JPH09243903A - Automatic focusing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform focusing in a short time even in the case of photographing a low contrast subject such as a white subject in a white background or a black subject in a dark scene. SOLUTION: An electrical signal outputted from a charge coupled element(CCD) 11 is nonlinearly digitally converted by a nonlinear converter 23. By such nonlinear digital conversion, the contrast of an image having uniform luminosity is emphasized. A digital signal on which the contrast is emphasized is supplied to a focus evaluating device 20. The evaluating device 20 accurately detects the maximum value of a high frequency component amount within a focus evaluating range by using the digital signal on which the contrast is emphasized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic focusing device used in an automatic focusing image pickup device such as an electronic still camera, and more particularly to converting an image formed through a focusing lens into an electric signal corresponding to its illuminance. The present invention relates to a device provided with a photoelectric conversion sensor and a focus establishing means for moving the focus lens to establish focus of an image based on a high frequency component included in the electric signal.

[0002]

2. Description of the Related Art Conventionally, a video camera employs a focusing device that determines the image formation state of an image by using a high frequency component included in a video signal. In this focusing device, when the image is in focus, the high-frequency component contained in the electric signal from the photoelectric conversion sensor such as CCD (charge-coupled device) is larger than when the image is out of focus. It's being used. Along with the movement of the focus lens, the focus lens position where the amount of high frequency components of the video signal becomes maximum in the focus evaluation range divided into a part of the image is detected.

[0003]

With the above-described focusing device, it is difficult to focus an object having a low contrast. This is because an increase rate of high-frequency components is small for a subject with low contrast even during focusing. As a result, when a black subject is photographed in a dark scene or a white subject is photographed on a white background, it takes a long time to focus, or the focusing itself cannot be performed.

In a dark scene, it is possible to increase the gain of the image signal to amplify the increase rate of the high frequency component.
In this case, noise increases as the gain increases, which may cause a malfunction. Moreover, if there is a bright portion in a part of a dark scene, the increase in gain may cause saturation of other circuits such as the A / D converter. On the other hand, when a white subject is photographed on a white background, it cannot be dealt with by adjusting the gain.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a focusing device capable of performing focusing in a short time even when photographing a subject having a low contrast.

[0006]

In order to achieve the above object, according to the present invention, a photoelectric conversion sensor for converting an image formed through a focusing lens into an electric signal according to its illuminance, and the electric signal. Based on the high frequency components contained in
In an automatic focusing apparatus including a focusing establishing unit that moves the focusing lens to establish focusing of an image, the focusing establishing unit includes a brightness detecting unit that detects the brightness of the image, and the photoelectric conversion unit. A non-linear converter that converts an electric signal from the sensor according to a non-linear conversion rule, and a focus evaluator that determines the image formation state of an image from the focus evaluation signal obtained by the conversion are detected by the brightness detection means. The conversion rule of the non-linear converter is converted based on the brightness, the degree of change of the electric signal is emphasized, and the result is supplied to the focus evaluator.

[0007]

Preferred embodiments of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 shows the overall structure of an electronic still camera to which the automatic focusing device according to the present invention is applied. The electronic still camera SC includes a focus lens 10 that captures an object OB at an appropriate lens position, and a CCD 11 as a photoelectric conversion sensor that converts an image formed through the focus lens 10 into an electric signal according to its illuminance. CCD1
The image captured by 1 is digitized by, for example, the A / D converter 12 and recorded in a desired recording medium 13 such as a memory card.

The automatic focusing device 14 is the above-mentioned CCD.
11 and a focus establishing unit 15 that moves the focus lens 10 to establish the focus of the image. As is clear from FIG. 2, the focus establishing means 15 includes a CCD driver 16 that supplies a drive signal for the CCD 11 and a focus lens 10.
Lens drive mechanism 17 for moving the lens back and forth along the optical axis
And the CCD driver 16 and the lens driving mechanism 1
7 and a controller 18 for controlling 7. Under the control of the controller 18, according to the drive signal from the CCD driver 16, a current having a magnitude corresponding to the illuminance of each pixel of the image is output from the CCD 11 as a serial signal series. The signal series output from the CCD 11 is supplied to the focus evaluator 20 through the signal processing means 19. The focus evaluator 20 determines the image formation state of the image based on the high frequency components included in the signal sequence.

The signal processing means 19 includes an amplifier 21 for amplifying the electric signal from the CCD 11, an A / D converter 22 for linearly converting the amplified electric signal into a digital signal, and the obtained digital signal for image conversion. And a non-linear converter 23 that performs conversion according to a non-linear conversion rule according to brightness.
The A / D converter 22 guarantees a linear digital output with respect to the input light to the CCD 11. The non-linear converter 23 includes, for example, an arithmetic unit or a LUT (look-up table) in which the bit width of the output signal is smaller than the bit width of the input signal.
Is used. The conversion rule of the non-linear converter 23 is switched by a control signal from the controller 18.
The brightness detector 24 connected to the A / D converter 22 detects the brightness of the image from the obtained digital signal and supplies the average value of the brightness to the controller 18.

The digital filter 25 passes an optimum electric signal corresponding to the operation mode of the automatic focusing device 14. In the focus evaluator 20, as shown in FIG. 3, a focus evaluation range 31 is divided into a part of the image 30, and the absolute value of the signal from the digital filter 25 is the focus evaluation range 31.
Is summed up with. The summed signal values are supplied to the controller 18 as focus evaluation values. The controller 18 may, for example, in an operating mode that performs a rough focus,
The digital filter 25 operates as a wide pass filter. By adopting a wide-pass filter, a linear output change according to the subject distance can be obtained even for a subject with low contrast. In addition, the controller 18
Operates the digital filter 25 as a high-pass filter, for example, in an operation mode in which fine focusing is performed. By adopting a high pass filter,
It is possible to efficiently capture the high frequency component at the time of focusing.

Next, the operation of this embodiment will be described. As shown in FIG. 3, consider a case where it is desired to capture a person as a subject in the focus evaluation range 31. First, the controller 18
Enter an operating mode that performs a rough focus. Under the control of the controller 18, the lens driving mechanism 17 drives the focusing lens 10 at a constant speed from the closest point to infinity. During this driving, the controller 18 instructs multiple times of photographing at arbitrary intervals. The CCD driver 16 drives the CCD 11 for each photographing, and the CCD 11 outputs an electric signal.

The output electrical signal is amplified and digitally converted, and then input to the non-linear converter 23. At this point, the non-linear converter 23 is set to the standard conversion rule for the input light as shown in FIG.
Subsequently, the digital signal that has passed through the digital filter 25 set as the wide band pass filter is input to the focus evaluator 20. The focus evaluator 20 sums up the digital signal values within the focus evaluation range 31 and outputs the focus evaluation value. The controller 18 selects an arbitrary range including the maximum focus evaluation value from a plurality of times of shooting.

As described above, by roughly performing the focusing while moving the focusing lens 10 over the entire focal length, it is possible to specify the range of the focal length to be surely focused.

Subsequently, the controller 18 enters an operating mode in which fine focusing is performed. Focus lens 10
Is moved to one of the boundaries of the range containing the maximum value. In this operation mode, the maximum value of the focus evaluation value can be derived by repeating the photographing over the entire area of the range in fine steps, or the position where the focus evaluation value decreases with the movement of the focus lens 10 is sought to find the focus. The maximum value of the focus evaluation value can be derived by reversing the lens 10. At this time, the digital filter 25 is set to a high pass filter, and only the high frequency component is focused on the focus evaluator 2.
0 is supplied. The focus lens 10 is moved, and the lens position having the maximum focus evaluation value is selected as the focus point.

In the focus establishment means 15, the brightness detection means 24 always detects the average value of the brightness within the focus evaluation range 31 at the time of entering the operation mode for executing fine focusing. The controller 18 determines that the focus evaluation value obtained during the rough focus adjustment is equal to or greater than a predetermined threshold value, and that the maximum focus evaluation value obtained by the movement of the focus lens 10 is the maximum value. When the difference from the minimum value is equal to or more than a predetermined threshold value, it is determined that the contrast sufficient for focusing is obtained in the focus evaluation range 31. Therefore, focusing is continued as it is. If the obtained focus evaluation value is smaller than the threshold value or the difference between the maximum value and the minimum value is smaller than the threshold value, the controller 18
Determines that the contrast of the focus evaluation range 31 is insufficient.

Here, for example, the entire image 30 is bright,
Suppose a white subject OB is selected against a white background. At this time, the controller 18 obtains a focus evaluation value smaller than the threshold value. This is because the contrast is low.
Therefore, the controller 18 converts the conversion rule of the non-linear converter 23 into that shown in FIG. In this conversion rule, based on the average value obtained by the brightness detector 24, the input is linearly converted only in a narrow range across a relatively high average value. As a result, the contrast is emphasized in the vicinity of the average value of the brightness corresponding to the brightness of the image 30, the variation in the focus evaluation value becomes large, and the focus is easily established.

On the other hand, for example, the entire image 30 is dark,
It is assumed that the contrast between the background and the subject OB is small. At this time, the controller 18 obtains a focus evaluation value smaller than the threshold value. This is because the contrast is small in this case as well. Therefore, the controller 18 converts the conversion rule of the non-linear converter 23 into that shown in FIG. According to this conversion rule, the input is linearly converted only in a narrow range across a relatively low average value. As a result, image 3
The contrast is emphasized in the vicinity of the average value of the brightness corresponding to the darkness of 0, and the focus can be easily established. However, at the final capture of the image, the non-linear converter 23 is always returned to the normal standard conversion rules.

[0019]

As described above, according to the present invention, since the degree of change in the electric signal output from the photoelectric conversion sensor is emphasized, it is possible to perform highly accurate focusing even for an image with low contrast. Becomes

[Brief description of drawings]

FIG. 1 is an overall configuration diagram of an electronic still camera to which an automatic focusing device according to the present invention is applied.

FIG. 2 is a block diagram showing a circuit configuration of focus establishment means.

FIG. 3 is an overall view of an image showing a focus evaluation range.

FIG. 4 is a graph showing conversion rules of a non-linear converter.

[Explanation of symbols]

10 focus lens, 11 CC as photoelectric conversion sensor
D, 14 automatic focusing device, 15 focusing establishment means,
20 focus evaluator, 23 non-linear converter, 24 brightness detector as brightness detection means, electronic still camera as SC autofocus imaging device.

Claims (1)

[Claims] 1. A photoelectric conversion sensor for converting an image formed through a focusing lens into an electric signal according to its illuminance,
An autofocusing device comprising: a focusing establishing unit that moves the focusing lens to establish focusing of an image based on a high-frequency component included in the electric signal; Brightness detecting means for detecting the following, a non-linear converter for converting an electric signal from the photoelectric conversion sensor according to a non-linear conversion rule, and a focus evaluator for judging an image forming state from a focus evaluation signal obtained by the conversion. And a conversion rule of the non-linear converter is converted based on the brightness detected by the brightness detection means, and the degree of change of the electric signal is emphasized and supplied to the focus evaluator. Focusing device.