JPH0385875A - Automatic focus camera - Google Patents

Abstract

PURPOSE:To attain automatic focus with high focusing accuracy even under low illuminance by eliminating an infrared ray cut-off filter from an optical path only under low illuminance. CONSTITUTION:When it is judged that an object illuminance is low to a degree that a proper exposure cannot be obtained when the shutter speed is not set longer than 1/60sec (one field period) even when an iris mechanism 31 is fully opened based on a photometry data, a low illuminance decision signal LP is fed to a motor 36 from a microcomputer 22. When the low illuminance decision signal LP is received, the motor 36 drives a rotary lever 37 by 180 deg. as shown in the arrow, and the infrared ray cut-off filter 34 is eliminated from the optical path in interlocking therewith. Thus, highly accurate focusing is attained even under low illuminance.

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Industrial Application Field The present invention relates to an auto-7 focus device for a camera that automatically aligns the focus based on a luminance signal in a captured video signal obtained from an image sensor. .

(b) The method of using the image signal itself from the image sensor f to determine the focus control in the autofocus device of the ↑ impact-receiving camera described by Ashigora essentially has parallax1", and the It has many advantages, such as being able to focus accurately even when the depth of field is shallow or for distant objects.Furthermore, it does not require a special sensor for Auto 7 Orcus, and is extremely simple mechanically.

Conventionally, this auto7 orcus method fl -M is
HK Technical Report "5-io, Vol. 17, No. 1, No. 86, page 21, it is written by Ishida et al., "Automatic focus adjustment of a video camera using the mountain climbing servo system," and the so-called mountain climbing servo system is described. It is known as the control ρp. This mountain-climbing servo control is a method in which the lens is moved back and forth in the optical axis direction so that the amount of high-frequency components of the video signal is maximized.

Japanese Unexamined Patent Publication No. 125910/1983 (GC12B7'11
) discloses an example of the above-mentioned so-called mountain climbing auto 7 orcus system. Here, the gist of this prior art will be explained using FIGS. 2 and 3. Fig. 2 is a circuit block diagram of a conventional 6-piece body.
It is input to the llIi value generation circuit (5). The focus evaluation value generation circuit (5) is completely turned off as shown in Figure 3. The vertical sync signal 5 (VD) and horizontal sync signal (HD) separated from the video signal by the sync separation circuit (5a) are input to the gate control circuit (5b) for setting the sampling area as the 7 orcus area. The gate control circuit (5b) outputs a vertical synchronizing signal (VD) and a horizontal synchronizing signal (
A rectangular sampling area is set in the center of the screen based on the fixed oscillator output that drives the HD) and image sensor, and a gate circuit ( 5c).

The luminance signal h corresponding to the range of the focus area extracted by the gate circuit (5C) passes through a high-j pass filter (H, P, F) (5d) to separate only the high frequency components. Amplitude detection is performed in the next stage detection circuit (5e). This detection output is converted into a digital value by an A/D conversion circuit (5f) at a predetermined sampling period, and is sequentially input to an integrator (5g).

Specifically, this integrator (5g) consists of an adder that adds the A/D conversion data and the launch data of the latch circuit in the subsequent stage, and a latch that latches this added value and is reset every 17f. It is a so-called digital integrator that is derived from the circuit,
The first data immediately before the reset, that is, the sum of all A/D converted data for the first field period, is output as a focus evaluation value. Therefore, the focus evaluation value generation circuit extracts the luminance signal within the focus area in a time-division manner, digitally integrates this high-frequency component over one field period, and outputs this integrated value as the focus evaluation value for the current field. I will do it. Immediately after the autofocus operation starts,
The initial focus evaluation value is held in the maximum value memory (6>) and the initial value memory (7).Then, the focus motor control circuit (1o) is controlled by the lens (2) via the focus ring (2).
The second
Monitor the comparator (9) output. The second comparison (9) is 7
Focus evaluation value and initial value memory after orcus motor drive (7
) and outputs their magnitude.

The focus motor control circuit (10) includes a second comparator (9).
) rotates the focus motor (3) in the initial direction until the output is large or small, and the current focus evaluation value is larger than the initial evaluation value and the preset fluctuation range. If an output is made, the rotation direction is maintained as it is, and if an output is made that the current 1: 1 value is smaller than the above fluctuation range compared to the initial evaluation value, the focus motor ( 3) Reverse the rotation direction,
Monitor the output of the first comparator (8).

The first comparator (8) compares the previous maximum focus evaluation value held in the maximum value memory (6) with the current focus evaluation value, and the current focus evaluation value of 66゛ is stored in the maximum value memory (6). There are two types of comparison signals (first mode) that are larger than the content of
PI) (P2) is output. Here maximum value memory (6)
is updated based on the output of the first comparison gain (8) if the current focus evaluation value is larger than the content of the maximum value memory (6), and is always updated to the maximum focus evaluation value up to now. The value is retained.

(13) is the 7 orcus ring (1) that supports the lens (1).
2) 7 orcus ring f-'7. This is a position memory that receives the position signal and stores the focus ring position, and similarly to the maximum value memory (6), the first comparator (8)
) is updated so that the focus ring position when the maximum evaluation value is reached is always maintained. Here, it is a well-known technique that the focus ring (2) is rotated by a focus motor (3), and the lens (1) moves forward and backward in the optical axis direction in accordance with this rotation.

Note that the 7 focus ring position signal is output by a potentiometer that detects the focus ring position, but the 7 focus motor (3) is a stepping motor, and the amount of rotation of this motor toward the near point and zero point is controlled by the positive and It is also possible to use a negative step amount and express f-ti of 7 orcas rings or 7 orcas smokes using this step amount.

The focus motor control circuit (10) includes a second comparator (9).
) The output of the first comparator (8) is monitored while rotating the focus motor (3) in the direction determined based on the output. 8) When the output indicates the second mode in which the current evaluation value is smaller than the preset first threshold value (Δy) compared to the maximum evaluation value (reaches Q in FIG. 4). At the same time, the focus motor (3) is reversed. After this reversal, the contents of the position memory (13) and the current 7-occus ring position signal are compared in a third comparison 5 (14), and when they match, that is, the 7-occus ring (2) is the focal point evaluation value. The focus motor control circuit (
10) works. At the same time, 7 units motor control circuit (
10) outputs a lens stop signal (LS) to complete the focusing operation.

By the way, in general video cameras that use a solid-state image sensor (CCD), in order to bring the spectral sensitivity characteristics of the image sensor closer to human visibility characteristics, an infrared cut filter (hereinafter referred to as a red ray cut filter) is installed in the optical path between the lens and the image sensor. (referred to as an outer cut filter) is inserted.

The transmittance characteristic of this infrared cut filter is a curve like the one-dot chain line (A) in FIG. In FIG. 5, the horizontal axis represents the wavelength of the incident light, and the vertical axis represents the relative sensitivity. Also,
The solid line (B) in Figure 5 shows the spectral sensitivity characteristics of the CCD, and the infrared cut filter (4
By inserting 5) as the second factor, the substantial spectral sensitivity characteristic of the CCD becomes as shown by the chain line (C). This chain line (
It can be seen that the T spectral sensitivity characteristic shown in C) is close to the visibility characteristic of the human eye shown in FIG. Said Oh I・
Although the focus device is suitable for use in video cameras, this device can also be applied to imaging devices such as so-called electronic still cameras, as described in the Technical Report of the Television Society of Japan dated December 22, 1988. An example of this is disclosed in ``-P Still Camera Development''.A block diagram of this circuit is shown in FIG. 7, and each circuit operation is similar to the flowchart in FIG. Here, this prior art will be explained.

When the release 5W (21) is pressed, the microcomputer (22) turns on the power of all circuits. In this power ON state, the spindle motor (24) is started by the spindle motor control circuit (23) and spindle servo is applied so that the spindle motor (24) rotates at a constant speed (3600 rpm). At the same time, photometry is performed by a photometry circuit (25) using a photodiode.

The microcomputer (26) is prepared in advance with an appropriate aperture value and shank speed for each photometric value, so when the photometric value is input from the photometry circuit (25), the appropriate aperture value and shutter speed are set. The speed is uniquely determined by the microcomputer (26). (When lfi is determined, the mechanical aperture mechanism (31) is controlled by the microcomputer (22).
), an aperture command is issued, and the aperture mechanism (31) optically apertures the incident light so as to maintain the aperture value.

The timing generator (27) also includes an imaging circuit (4).
), and since the electronic still camera of this embodiment employs an electronic shutter, the timing generator (27) also controls the shutter speed of this image sensor. It will be held at At prompt 9, as mentioned above, the microcomputer (22) runs Jabutaspi 1! Once this shank speed is determined, the timing generator (27) controls the driving of the image sensor based on this shank speed.

Once the proper exposure condition is secured in this way, the microcomputer (2
2) causes the autofocus circuit (30) within the chain line to perform the focusing operation as described above to bring the lens (1) into focus. The imaged video signal obtained after completion of this focusing is processed as a main exposure output by the camera processing circuit (28) as a video signal such as γ correction, and further processed by the recording processing circuit (29). Signal processing such as FM modulation is performed, and the signal is recorded on the video flow controller (26), thereby completing a series of recording operations.

(c) Problems to be Solved by the Invention In order to obtain high focusing accuracy with a camera using the auto7 orcus method that utilizes the high-frequency I component of the luminance signal in the video signal, it is necessary to extract the focus evaluation value. The easier it is, that is, the larger the amplitude of the video signal output from the imaging circuit is, the more convenient it is.

In general, the above-mentioned autofocus system can perform focusing operations with relatively high accuracy even in low illuminance, but in extremely low illumination where the amplitude of the video signal is less than 5% of the optimum illuminance. The focus evaluation value becomes significantly smaller and the focusing accuracy decreases.

(d) Means for Solving the Problems The present invention removes the infrared cut filter from the optical path between the lens and the image sensor during the focusing operation period in ultra-low illumination conditions, and replaces it with an infrared cut filter. It is characterized by inserting an optical member such as glass or transparent plastic with a thickness that can compensate for the optical path length between the lens and the image sensor when a filter is inserted.

(e) Effect The present invention is capable of obtaining sufficient image amplitude by extracting the video signal at the wavelength that was removed by the infrared cut filter even in ultra-low illumination conditions as described above, and the focus evaluation value can be set to a large value, making it possible to perform highly accurate focusing operations.

(f) Example An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows the entire circuit prongs of this embodiment. In FIG. 1, the same parts as in FIG. 7 are designated by the same reference numerals, and their explanation will be omitted.

In the figure, what is different from Figure 7 is the R degree value generation circuit (4
1), L E D (32), infrared force/notofino/reduction mechanism (33), and strobe (50) add 14 t. The brightness evaluation value generation circuit (41)
As shown in the figure, L P F (41a), A/D converter (41
b) and the product 12B(4tC), L
The P F (41a) extracts the low frequency 62 minutes of the luminance signal in the video signal from the image pickup circuit (4) and converts it into A/D! (41b) performs A/D conversion at a predetermined sampling period, and then integrates this A/D conversion value! (41c) 1
It is integrated over a field period, that is, it is digitally integrated for one field, and this integrated value is output to the microcomputer (22) as a brightness evaluation value indicating the average brightness level in the field. Note that the gate circuit (5C) of the focus evaluation value generation circuit (5) is connected to the Mi stage of L P F (41a).
) may be inserted to detect only the brightness level at the center of the screen.

■, El) (32) emits infrared light only when the brightness evaluation value is below a predetermined value (Z), that is, when the microcomputer (22) determines that there is no proper exposure condition and the average brightness level is low. do.

The infrared cut filter insertion/removal mechanism (33) is controlled by an infrared cut filter (34), a glass plate (35), a motor (3G), and a rotating lever (37). That is,
As shown in Figure 10, the shaft (36) of the motor (36)
A) is fixed at the center of a rotating lever (37) to which an infrared cut filter (34) and a glass plate (35) are fixed at both ends. Then, while the proper exposure state is maintained, the infrared cut filter (34) is placed in the optical path between the lens (1) and the CCD that controls the imaging circuit (4) so that it is perpendicular to the round axis. When the subject illuminance is extremely low and proper exposure cannot be obtained, the motor (36) is driven and the co-moveable lever (37) is rotated 180 degrees to remove infrared light. 5. Based on the command from the microcomputer (22), the filter (34) is taken out of the optical path, and the glass plate (35) is then inserted into the optical path to stop the motor (36) at this position. It works.

Next, the operation of the apparatus of this embodiment will be explained with reference to the flowchart shown in FIG.

Based on the photometric data obtained from the photometric circuit (25), the microcomputer (22) determines the appropriate aperture value and shutter speed. is less than 1/60sec, so
Infrared power y in the optical path between lens (1) and CCD (4a)
A rotating lever (37) is attached to the column into which the filter (34) is inserted.
) is fixed11. The incident light that has passed through the lens (1) passes through the infrared cut filter (34), where the S-wavelength light is photoelectrically converted by the CCD (4a) and output as a video signal. As in the rice production example, the lens position was set at the position where the integral value for one field of the high-frequency component of the luminance signal in image 4A is maximum, taking into consideration the visibility characteristics of the human eye. Automatic focusing operation (AF operation) to maintain is performed (S
TEP (100)), the captured video signal obtained after completion of this AF is recorded on a video floppy as the main optical output. Also, when the subject is extremely dark, the appropriate exposure value may be
I may not be able to do it. That is, based on photometric data,
Even if the diaphragm flft (31) is fully opened, the brightness of the subject is such that the correct exposure value cannot be obtained unless the shutter speed is longer than 1/60 sec (1 field period). If it is determined that 1 is low, the microcomputer (
22), a low illuminance determination signal (LP) is supplied to the motor (36).

When this low illumination judgment signal (1, P) is received, the motor (3
6) The rotating lever (37) is 18 as shown by the arrow in Figure 10.
The infrared cut filter (34) is removed from the optical path, and the glass plate (3) symmetrically arranged around the shaft (36a) is rotated by 0°.
5) is inserted into the optical path (STEP (10)).

Here, the thickness (t,) of the glass plate (35) is
The thickness of the throat filter (34) is (tl), the refractive index O1,
), and the refractive index (n, ) of the glass plate (35) itself is:
It is shown by the following formula.

nl-1n warehouse 1,: X Xt, ...(
a) n+ n, -1 Equation (a) is the optical path length (Ll) between the lens and the CCD when the infrared cant filter (34) is inserted between the lens (1) and the CCD (4a). This corresponds to the conditional expression for equalizing the optical path length (L,) between the lens and the CCD when the glass plate (35) is inserted instead of the infrared power/nod filter (34).

In addition, if the optical path length can be made equal in this way,
An optical member such as plastic may be substituted for the glass plate.

In this way, even when the glass plate (35) is inserted into the optical path, the optical path length does not change at all, and the CCD
In (4a), light with a long wavelength that was removed by the infrared cut filter (34) is also photoelectrically converted. Therefore, in order to give priority to highly accurate focusing operation over the visibility characteristics of the human eye, the high-frequency components of the luminance signal in the video signal formed by the photoelectric conversion output, which does not include long wavelength light, are 1
The integral value of f1-)-d is obtained as the focus evaluation value.

By the way, in a situation where the glass plate (35) is inserted into the optical path, if the brightness evaluation value for one field reaches a predetermined value, the microcomputer (22) calculates the focus evaluation value necessary for AF operation. 5TEP (102)) to determine that the amplitude of the video signal necessary for detection has been obtained in the f minute, and move the lens to the position where the obtained focus evaluation value is maximum (AF operation that maintains Iilt). (STEP (+03)).

・On the other hand, if the brightness evaluation value does not reach the predetermined value of 1 under the same situation, the microcomputer (22 inches) should judge that the amplitude of the video signal is not sufficiently obtained.STEP (102)
Issue a light emission command to L E D (32), and L E D
(32) lights up ( S T E P (103)
). In addition, this +-E D (32)・\ light emission command is
L E D (32) until the brightness evaluation value reaches the predetermined value
Control so that the amount of light emission gradually increases ( S T E
P (104)).

When the subject is illuminated with external light emitted from L E D (32) in the G light, the video signal amplitude increases,
The brightness evaluation value will reach a predetermined value.

In this way, when the brightness evaluation value reaches a predetermined value, this L E
D (32) A based on the focus evaluation value obtained in the lit state
The F operation is performed (STEP (105)), and when the lens (1) reaches the in-focus position where the focus evaluation value is maximum and the focusing operation is completed, the L E D (32) is extinguished. (STEP (106)) 6 glass plates (
35) is inserted, when the AF operation is completed and the lens (1) reaches the focusing position, the microcomputer (22) drives the motor (36) to move the rotating lever (37) as described above. The infrared cut filter (34) was inserted into the optical path in place of the glass plate (35) by rotating it 180° in the opposite direction (
After S T E P (107)), strobe light (50
) is emitted, and the captured image signal obtained immediately after this is supplied as the main exposure output to the process circuit (28) and the recording signal processing circuit (29), and is recorded on the video 70/nobby (26).

In this way, if the subject is in extremely low illumination and it is not possible to obtain an appropriate exposure condition, the infrared cut filter (34) can be removed from the optical path only while the AF operation is being performed to achieve high precision AF operation. This makes it possible to insert an infrared cut filter (34) only during main exposure and record a video signal that matches the visibility characteristics of the human eye.

In this embodiment, the lens (1) was moved back and forth in the optical axis direction for AF operation to reach the in-focus position, but the lens itself was fixed and the focus morph control circuit (10) output was adjusted accordingly. It is also possible to use a bimorph or the like to move the CCD (4a) back and forth in the optical axis direction to reach the in-focus position.

In addition, the strobe (50) is exactly the same as the one installed in a normal still camera, and emits light momentarily when the AF operation is completed to compensate for the proper exposure state during the main penetration. play a role.

(G) Effects of the Invention Since the present invention is configured as described above, it is possible to achieve autofocus with high focusing accuracy even in low illumination while matching the recorded video signal with the visibility characteristics of the human eye. .

[Brief explanation of drawings]

1, 9 to 11 relate to one embodiment of the present invention, in which FIG. 1 is an overall circuit block diagram, FIG. 9 is a main part circuit block diagram, and FIG. 10 is a main part mechanism explanatory diagram. , FIG. 11 is a flowchart. In addition, Figs. 2, 3, and 7 are circuit block diagrams of the conventional example, Fig. 4 is a diagram explaining the operation of mountain climbing AF, Figs. The figure is a flowchart of a conventional example.

Claims (1)

[Claims] (1) A focus control means that performs a focusing operation using an infrared cut filter inserted in an optical path between a lens and an image sensor, and a high-frequency component of a luminance signal in a captured video signal obtained from the image sensor. An autofocus camera comprising: The infrared cut filter is removed from the optical path only during low illumination.