JPH05260359A - Automatic focusing device - Google Patents

Abstract

PURPOSE:To obtain the automatic focusing device by which more accurate focus processing is attained even to an object with a low brightness. CONSTITUTION:The device is made up of a lens 1, a half mirror 2 by which light of an object is separated into a side of a photometry element 5 and an image pickup element 3, the image pickup element 3, an image pickup processing circuit 4, a BPF circuit 8 extracting the contrast, an arithmetic operation processing circuit 10, a focus motor 12 and the photometry element 5. When the brightness of the object is low, pre-sampling is implemented by fixing the lens 1 to a prescribed position to detect a noise level of the contrast data and to obtain an arithmetic operation coefficient corresponding to the level. Then the contrast obtained by driving the lens 1 one by one step is divided by the arithmetic operation coefficient to obtain a correction contrast. The focal position is detected based on the value.

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, and more particularly to an automatic focusing device for detecting a focusing position based on contrast data based on a video signal.

[0002]

2. Description of the Related Art Conventionally, as one of automatic focusing methods of an image pickup apparatus using an image pickup device, contrast information of an object for evaluating the degree of focusing is calculated based on a video signal when the focusing lens is extended or retracted. Then, there is a method of determining the lens position where the value shows a peak as the in-focus position, and driving the focusing lens at this position. This method is called the "mountain climbing method", and is based on the NHK Technical Research Report (1965, Volume 17, No. 1 (total 86)).
The details are described on pages 21 to 37). In the processing operation of the focus control device to which the "mountain climbing method" is applied, when the subject has low brightness, the value of the contrast information of the subject decreases due to the low video signal level, and the focus position can be determined. It could be very difficult. Therefore, conventionally, the gain of the video signal is increased to obtain the contrast data and the focus position is detected.

[0003]

However, in the above-described conventional focus control device, when the gain of the video signal of the low-brightness subject is increased and processed, noise is also amplified at the same time, and the accurate focus position is obtained. Was difficult to detect. The noise includes dark current of the image sensor, signal transfer noise, signal amplification noise, and the like. FIG. 6 is a diagram showing changes in the lens position and the contrast value E, which is contrast data, when the gain is increased for the low-illuminance subject. The lens position will continue to be indicated by the number of lens driving steps j, and j = 0 is the number of focusing steps at infinity,
Further, jmax is the closest focusing step number, and jp is the driving step number of the contrast value peak Es. In the above "mountain climbing method", it is necessary to confirm whether the contrast value E is increased by a predetermined threshold value or more from the past maximum value, and then continuously decreased by a predetermined number of times with respect to the number of lens driving steps. become. However, as shown in FIG. 6, when noise more than the change in the true contrast value is included, the noise is simply increased.
Subsequently, it was not possible to check the state of decrease, and it was therefore difficult to accurately detect the driving stage number jp at the peak point.

The present invention has been made to solve the above-mentioned problems, and in an automatic focusing apparatus using contrast data, the subject has low brightness and the contrast data contains a lot of noise. It is an object of the present invention to provide an automatic focusing device capable of detecting an accurate focusing position of a lens even in the case where it occurs.

[0005]

An automatic focusing apparatus according to the present invention uses a plurality of predetermined contrast data based on a video signal formed by the image pickup means under a same photographing condition before evaluating the focused state. A first corresponding to a pre-sampling means for obtaining each at a time point, and one predetermined value for selecting a difference between the maximum value and the minimum value of the plurality of contrast data obtained by the pre-sampling means as its lower limit value. And a second data corresponding to a quotient obtained by dividing the contrast data obtained over time based on the video signal formed by the image pickup means by the first data Is provided as a reference data for focusing state evaluation to the automatic focusing means.

[0006]

Before the evaluation of the in-focus state, the pre-sampling means obtains the contrast data a predetermined number of times under one photographing condition, and the first data generating means makes the maximum and minimum values of the data. The first data is obtained with the lower limit of the difference of Then, the quotient obtained by dividing the contrast data obtained over time by the image pickup means by the second data generation means by the first data is obtained as second data, and the second data is used for focusing state evaluation. Is supplied to the automatic focusing means as reference data. Then, the automatic focusing means detects the focusing position of the lens based on the reference data.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the illustrated embodiments. FIG. 1 is a main block configuration diagram of an electronic image pickup device incorporating an automatic focusing device according to an embodiment of the present invention. As shown in the figure, the present apparatus mainly includes a focusing motor 12, a focusing lens 1 that is driven to be focused by the focusing motor 12, the captured subject light on the photometric element 5 side and the image pickup element 3 side. A half mirror 2 that divides the image into two, an image pickup device 3 that converts a subject image into an electric signal, an image pickup processing circuit 4 that performs image pickup signal amplification and sample hold processing, and an output that outputs a video signal to a video recording / reproducing system (not shown). The terminal portion 7,
A BPF circuit (bandpass filter) 8 for extracting high-frequency components in a video signal and a contrast value for analog / digital conversion are used as contrast values for obtaining contrast data which is information for evaluating the degree of focusing. A first A / D conversion circuit 9, a photometric element 5 for photometrically measuring the subject light split by the half mirror 2, a second A / D conversion circuit 6 for analog / digital conversion of the photometric output, and a CP
Arithmetic processing circuit 10 including U, ROM, RAM, etc.
And a motor drive circuit 11 for driving the focusing motor 12. The pre-sampling means for obtaining the contrast data detected prior to the focus position detection is composed of the image pickup processing circuit 4, the BPF circuit 8, the first A / D conversion circuit 9 and the arithmetic processing circuit 10. Further, the arithmetic processing circuit 10 uses the first data,
In addition, a first data generating unit that generates second data, a second data generating unit, and an automatic focusing unit that determines a focusing position based on the second data are built-in, and further, an imaging signal. The gain designation at the time of amplification, the level of the photometric output, the drive control of the motor drive circuit 11 for driving the lens, and the like are performed.

The focusing operation of the image pickup apparatus having the above-described structure will be described with reference to FIGS. 2 and 3. First, the light amount of the subject light is detected by the photometric device 5, and the second A / D conversion circuit 6 is used to detect the light amount. And is taken into the arithmetic processing circuit 10 as a digital value. The brightness level of the subject is determined based on the data, and when it is determined that the brightness is not low, the focusing process with the normal contrast value is performed. However, in the case of this embodiment, the contrast data is calculated by setting the calculation coefficient Ed which is the first data described later to 1. However, when the subject has low luminance, the gain of the image pickup processing circuit 4 is increased and the contrast value E for several fields (in this case, 9 fields) is pre-selected as shown in FIG. Based on the video signal from the image pickup processing circuit 4 which is a sampling means, it is taken into the arithmetic processing circuit 10 via the BPF circuit 8 and the first A / D conversion circuit 9. Therefore,
Of the contrast values E, the maximum value Emax and the minimum value Emi
The calculation coefficient Ed which is the first data is selected with the lower limit of the noise width Epp which is the difference value of n. This calculation coefficient Ed
The lower limit value of is increased or decreased according to the magnitude of the noise component in the captured contrast value E.

Subsequently, the focusing lens 1 is extended one step at a predetermined focusing speed from j = 0 step (infinity in-focus position), and the contrast value Ej (see FIG. 3) is obtained from the BPF circuit 8, the first A /. It is taken into the arithmetic processing circuit 10 as a digital value via the D conversion circuit 9. At the same time, the arithmetic processing circuit 1
At 0, the contrast value Ej, which is data including noise, is divided by the calculation coefficient Ed, and the corrected contrast value Sj, which is the second data, is calculated. The correction contrast value Sj gives a correction curve less affected by noise as shown in FIG. 3 because the divisor is the arithmetic coefficient Ed corresponding to the size of the noise component. Then, the arithmetic processing circuit 10 simultaneously searches for the peak point a (see FIG. 3) of the corrected contrast value Sj, and once the peak point is detected, the feeding operation is temporarily stopped.
Then, the focusing lens 1 is moved at high speed to the driving step number jp corresponding to the peak point a through the motor drive circuit 11 and the focusing motor 12 to end the focusing operation.

Next, the focusing processing operation of the automatic focusing device of this embodiment will be described in more detail with reference to the flowchart of FIG. When the focus processing command is output, the arithmetic processing circuit 10 in FIG. 4 starts the processing operation and first checks whether or not the subject has low brightness (step S1). If the brightness is not low, the process jumps to step S7 to set the arithmetic coefficient Ed to 1, and further jumps to step S6 to call a subroutine AF process described later. When the calculation coefficient Ed is set to 1, the same processing as the conventional focusing operation is executed. On the other hand, if the subject has low brightness, the process proceeds to step S2 and the subsequent steps. In step S2, the gain of the image pickup signal system of the image pickup processing circuit 4 is increased to correspond to a low-luminance subject. continue,
The pre-sampling operation is performed with the focusing lens 1 fixed in a predetermined position. That is, the contrast value E for n fields is loaded into the arithmetic processing circuit 10, and the maximum value of the contrast value E is loaded as Emax and the minimum value is loaded as Emin (step S3). Maximum value E
The difference between max and the minimum value Emin is set as the noise width Epp (step S4). For example, when Emax is 28 and Emin is 23, the value of Epp is 5. Next, the noise width Epp is set to the lower limit value, the power of 2 (2 m power) is set to the upper limit value, and the calculation coefficient Ed is selected (step S5). For example, if the value of Epp is 5 as described above, and m is 3, then 8 is selected as the value of Ed.

Succeedingly, in a step S6, a subroutine "AF process" is called. The AF process is a process of obtaining the number of drive steps of the in-focus position based on the already set calculation coefficient Ed as described later and driving the focusing lens 1 to the in-focus position. After this "AF process", this routine is ended. In the calculation of the Ed value in step S5, the value of the m-th power of 2 is used as the upper limit value, but this is to speed up the division calculation inside the microcomputer. However, the present invention is not limited to this, and for example, the maximum value Emax or the minimum value Emin may be used as the upper limit value. In that case, the calculation coefficient Ed is given by the following equation. That is, Epp ≦ Ed ≦ Emax or Epp ≦ Ed ≦ Emin.

Next, the processing operation of the subroutine "AF processing" will be described with reference to the flowchart of FIG. First, in step S11, the step number j is 0 step corresponding to the infinity in-focus position, and the descent detection number t is, for example,
Set to 3. Then, the contrast value at the stage number j, in this case, 0 stage is taken in as Ej, that is, E0, and the value is divided by the arithmetic coefficient Ed. The quotient is stored as the corrected contrast value Sj, that is, S0 (steps S12 and S13). In step S14, the focusing lens 1 is driven in the normal direction by one step. Further, the contrast value at the stage number j + 1 is taken in as Ej + 1, and the value is divided by the calculation coefficient Ed. The quotient is stored as the corrected contrast value Sj + 1 (steps S15 and S16). In step S17, the corrected contrast values Sj and Sj + 1
To compare. When the value Sj + 1 is greater than or equal to the value Sj + 1, it is determined that the contrast value is increasing, the number of steps j is incremented (step S19), and the process returns to step S14. Then, the normal rotation drive is continued. If the value S
If j is larger, it is determined that the descent has started, and the process proceeds to step S18 to set the continuous descent count number L to 1.
Further, the value Sj at that time is fetched as the correction maximum value Smax (step S20).

Then, in step S21, the count number L is compared with the drop detection number t (t = 3 in this case). Then, until the values L and t become equal, that is,
Corrected contrast value Sj + 1 or Sj + three consecutive times
Until the value decreases by 2, the process proceeds to step S22, and the focusing lens 1 is driven in the normal direction by one step. Further, the contrast value at the number of steps j + 2 is taken in as Ej + 2, and the value is divided by the calculation coefficient Ed. The quotient is stored as the corrected contrast value Sj + 2 (steps S23, 24). In step S25, the corrected contrast values Sj + 2 and Sj
Compare max. If the value Sj + 2 is larger, it is determined that the contrast value has started to rise again, and the process returns to step S14. If the value Sj + 2 is smaller or equal, it is determined that the descent is continued, and the process proceeds to step S26.
The continuous falling count number L is incremented, and at the same time, the stage number j is also incremented. Then, the process returns to step S21. In the determination in step S21, when the count number L becomes equal to the drop detection stage number t, that is, t
When the corrected contrast value Sj + 2 decreases continuously, the process jumps to step S29. This step S2
In 9, the focusing lens 1 is moved at high speed to the number of steps at which the maximum corrected contrast value Smax is obtained, that is, the number of focusing steps. This completes this subroutine.

As described above, the automatic focusing apparatus according to the present embodiment reduces the noise level of a low-luminance object by pre-sampling in which the contrast data is captured while the focusing lens 1 driving position is fixed. The degree is calculated as the calculation coefficient Ed, and the contrast data including noise is divided by the calculation coefficient Ed to obtain the corrected contrast data that can obtain a more accurate peak point, and the in-focus position is detected by the corrected contrast data. By doing so, even if the subject has low brightness, it is possible to detect the in-focus position more accurately. In addition, the above-mentioned embodiment is a "mountain climbing method".
However, the automatic focusing device of the present invention is not limited to this, and the entire lens drive range, or contrast information of a predetermined drive range,
Of course, the present invention can be applied to a focusing method in which the peak position is obtained based on the information once captured. Further, the subject matter of the present invention can be applied not only to an electronic camera but also to a movie camera or a camera using a silver salt film.

[0015]

As described above, the automatic focusing device of the present invention,
The first data corresponding to the degree of noise is obtained by the pre-sampling operation, and the contrast data including the noise is divided by the first data to obtain the second data with which a more accurate focus position can be obtained. I did it. Therefore, even if the subject has low brightness or the like and the contrast data contains a lot of noise, it is possible to detect the in-focus position more accurately.

[Brief description of drawings]

FIG. 1 is a block configuration diagram of an electronic image pickup apparatus incorporating an automatic focusing device according to a first embodiment of the present invention.

FIG. 2 is a diagram of contrast values obtained by a pre-sampling operation in the automatic focusing device shown in FIG.

FIG. 3 is a diagram of a contrast value including noise and a corrected contrast value for each driving step number in the automatic focusing device of FIG. 1;

FIG. 4 is a flowchart of focusing processing of the automatic focusing apparatus shown in FIG.

5 is a flowchart of a subroutine "AF process" called in the focusing process of FIG.

FIG. 6 is a diagram of a contrast value including noise for each drive step number in a conventional automatic focusing device.

[Explanation of symbols]

4 …………………… Image pickup processing circuit (pre-sampling means) 8 …………………… BPF circuit (pre-sampling means) 9 …………………… 1st A / D converter circuit (pre-sampling means) Sampling means 10 ... Arithmetic processing circuit (pre-sampling means, first data generating means, second data generating means,
Automatic focusing means) E, Ej …………………… Contrast value (contrast data) Ed ………………… Calculation coefficient (first data) Sj, Sj + 1, Sj + 2 …… Corrected contrast Value (second data)

Claims (1)

[Claims] 1. Pre-sampling means for respectively obtaining contrast data based on a video signal formed by the image pickup means at a plurality of predetermined times prior to the evaluation of the in-focus state under the same photographing condition, First data generating means for obtaining first data corresponding to one predetermined value for selecting a difference between the maximum value and the minimum value of the plurality of contrast data obtained by the pre-sampling means as the lower limit value thereof; , The second data corresponding to the quotient obtained by dividing the contrast data obtained over time based on the video signal formed by the image pickup means by the first data is used as the reference data for the focus state evaluation. An automatic focusing device comprising: a second data generating unit for supplying the focusing unit.