to this extension and passes through Hto, + I), we consider the point where the two intersect to be the true coincidence of the two images. . In this way, H as shown in Figure 11, (12, -1)
In the case of ≧Hk (1,000), the length β between a and the true matching point q is expressed by the following equation from the geometric configuration of the figure.

As shown in Figure 12, Hko, -1) < H, (s, + i)
In the case of , it becomes .

In the circuit shown in Figure 9, the interpolation calculation circuit (138) performs (22)
Equation (23) is calculated.

Furthermore, the equation (21) is corrected by the interpolated value β as shown in the following equation.

D'* -Dh±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. corresponds to In the manner described above, the interpolation calculation circuit (138) calculates the interval D' between the two images in the standard part (62) and the reference part (64).

Next, the deviation amount e of the image of the photographing lens from the in-focus position is determined by the deviation calculation circuit (140) using the interval D'. The distance between the two images when in focus. Then, the movement fkh of the image in FIG. 5 is given by the following equation.

h −−(D' −D,) ・・・・・・ (
25) Here, h<0 indicates the front bin, and h>O indicates the back bin.

In the case of the imaging system shown in FIG. 5, the values are D and -2H, but in reality they vary slightly due to assembly errors, so it is preferable to set appropriate values during assembly and adjustment.

Now, when the amount of movement is determined, the amount of deviation e is calculated based on equation (3).
is calculated, but the magnification σ is determined in advance according to h, for example, the first
Numerical values as shown in the table are determined experimentally and prepared in the ROM (142), and are used to calculate the deviation amount e.

Table 3 As described above, the direction and amount of displacement of the photographic lens with respect to the subject 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. ccD (104) is a transfer pulse generation circuit (1
44), and the internal transfer line is always in a data transfer state. CCD (1
04) is the terminal (P) of the microcomputer (146).
I? ) The charge in each pixel is cleared by the clear pulse output from the pixel. Therefore, the time when the charge is cleared becomes the time when charge accumulation starts. With the start of this charge accumulation, C
From the terminal (q,) of the OD (104), a ramp voltage whose rate of decline varies over time according to the brightness of the subject is output. This voltage is compared with a predetermined constant voltage VS by the comparator circuit (148), and when it drops to this voltage, the comparator circuit (148)
48) outputs a "high" voltage. In response to this "high" voltage, a shift pulse is output from the terminal (P,), and in response to this, the charge accumulated in each pixel of the CCD (104) is transferred to the transfer line. For ccD (104), after a clear pulse is given to the terminal (q,), the terminal (q,
) is the charge accumulation time until a shift pulse is applied. In addition to the pixels shown in FIG. 6, COD (l O4) includes a plurality of pixels used as dummy pixels and a plurality of pixels for obtaining a dark output. CCD (10
4) first outputs a dummy signal and a dark signal from the output terminal (q,) when a shift pulse is applied, and then outputs a required pixel signal. Note that the output of the CCD is equal to the power supply voltage Vcc
When , changes, this change is superimposed, so it is input to a circuit (150) for canceling and removing this change. This voltage fluctuation removal circuit (150) is supplied with a voltage obtained by dividing the power supply voltage Vcc by resistors (154) and (156) at an input (152), and outputs a voltage according to the difference between the two inputs. When outputting the pixel signal, one of the initial dark signals of the integral data output of C0D(lO4) is sampled and held in the sample and hold circuit (158), and the subsequent pixel signal RoL is sampled and held in the sample and hold circuit (158) by the subtraction circuit (160). 158) is reduced by the dark signal. 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 an amplification 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 a brightness level detection circuit (164) as a change in the slope voltage per fixed time using the slope voltage from the terminal (q,), and this change is used as a signal indicating the brightness level. The amplified pixel signal is applied via a multiplexer (166) to an input (170) of a voltage comparison circuit (168) constituting a digitization circuit. The digitization circuit includes a voltage comparison circuit (168),
A digital-to-analog converter circuit (72) and a microcomputer (146) programmed to supply an 8-bit binary number to a D-A converter circuit (172) and to store comparison results, for example, perform sequential comparisons. It is marked *X as an A-D conversion circuit of the format. The digitized pixel signal is stored in a memory at a predetermined address according to the pixel addresses Ri and Li. 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 to this, the microcomputer (146)
Step 6) moves on to the CCD initialization program. Before focus detection starts, COD (104)
Charges are accumulated in the transfer lines and pixels above the normal pixel signal level, but before extracting the pixel signal,
This unnecessary charge is cleared from the transfer line and pixels.

This clearing operation is the initialization of the CCD.

In this initialization, a clock pulse with a cycle shorter than that during normal pixel signal transfer (for example, 1/16 of the normal pixel signal) is given to the COD to repeat the transfer operation faster than usual multiple times (for example, 10 times), and in this way the pixel signal is transferred. Leave the line empty. 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) is a microcomputer (14).
Transfer pulses 11.6) are generated using clock pulses of a constant period from the terminals (p, s) of 11.6). -1*I- is generated.

The transfer pulse whose cycle is shorter than normal is when the flip 70 (176) is in the reset state and its output is “high”.
voltage, the clock pulse is generated by changing the frequency division ratio of the clock pulse by a predetermined value within the transfer pulse generation circuit (144) in accordance with this "high" voltage. The flip 70 tube (176) is reset by a pixel charge clear pulse from the microcomputer (146) and set by a shift pulse. Furthermore, the shift pulse causes the transfer pulse generation circuit (144) to enter a state in which it generates a normal transfer pulse. Furthermore, C0D(1
04), the time from the generation of the charge clear pulse to the generation of the shift pulse is defined as the charge accumulation time, during which time the transfer pulse generation circuit (144) outputs a transfer pulse with a shorter cycle than normal. However, since the signal outputted from the CCD (104) via the transfer line during the charge accumulation period 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 (146) moves to the program for focus detection described above. First, when a clear pulse is output, the CCD (104) starts accumulating charges. At the same time, the COD (104) terminal (
q2) outputs a ramp voltage that drops from a predetermined voltage at a rate according to the subject brightness, and when this voltage drops to a predetermined level Vs, the output level of the voltage comparison circuit (148) changes from "low" to "high". ``Inverts to voltage. This "high" voltage is used as an interrupt signal, and when the microcomputer (146) receives the interrupt, it outputs a shift pulse from the terminal (Pea). CCD (
The charges accumulated in each pixel of 104) are transferred in parallel to the transfer line, and then transferred in series to the output terminals (q,
) are sequentially output as voltage signals. This voltage signal is digitized as described above and stored in a predetermined memory. When the capture of pixel signals is completed, the terminal (P
, ), for example, a “high” voltage signal is temporarily output,
In response, the multiplexer (166) selects and outputs the constant voltage from the constant voltage circuit (178), and this constant voltage is digitized by the digitization circuit (108) and stored in a predetermined memory. As mentioned above, the distance between the two images formed on the standard part and the reference part during focusing does not match the design value due to assembly errors in the optical system, so this data is used to correct this error. used as. Constant voltage circuit (178) is constant current circuit (180)
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 this standardized correlation value is used to determine whether or not the image is photographed, the best correlation value with high accuracy obtained for a subject with high brightness or high contrast is compared with the best correlation value obtained for a subject with low brightness or low contrast. 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 drawings]

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 amount of movement of the image on the sensor, and FIG. 9 (AXB) is a diagram showing an example of the pixel configuration of the line sensor of the focus detection device according to the present invention. 1 is a block circuit diagram showing the configuration of the signal processing circuit of the focus detection device according to the present invention, FIGS. 1O, 11, and 12 are graphs for explaining the operation of the signal processing circuit, and FIG. A graph showing the magnification of the optical system of the focus detection device, FIG. 14 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. 815 is a flowchart of the operation of the signal processing circuit. It is a flowchart which shows. 2.22...Photographing lens, 12.14, 62.64.
104...Line sensor (CCD), 4.36°52
...Condenser lens, 6,40,54.56...
Imaging lens, 67.69...Subject brightness monitor photodiode

Claims (1)

[Claims] In a focus detection device that detects the focus state of an objective lens by detecting the correlation between first and second images formed by 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;
A second line sensor that receives a second image and outputs a second image signal according to the light distribution pattern of this image, and calculates the correlation between the first image signal and the second image signal, and calculates the correlation between the first image signal and the second image signal. a correlation means for selecting a correlation value with the best correlation; a contrast calculation means for calculating the contrast of a subject image; a normalization means for normalizing the best correlation value by this contrast value;
A determination means for determining whether or not focus detection is possible based on the value of the standardized best correlation value, and a determination means that operates when focus detection is possible and detects the amount of focus deviation from the expected focus position of the objective lens. A focus detection device comprising: means for calculating based on the calculation result of the correlation means.