JPH0473563B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to an autofocus device used in a movie camera, still camera, or single-lens reflex camera that uses a solid-state image sensor or an image pickup tube as an image sensor.

BACKGROUND TECHNOLOGY AND PROBLEMS This type of autofocus device includes active methods such as a triangulation method using ultrasound and infrared irradiation, and passive methods using contrast detection, coincident image detection, etc. Among these, the following is known as an example of a passive autofocus device.

Figure 1 shows the overview, and this is TTL
This is the type used in single-lens reflex cameras.

In FIG. 1, light passing through a lens 1 is incident on a sub-mirror 2, and the transmitted light is incident on a film surface 3. On the other hand, the light reflected by the sub-mirror 2 enters the beam splitter 4. For this beam splitter 4, one row of picture elements (sensor ) are provided with a sub-imaging device S ₁ and a sub-imaging device S ₂ each consisting of a one-dimensional array of solid-state imaging devices, such as CCDs. In the figure, broken lines 6 and 7
The sub image sensor S ₁ for the film surface 3 is shown by
At the equivalent position of S ₂ , they are spaced an equal distance l from the front and back of the film surface 3 .

Although not shown, a main mirror is provided between the sub-mirror 2 and the lens 1 so as to be orthogonal to the surface direction of the sub-mirror 2, and the incident light is directed to the sub-imaging elements S ₁ and S to the sub-mirror 2. ₂ , the light is made to enter the focusing plate provided on the opposite side.

By the way, whether or not the image is in focus can be evaluated by whether or not the image is clear. It is well known that whether or not an image is clear can be determined by determining the spatial frequency of the image and evaluating the MTF (modulation transfer function) with respect to the amplitude. Considering the conditions that it can be done and is inexpensive, it is difficult in practice.

On the other hand, in general, whether an image is clear or not can often be determined by whether the outline of the subject is sharp.

This contour information is obtained by differentiation, but in computer processing, it is obtained by using the difference in output between picture elements (sensors) of the sub-imaging device.

Here, for the sake of simplicity, half of the screen is white,
If we consider a subject whose brightness changes gradually, with the remaining half being black, as shown in Figure 2,
Contrast E is strongest where the object is accurately focused (focus point), and contrast E decreases even if the object is shifted from this position to the front or back.

Considering the evaluation function that represents this contrast,
This means that when an image is obtained using the one-dimensional solid-state image sensors S ₁ and S ₂ as described above, the n-th image and the n+
It can be expressed as the difference between the output In of the first sensor and In+1. That is, E= _o-1 〓 ⁿ⁼⁰ (In+1-In)...(1). Considering a sufficiently long array of one-dimensional image sensors, the array direction is set to x, and E=∫dI/dXdx...(2), which simply expresses the integral of I and is unrelated to sharpness. I get used to it. In order to avoid this drawback, some method must be used to perform nonlinear processing on In+1-In in equation (1). Conventionally, for this nonlinear processing, analog calculations are performed or the nonlinearity of the image sensor is utilized.

Assume that the contrast evaluation function E is obtained by performing the above-described nonlinear processing. This evaluation function E
is applied to the outputs of the above-mentioned sub-imaging devices S ₁ and S ₂ , and the function curve is obtained by plotting the focus shift on the horizontal axis and the contrast on the vertical axis, as shown in FIG. 3. In FIG. 3, curve 8 is the contrast C ₂ curve of element S ₁ , and curve 9 is the contrast C ₂ curve of element S _2.
Each curve is shown.

That is, since the sub-imaging elements S ₁ and S ₂ measure the contrast before and after the original focus, as the lens 1 is moved and focusing is performed, two contrast curves whose phases are shifted are obtained.
As described above, this device is configured so that the correct focus position is located halfway between the front focus side image sensor S ₁ and the rear focus side image sensor S ₂ , so that the contrast curves 8 and 9 The in-focus position is when the values are the same. Further, if contrast C ₁ >contrast C ₂ , it indicates that the current focus is on the front, and if contrast C ₁ <contrast C ₂ , it indicates that the current focus is on the back.

In this way, autofocus is possible by using the contrast information of the outputs of the two image sensors S ₁ and S ₂ placed at positions shifted forward and backward relative to the original focus.

Incidentally, an actual contrast curve is not an ideal one as shown in FIG. 3, but usually has a plurality of maximum values. This is because there are a plurality of subjects within the field of view of the imaging lens, and the contrast curve becomes maximum at the focal position of each subject. For example, as described below, the 9th
As shown in the figure, there is a human 22 and a pillar 2 in front of the camera 21.
3. When the bookshelves 24 are arranged in sequence, there are three maximum values of contrast as shown in FIG.

Conventionally, in consideration of the fact that there are multiple maximum values as evaluation values of sharpness, in that case, the position of the maximum maximum value is used as the focal point,
A spatial filter was applied to lower the spatial frequency, or the image was blurred to broaden the evaluation function to make it easier to determine the local maximum value.

However, as shown in Figure 9, the person 22 you want to photograph
If there is a background of a bookshelf 24 with a sharp contrast behind the bookshelf 24, if the method focuses on the maximum value, the maximum value at the position of the bookshelf 24 will be the maximum, so focusing on that bookshelf will not be possible. There was an inconvenience.

Another drawback is that when the spatial frequency is lowered or blurred, fine patterns cannot be brought into focus.

Purpose of the Invention In view of the above points of this invention, when there are multiple objects at different distances within the same angle of view and there are multiple maximum values as sharpness evaluation values, it is possible to always This allows autofocus to be performed in accordance with the photographer's intentions.

Summary of the Invention The present invention provides an autofocus device that autofocuses the sharpness of an image as an evaluation function using its spatial frequency. , find the local maximum value of sharpness corresponding to multiple objects existing within the camera's field of view and at different distances from the camera, store the focal position information corresponding to this maximum value in memory in advance, and then An autofocus device that focuses on a desired subject by selecting a focus position corresponding to one of a plurality of maximum values using a focus select switch provided on an operation unit,
This makes it possible to obtain autofocus that meets the photographer's intentions.

Embodiment Hereinafter, an embodiment of the present invention will be described with reference to FIG. 4 and subsequent figures. This embodiment is an example of a novel autofocus device that overcomes other drawbacks of the conventional example described above.

That is, in the case of the above-mentioned autofocus device, since the lens system is moved during distance measurement, the operating speed is slow and the movement is unnatural.

Also, if the focus is greatly out of focus and it becomes the undetectable area in Figure 3,
The drawback is that distance measurement is no longer possible.

Also, since a beam splitter is used,
Another drawback is that the amount of incident light decreases depending on the number of stages, and the operating range of autofocus is narrow.

This example is a new autofocus device that has fast distance measurement speed and can respond quickly even when the distance suddenly changes from infinity to close-up.

FIG. 4 shows an optical system according to an example of the present invention.
1 is a lens, 12 is a main mirror, 13 is a sub-mirror, 14 is a main image sensor consisting of, for example, a CCD, 1
5 is a sub-imaging device consisting of, for example, a CCD;
The sub-imaging device 15 is used for autofocus distance measurement.

The sub-imaging device 15 may be a one-dimensionally arranged line sensor similar to the conventional example described above, but in this example, it is an image capturing device in which sensors are two-dimensionally arranged. In addition, this sub-image sensor 15
Due to its purpose of use, it does not have to be the same size as the main image sensor 14, and may be a partial one that cuts out a part of the field angle of the main image sensor 14. The number of picture elements (sensors) of this sub-imaging device 15 is, for example, 64 vertically by 64 horizontally.

Light that has passed through the lens 11 using the TTL method enters the main image sensor 14 via the main mirror 12. A part of the light passing through the lens 11 is also reflected by the main mirror 12 and guided to the viewfinder. Further, a part of the light that has passed through the main mirror 12 is reflected by the sub mirror 13 and is transmitted to the sub image sensor 1.
5.

In this example, the lens 11 is used for distance measurement for autofocus as in the conventional case.
Instead of moving the sub-imaging device 15, the sub-imaging device 15 is moved by ±Δd in the optical axis direction. Therefore, in this example, the sub-imaging device 15 is mounted on an actuator as shown in FIG.

This actuator includes, for example, an annular body 1 which is magnetized to an N pole and placed at a certain distance from the cylinder 16 so as to surround a cylinder 16 which is magnetized to an S pole.
7 are formed, and a voice coil 19 is inserted into a groove 18 formed between the cylinder 16 and the annular body 17. This voice coil 19 is attached to a base 20 to which the sub-imaging device 15 is fixed, and depending on the current supplied to this voice coil 19, the sub-imaging device 15
The piston moves in the direction shown by the arrow in the figure (the optical axis direction shown by the arrow in FIG. 4).

In this case, the origin position of the sub image sensor 15 is at the same position as the focal length of the main image sensor 14, and the actuator causes the sub image sensor 15 to scan a movement amount D corresponding to the total focal length of the optical system. make a piston motion as if Scanning in this case is performed in a step-driven manner using a staircase wave as shown in FIG.
5 is made to take 16 stepwise positions during its movement amount D. In other words, if it takes time T for the amount of movement D, the sub-imaging device 15 will move by T/
It moves its position in the optical axis direction every 16=τ time, and there are 16 positions d ₀ , d ₁ , d ₂ ...
d ₁₅ , and stops at each position for a time τ. Here, d ₀ is the nearest focal point of the optical system,
d ₁₅ corresponds to the focal position at infinity.

Then, at each focal position of d ₀ to _{d 15} , the sharpness of the image of the sub image sensor 15 at each position is determined at each stop time τ, and information representing the sharpness at each position is obtained. is stored in memory. Then, among the sharpness information stored in the memory, a maximum position, that is, a position with high sharpness compared to other positions, is found, and based on this, the main image sensor 14 or lens 11 is moved to take appropriate action. The focal point is placed at the focal point. In this way, autofocus is achieved. In addition, the amount of movement D
The scanning speed of the above-described distance measurement by the sub-imaging device 15 to find the focal point position of maximum sharpness in the image is, for example, about 3 times/second.

By the way, in this example, d ₀ of the sub-image sensor 15
The arithmetic processing for determining the sharpness of the object at each position of ~ _d15 takes advantage of the fact that the image sensor is used as the sub-image sensor 15.
Two-dimensional differential processing of the image is used. FIG. 7 shows some picture elements (sensors) of the sub-imaging device 15, and the value of the two-dimensional differential between a certain picture element (i, j) and the surrounding eight picture elements in the figure is shown. If dl, the total amount of differential value is expressed as 〓= _inax 〓 ⁱ⁼⁰・_jnax 〓 ^j=0 dl. Then, the total amount of differential values at each focal position of the optical system from d ₀ to _{d 15} is stored in the memory as information on the sharpness of the object at each position.

For example, as shown in FIG. 9, in front of the camera 21,
If the person 22, the pillar 23, and the bookshelf 24 are lined up in sequence, and the positions of the person 22, the pillar 23, and the bookshelf 24 are clearly imaged at positions d ₃ , d ₇ , and d ₁₀ , for example, as shown in FIG. As shown in , the total amount of differential value 〓 is
It reaches a maximum at positions d ₃ , d ₇ , and d ₁₀ . In other words, if there are multiple objects within the field of view of the imaging lens 11, the total differential amount becomes maximum at each focal position of the camera's optical system according to the number and position thereof, and is stored in the memory. , the lens 11 or the image sensor 1 is placed at each focal point that becomes the maximum point.
By moving 4, each subject will be in focus.

When a plurality of objects at different distances from the camera come within the angle of view, it is normal to want to focus on a desired one of them. In this way, if there are multiple maximum points, in order to focus on the desired subject,
You can do it like this:

In other words, since the desired subject to be focused is often the closest to the camera, first focus on the closest subject, the person 22 in the example of FIG. If the object is not there, you can move it to another maximum point so that you can always focus on the desired object.

FIG. 10 shows a block diagram of an example of the control system of the apparatus of the present invention in consideration of the above.

In FIG. 10, 31 is a focus control circuit, which is composed of an arithmetic unit, a register, a digital comparator, a counter, etc.

From this focus control circuit 31, a stepped signal for causing the sub-imaging element 15 to scan for distance measurement as shown in FIG. 6 is obtained in the form of a digital signal. The counter described above is used for this purpose and is supplied with a clock having a period τ. The output of this counter is then supplied to the D/A converter 32 to be converted into an analog signal, which is then supplied to the voice coil 19 of the actuator shown in FIG. is brought to the focal position. At this position, the image output obtained from this sub-imaging device 15 is supplied to the logarithmic compression circuit 33. This logarithmic compression circuit 33 is for converting the distance characteristic of light into a linear one since it is logarithmic.

The output of the logarithmic compression circuit 33 is converted into a digital signal by the D/A converter 34 and then supplied to the arithmetic circuit 35 to obtain the total amount of differential values of the two-dimensional differential of the image, which is stored in the memory 36. , is stored as an evaluation value of the sharpness of the image at that position at an address that corresponds 1:1 to that position.

Next, when time τ has elapsed, the counter in the focus control circuit 31 increments and its count value changes. Then, D/A
The output of the converter 32 changes, and the position of the sub-imaging element 15 is moved in the optical axis direction. Then, at this changed position, the same operation as above is performed,
The total amount of differential values of the image output, which is the evaluation value of sharpness at that position, is written into the corresponding address of the memory 36. The above operations are performed at all of the above-mentioned 16 positions d ₀ to d ₁₅ , and thereby the stored contents of the memory 36 correspond to those shown in FIG. 8. This is repeated below.

A local maximum value is detected for the 16 pieces of information in the memory 36, and if it is a local maximum value, a marker is attached to the information. That is, memory 3
6, when writing information on the sharpness of each position, a digital comparator installed in the focus control circuit 31 compares the information on the previous and following positions with the information on that position and determines the maximum value. When it is detected, a marker (for example, one bit is used as this marker and is set to "1", for example) is added to the information and written. Alternatively, after all the information has been written to the memory 36, it may be read out sequentially, the maximum value is found using a digital comparator, a marker is attached, and the information is written at the original position again.

When the subject is as shown in Figure 9 relative to the camera position, the sharpness information at positions d ₃ , d ₇ , and d ₁₀ becomes maximum, and as mentioned above, the autofocus is first applied to the subject in the foreground. If the focus control circuit 31 selects a focus control circuit 31, information on this position, such as address information, is obtained from the focus control circuit 31 according to the maximum value marker added to the information on the position _d3 , and this is converted into an analog signal by the D/A converter 37. This analog signal causes the imaging lens 11 in this example to be moved so that the optical system is at the focal position of _d3 . If this position _d3 is not the focus position for the desired subject, operating the focus select switch 38 moves to the address position with the next maximum value marker, and the address information is transferred from the focus control circuit 31. obtained,
The lens 11 moves to a position that follows it.

The focus select switch 38 is placed near the lens of the camera body 40, for example, as shown in FIG.
When you turn down , the maximum value marked with a marker in the memory 36 will move sequentially toward, for example, a larger address, and the focus position will sequentially move, for example, to a subject position that is farther away from the camera.
When the switch 38 is turned to the F side in the figure, the memory 36
The focus position is sequentially moved to the local maximum value position even though the address is small, and the focus position is sequentially moved in the direction closer to the camera.

As shown in Fig. 8, if you perform a focus selection operation on the maximum value mark and mark it with numbers, for example, by selecting the number of the maximum value, you can easily find the target object. You can follow it even if it moves.

Note that instead of moving the lens system 11 using the output of the D/A converter 37, the main image sensor 14 may be moved.

Furthermore, the present invention is of course applicable not only to a system in which the sub-imaging element is moved for distance measurement as in the above example, but also to a conventional system in which the lens system is moved for distance measurement.

However, according to this example described above, instead of moving the lens system during distance measurement as in the conventional method, the image sensor, which has a small inertial mass, is moved, and the other lens systems are kept fixed until the in-focus point is found. This increases the distance measurement speed and prevents unnatural movements. Furthermore, in this example, since the in-focus point is detected at the same time for all focal lengths, there is an effect that the response is quick even if the subject position suddenly changes from infinity to close-up.

It also moves the sub-imaging device for focusing.
Since the lens system is not used for distance measurement, any interchangeable lens can be used as the lens system.

In the conventional case, the image sensor for distance measurement is a one-dimensional line sensor, so distance measurement is possible for images where the contrast changes in the arrangement direction of each picture element, but the contrast in the arrangement direction When there is no change, that is, when the line sensor is in the horizontal direction,
Distance measurement becomes impossible for objects where the contrast does not change in the horizontal direction and only changes in the vertical direction.

On the other hand, as described above, when a contrast change is determined two-dimensionally using an image sensor as an image sensor for distance measurement, there is no directional dependence as described above. In addition, since the evaluation function is also a two-dimensional differential, there is no need to take the trouble of performing non-linear processing as in the conventional example shown in Figure 1.
Since dimensional contrast detection is performed, accurate contrast detection in all directions can be performed.

Effects of the Invention As described above, according to the present invention, it is possible to detect the presence of a plurality of objects at different distances from the camera within the angle of view of the camera, based on the evaluation value of sharpness at each focal position of the optical system. The maximum value of the sharpness is determined in response to the sharpness, the focal position information corresponding to this maximum value is stored in memory in advance, and the focus select switch is selected from among the plurality of maximum values of the sharpness evaluation value. Since a desired subject can be focused by selecting a focal position corresponding to one local maximum value, it is possible to easily focus on any subject desired by the photographer.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an optical system of an example of a conventional autofocus device, FIGS. 2 and 3 are diagrams for explaining the same, and FIG. 4 is a diagram showing an example of an optical system of the present invention. Figure 5 is a diagram showing an example of the configuration of the main part.
6 to 9 are diagrams for explaining the control system, FIG. 10 is a block diagram of an example of the control system of the present invention, and FIG.
The figure is a diagram for explaining the configuration of a part thereof. 11 is an imaging lens, 12 is a main mirror, 13
14 is a main image sensor, 15 is a sub-image sensor, 36 is a memory, and 38 is a focus select switch.

Claims (1)

[Claims] 1. In an autofocus device that autofocuses the sharpness of an image as an evaluation function using its spatial frequency, based on the sharpness evaluation values at a plurality of focal positions of the optical system. Then, the maximum value of the above-mentioned sharpness corresponding to a plurality of objects existing within the field of view of the camera and at different distances from the camera is determined, and the focal position information corresponding to this maximum value is stored in memory in advance. , one of the multiple maximum values is selected by the focus select switch provided on the operation section of the camera.
An autofocus device characterized in that a desired subject is brought into focus by selecting a focus position corresponding to two local maximum values. 2. The focus select switch is configured to sequentially select from a focus position corresponding to a subject located close to the camera to a focus position corresponding to a subject located far away from the camera. autofocus device.