JPH0683387B2 - Focus detection device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an apparatus for performing focus detection based on an output signal of an image pickup unit, and particularly, with high sensitivity of a video camera or the like, highly accurate focus detection can be performed. Focus detection device capable

(Prior Art) As a so-called TTL-passive automatic focus adjustment device mainly used for video cameras, which detects a focus from a luminance signal in a video signal, "NHK Technology Research" Vol. 17 No. 1 (Vol. ) (Published in 1965), "The automatic focus adjustment of the TV camera by the hill climbing servo system" and various systems have been proposed. Many of these methods use the phenomenon that the higher the high frequency component in the luminance signal becomes as the image of the subject becomes sharper, and this high frequency component is taken out by some kind of signal processing means, and the position where this peaks is taken as the focus. This is due to the principle of

By the way, as will be described in detail later, in the conventional video camera, since the depth of field is deep on the short focus side, even if there is a high probability of near-far conflict on the short focus side, it will be described later in FIGS. 10 and 11. Although a constant range-finding field was set as shown by 24 in the figure, it has become necessary to obtain accurate focus even on the short focus side as the sensitivity of the camera increases.

(Object) To solve the above-mentioned problems, the present invention provides a focus detection device that performs focus detection based on an output signal of an image pickup unit, which is likely to occur under a certain distance measuring field of view. It is an object of the present invention to provide a means capable of reducing an erroneous distance measurement due to and performing highly accurate focus detection.

(Description by Embodiments) Means taken in the present invention for achieving the above-mentioned object will be illustrated and described below with reference to the embodiments shown in the drawings. The following description is an example in which the present invention is applied to a focus detection device of a type that performs focus detection from a high frequency component of a luminance signal in an output signal of a two-dimensional image sensor, a conventional focus detection device and a focus detection device of the present invention The configuration of one embodiment and the order of the focusing process are performed.

(Conventional Focus Detection Device) (FIGS. 1 to 11) FIGS. 1 to 4 schematically explain the principle of focusing the position where the high frequency component in the luminance signal has a peak. It is a thing. FIG. 1 shows a state of an image on an image forming surface when a subject having a black and white stripe is picked up by a video camera, and a portion in which a vertical line is drawn corresponds to a black portion. . When the image plane is in focus, it becomes as shown in FIG. 7A, whereas when it is out of focus, the black and white boundary of the subject is blurred as shown in FIG. FIG. 2 shows the luminance signal (Y) during output of the image sensor in each of the above states.
Signal), and naturally the in-focus state (A) has a larger output difference depending on the position than (B). That is, the closer to the in-focus state, the higher the contrast. Several proposals have been made for the means for processing the Y signal. For example, when this signal is differentiated to be an absolute value, the peak of the differential signal at the in-focus position as shown in FIGS. 3 (A) and 3 (B). There are some that utilize the phenomenon that maximizes. In order to further improve the S / N, a means may be adopted in which the differential signal is squared and then integrated.

FIG. 4 shows the output of the high-frequency component detected as described above on the vertical axis, and the stop position of the lens group involved in the focus adjustment of the photographing lens on the horizontal axis. The high-frequency component has a peak at the in-focus position A, and the peak is off at the out-of-focus position B.

FIG. 5 shows an example of a sequence for executing the focus detection operation shown in FIGS. 1 to 4, and this sequence is
The focus detection operation of the present invention is also applied as an example. Simultaneously with the start of the distance measurement, the Y signal of the portion corresponding to the distance measurement visual field is taken out by means described later, and the high frequency component thereof is extracted. Then, the lens is
In this example, driving is performed in the direction of renormalization (generally, by renormalization, a distant object is focused). In FIG. 5, the symbol “→ F” indicates that the lens is driven in the retraction direction, and the symbol “→ N” indicates that the lens is driven in the retraction direction. In this state, the high frequency component is extracted again. The reason why the high frequency components at the two lens positions are extracted at the time of starting the distance measurement is to detect the direction (front pin or rear pin), and these two signals are compared.

FIG. 6 shows the relationship between the magnitudes of the high-frequency components A and B extracted in the above and and the state of the focus detection system. In FIG. 6 (A), A <B, Since it is in a state, it is necessary to further retract the lens (), and in the figure (B), it can be regarded as a mating pin because A≈B (), and in the figure (C), A> B and then Since it is in a pin state, it is necessary to further extend the lens (). As a result of the direction detection in this way, the high frequency component is extracted again at the new position where the lens is driven, and the high frequency component is compared with the value before driving (). At this time, the current value is used for comparison in the next cycle, so it is sampled and held (S / H), and is taken out at the next high frequency component extraction and used for comparison. In this way, when the condition of A≈B is reached, it is determined that the lens is in focus and the driving of the lens is stopped.

FIG. 7 shows an example in which the above-mentioned means for taking out the Y signal is incorporated in an actual camera. In the figure, 10 is a lens group involved in focus adjustment among photographing lenses, and 11 is a zoom system. A lens group that normally consists of a variator lens and a compensator lens, 12 is a lens group of an imaging system, 13 is a solid-state image sensor of an image sense type such as CCD, 14 is a clock signal generator, and 15 is a frequency divider. , 16
Is a CCD drive circuit, 17 is a horizontal and vertical sync signal generator, 18
Is an analog gate, and 19 is an automatic focus adjustment circuit. Specifically, direction detection is performed by the high frequency component as described above. Reference numeral 20 is a microprocessor, and the motor drive circuit 21 operates based on the command to drive the motor 22.

In the above configuration, the clock signal generated by the clock signal generator 14 is frequency-divided by the frequency divider 15 to generate a screen in 1 field 1/60 second based on the standard television signal in the NTSC or PAL system. Then, a signal that determines the read timing of the CCD 13 in one horizontal period is formed. Based on this signal, the CCD drive circuit 16 sequentially reads the optical information accumulated on the CCD 13. The read signal is added with a synchronization signal by the adder circuit 17A and further processed by a processing circuit (not shown) in a known manner to form a video signal.

On the other hand, the frequency divider 15 simultaneously outputs a signal for opening the gate only at a position corresponding to a predetermined distance measuring field of view (for example, a position shown by 24 in FIG. 8), and this part is passed through the analog gate 18. Only Y signal of auto focus control circuit
Sent to 19. The subsequent operation is as described above.

FIG. 9 shows an example of a signal at each position of D to G in FIG. 7, (D) is a raw signal just read from the CCD 13, and (E) is an adder 17A to this signal. The signal on which the synchronization signal is superimposed in (F) is the synchronization signal for automatic focus adjustment, and (G) is the Y of the portion corresponding to the distance measuring field 24 taken out for automatic focus adjustment. The signals are shown respectively.

FIGS. 10 and 11 show images in the window of the subject when the subject 32 is photographed by the video camera configured as described above, and FIG. Indicates the case where the subject 32 is photographed at a focal length that is relatively short. As can be seen from this figure, in general, when photographing with a short focal length, the proportion of the subject in the screen is often smaller than when photographing with a long focal length. Even if the image is taken at a long focal length, the subject distance is often distant if the subject is small. Considering these points, if the position of the distance measuring field 24 is set as shown in Fig. 10 and Fig. 11, the wrong distance measurement due to the perspective conflict will occur when shooting at the long focus side. Is unlikely to occur, but this tends to occur on the short focus side. Note that reference numeral 23 in the figure denotes an imaging visual field.

On the other hand, the distance measuring field position 24 shown in FIG. 10 and FIG. 11 can be made smaller for the purpose of preventing perspective conflict.
In this case, the probability of measuring the distance to a non-contrast portion of the subject is increased mainly on the long focus side, which is not preferable. Therefore, with conventional video cameras, the depth of field on the short focus side is deep, so even if there is a high probability of near-far conflict on the short focus side, the range of view as large as that shown in 24 above The position is set. However, as the sensitivity of the camera becomes higher, it is necessary to obtain accurate focus even on the short focus side if possible.

Further, FIGS. 10 and 11 can also be considered as the difference in subject distance. That is, FIG. 10 shows that when the subject is at a relatively short distance, the range captured by the distance measuring field of view 24 is almost all the subject 32. In the case of long distance, perspective competition may occur as shown in Fig. 11. Also in this way of thinking, since the depth of field becomes deeper as the subject distance becomes farther, it does not cause a big blur even if a perspective conflict occurs.
It was said that a range-finding field having a size of 24 is suitable. However, also in the above concept, as the taking lens has a higher magnification and the F value becomes smaller, higher distance measuring accuracy is required.

(Structure of Embodiment of Focus Detection Apparatus of this Invention) (Twelfth Embodiment
(FIG. 13, FIG. 13) This invention eliminates the above-mentioned drawbacks of the conventional focus detection device, reduces the false distance measurement due to the near-far competing object which tends to occur under a constant distance measuring field, and achieves high accuracy. The focus detection is performed. That is, when further examining the above-mentioned drawbacks of the conventional device, if the distance measuring field is too large, perspective competition may occur, and conversely, if the distance measuring field is too small, the contrast of the subject may not be well captured. It is because there is.

Therefore, according to the embodiment of the present invention, when the minimum distance measuring field of view is set at the start of shooting or at the time of detection of out-of-focus, and a signal component sufficient to enable the focus detection operation is not obtained from the output of the image pickup unit. Try to enlarge the distance measuring field. The minimum and maximum distance measuring fields depend on the design intent, but for example, the 12th
The visual field 25 in the figure is the minimum visual field, and the visual field 24 in FIGS. 10 and 11 is the maximum visual field. Comparing FIG. 11 and FIG. 12, it is clear that the distance measuring field 25 of FIG. 12 is superior when the subject 32 is small on the screen due to a long distance or a wide distance. Is.

FIG. 13 shows an embodiment of the focus detection device of the present invention,
Portions having basically the same configuration and function as those of the apparatus shown in FIG. 7 are designated by the same reference numerals as those in FIG. 7, and detailed description thereof will be omitted. Reference numeral 26 in FIG. 13 denotes a programmable logic array (PLA) which is an example of a gate control circuit and controls a signal for selecting a distance measuring field of view. At the start of shooting, PLA2
The gate signal from 6 corresponds to the minimum distance measuring field as shown by 25 in FIG. In this small distance measuring field of view, if the direction of the front focus, rear focus, etc. cannot be detected, that is, if the contrast cannot be obtained, the microprocessor 20 connected to the automatic focus adjustment circuit 19 cannot detect the focus. Feed back a signal indicating that to PLA26. Here, focus detection is impossible means that high frequency components A and B in FIG.
Does not substantially differ from A + B or A or B
Depending on the conditions such as not exceeding a certain level, the front pin,
It means that it is not possible to detect any state of the rear pin or the dowel pin.

PLA 26 receives the above signal and prolongs the time for opening gate 18. In this way, the distance measuring field of view is set to the minimum (for example, 25 in FIG. 12) at the start of shooting, and if focus detection is not possible in this field of view, the distance measuring field of view is gradually expanded until it becomes possible. Is.

(Focusing Process of One Embodiment of Focus Detection Apparatus of the Present Invention)
(FIGS. 12 to 15) FIG. 14 shows the Y signal, the gate signal and the Y extracted from the gate in the process until focusing in the embodiment of FIG.
FIG. 15 shows the manner in which the signal changes, and FIG. 15 also shows the relationship between the lens stop position and the high frequency component output.
In the same figure, the horizontal axis is the stop position of the range ring as in FIG. 6, N is the maximum position where the focusing distance is at the closest end,
∞ represents the maximum renormalization position, and the vertical axis represents the level of high frequency components.

First, FIG. 14 (A) includes a distance measuring field of view at the start of shooting.
The image signal in the horizontal period is schematically shown. In this case, the subject (area e) is at a distance of l in FIG. 15, but the distance n behind it is a background (area f) with high contrast.
Further, the stop position of the distance ring at the beginning of shooting is assumed to be the position where the distance m is the in-focus position. Figure 14 (B)
Indicates the gate signal output from the PLA 26 in this case. As described above, the gate 18 has a minimum ON period, and the Y signal of the subject extracted from the gate 18 has no contrast, so direction detection is possible. Without focus detection. The high-frequency component from the distance measuring field shown in the figure is
It is represented by curve 103 in FIG. A curve 103 indicates that a hill-climbing curve appears only in the vicinity of the focus because it detects only a portion on the subject where there is almost no contrast in the distance measuring field, and that it does not appear at other positions.
It should be noted that the fact that a mountain climbing curve appears near the focus means that
In the Y signal of FIG. 14A or 14C, it is shown that a small contrast that cannot be discriminated due to defocus is actually present. In this case, since the stop position of the lens is at the position m in FIG. 15, the high-frequency component obtained from the Y signal in FIG. 14 (C) is at the level without inclination indicated by the point C in FIG. Detection is impossible.

When it is determined that the focus cannot be detected, a signal indicating the fact is transferred from the microprocessor 20 to the PLA 26, and the gate signal has a longer ON period. FIG. 14 (D) shows the gate signal in this case, and FIG. 14 (E) shows the Y signal extracted by the control of this gate signal. The high frequency component corresponding to this new distance measuring field is the 15th
Shown by curve 102 in the figure. In FIG. 14 (E), the contrast portion is captured at both ends thereof, so that the high frequency component has a level indicated by E in FIG. Since focus can be detected sufficiently at this position, the lens 10 is pushed out by the operation of the above-described automatic focus adjustment system, and the focus is slightly more focused than in FIG. 14 (A), so the Y signal is The contrast of the background becomes gentle and the contrast of the subject becomes clear as shown in FIG. Since focus detection is possible in this distance measuring field, the PLA 26 outputs the same gate signal as in FIG. 7D, and the Y signal extracted by this is shown in FIG. The high frequency component output at this position is at the level indicated by G in FIG.

The Y signal extracted in the next step by repeating the above operation is shown in FIG. 14 (I), and the high frequency component becomes the level shown by the peak I of the curve 102 in FIG. It turns out that The Y signal in one horizontal period including the distance measuring field in the focused state is shown in FIG. 14 (H),
It can be seen that the subject is in focus and the background is blurred as compared with FIG. When the microprocessor 20 detects out-of-focus after reaching the in-focus state, it returns to the minimum distance measuring field (25 in FIG. 12) again and repeats the above-mentioned focusing process. To control.

A curve 101 in FIG. 15 shows a case where the distance measurement is always performed in the maximum distance measurement field without adjusting the distance measurement field as described above. It has shifted to the middle of the distance n. In the state shown in FIG. 11, there is a high possibility of erroneous distance measurement such as the curve 101, but according to the above-described embodiment of the present invention, accurate focus detection is possible as long as the subject has a detectable contrast. Can be done
The accuracy of the distance measuring system or the focus adjusting system is improved.

In the above-mentioned embodiment, the two-dimensional image sensor is used as the image pickup means, and the focus detection is performed by the high frequency component of the luminance signal in the output signal. It may be an image sensor.
Alternatively, it can also be applied to the case where focus detection is performed by the high frequency component of the output signal of one or a part of the columns of the image sensor including the interline CCD. Furthermore, the present invention is also applied to an apparatus that does not use high frequency components, such as an apparatus that divides the distance measurement visual field corresponding position in the imaging means into two parts and compares the outputs of these two parts to perform focus detection. can do. Further, in order to control the gate 18 in FIG. 13, a known gate control circuit can be used in addition to the programmable logic array 26 in FIG.

(Effect) As described in detail above, according to the present invention, in an apparatus that performs focus detection based on the output signal of the image pickup means, the extraction range of the output signal of the image pickup means is made variable, and at the start of shooting or when non-shooting is performed. When the focus is detected, it is operated in the minimum extraction range of the above-mentioned extraction ranges. , And highly accurate focus detection or automatic focus adjustment can be performed.

[Brief description of drawings]

1 (A) and 1 (B) are explanatory views that schematically show the state of the subject image on the imaging surface of the image pickup apparatus when focused and out of focus, respectively, and FIGS. 2A and 2B. FIG. 3B is an explanatory diagram showing the luminance signal during output of the image sensor in the states of FIGS. 1A and 1B, and FIGS. 3A and 3B are respectively FIG. 2A. And (B) are explanatory views showing signals obtained by differentiating the signals, FIG. 4 is a diagram showing the relationship between the lens position of the image pickup device and the high frequency component in the output of the image sensor, and FIG. 5 is the focus detection operation. Explanatory diagrams showing an example of the sequence, FIGS. 6 (A), (B) and (C) are explanatory diagrams showing the relationship between the state of the focus detection system and the high frequency components in the output of the image sensor, respectively. Is a block diagram of a conventional focus detection device, FIG. 8 is an explanatory diagram showing a standard distance measuring field position, and FIG. D), (E), (F) and (G)
Are waveform charts showing signals at D, E, F and G in FIG. 7, respectively, and FIG. 10 is a case where an object is photographed at a focal length of a relatively long focus in an image pickup apparatus using a conventional focus detection apparatus. 11 is an explanatory view showing a state of an image in the finder screen of FIG.
Similarly, FIG. 12 is an explanatory view showing a state of an image when the image is taken at a focal length from a relatively short focal point, and FIG. FIG. 13 is a block diagram of an embodiment of the focus detection apparatus of the present invention, FIG. 14 is a waveform diagram for explaining the operation of the embodiment of FIG. 13, and FIG. A luminance signal for one horizontal period is shown in FIG. 7B, which is also a gate signal, and in FIG.
FIG. 6D shows a gate signal when the ON period is long, FIG. 6E shows a luminance signal extracted by controlling the gate signal of FIG. The luminance signal in one horizontal period in a focused state, as compared with the case of FIG. 7A, the luminance signal taken out from the gate in FIG. 9G, and the horizontal signal in the focused state in FIG. FIG. 15 (I) is a waveform diagram showing the luminance signal during the period, and FIG. 15 (I) is a waveform diagram showing the luminance signal similarly taken out from the gate.
FIG. 15 is a diagram showing the relationship between the position of the lens and the high frequency component in the output of the image sensor in the embodiment of the figure, corresponding to FIG. 14; Explanation of code 10 …… A group of lenses that are involved in focus adjustment among the taking lenses,
13: a two-dimensional image sensor, which is an example of imaging means, 18
...... Analog gate, 19 ...... Automatic focus adjustment circuit, 20 ......
Microprocessor, 22 ... Motor, 24, 25 ... Distance measuring field, 26 ... Programmable logic array which is an example of gate control circuit.

Claims (1)

[Claims] 1. A focus detection device for performing focus detection by an output signal of an image pickup means for converting an object image formed on the image pickup screen into an electric signal, the output signal of the output signal on the image pickup screen of the image pickup means. The extraction range setting means for setting the angle of view of the extraction range, the focus detection means for detecting the focus state based on the image pickup signal corresponding to the extraction range, and the extraction range setting means based on the output of the focus detection means. Control means for controlling and gradually expanding the extraction range of the output signal on the imaging screen, wherein the control means detects the focus by the focus detection means in a state where the extraction range of the small angle of view is set. If the low contrast state is impossible, the extraction range set by controlling the extraction range setting means is gradually expanded to a large angle of view. When the focus detection means determines that the focus is largely out of focus in the extraction range of the enlarged large angle of view, the extraction range is forcibly reduced to a small angle of view, and the focus detection operation is performed. A focus detection device characterized by being configured to perform.