JPH0711621B2 - Automatic focus detection method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of use] The present invention is used for a photographing device such as a video camera, a still camera, etc., and in particular, spot light such as infrared light is projected onto a subject and its reflected light is reflected. The present invention relates to a so-called active automatic focus detection method that receives a focus detection signal by receiving light with a pair of light receiving elements.

[Prior Art] In such an active automatic focus detection method, a conventional example in which focus detection is performed based on a difference between outputs from a pair of light receiving elements will be described first with reference to FIGS. 3 to 6.

In FIG. 3, spot light is projected from the light emitting element 2 pulse-driven by the pulse oscillator 1 onto the subject S, and the reflected light reflected by the subject S passes through the condenser lens 3 to form a pair of light receiving elements 4 a and 4 b. Incident on. In this case, the light receiving elements 4a and 4b are movable as shown by arrows along with the movement of the taking lens (not shown). Outputs from the light receiving elements 4a and 4b are passed through amplifiers 5a and 5b, respectively, and unnecessary external light components are removed by the synchronous detection circuits 6a and 6b which are synchronized with the pulse oscillator 1, and are output to the integrators 7a and 7b at the next stage. Is entered.

The outputs Sa and Sb of the integrators 7a and 7b gradually increase with the elapse of time t, as shown in FIG. 4 (a). In this case, as shown in the figure, if, for example, Sa <Sb, the light intensity of the spot light incident on the light receiving element 4b is much larger than that of the spot light incident on the light receiving element 4a, then out of focus. is there. In the in-focus state, there is almost no difference between the outputs Sa and Sb, and Sa≈Sb. Outputs Sa and Sb from integrators 7a and 7b
Is further input to an adder 8 for adding both, and a subtracter 9 for obtaining the absolute value of the difference between the outputs Sa and Sb. The outputs of the adder 8 and the subtractor 9 are respectively predetermined threshold values V1 and V2 ( V1>
V2) is input to the comparators 10 and 11, and the outputs of the comparators 10 and 11 are connected to the arithmetic processing circuit 12, respectively.

FIG. 4 (b) shows the output of the adder 8, and this output Sa + Sb
Gradually increases with time t, and the comparator 10 outputs an H level at time t1 when the threshold value V1 is reached. (C) is the output of the subtractor 9, and the output | Sa−Sb | gradually increases with time t, and the comparator 11 outputs the H level at the time t2 when the threshold V2 is reached. These output H levels are used by the arithmetic processing circuit 12 to determine which time is earlier input or how soon it is input.

For example, when the time t2 when | Sa−Sb | = V2 is input earlier than the time t1, the relationship Sa = Sb is broken, that is, the state is out of focus. If the time t1 when Sa + Sb = V1 is earlier before | Sa−Sb | = V2, then Sa
It is possible to determine that the state is close to ≈Sb and that it is close to the focused state.

The above description can be restated as follows. That is,
As shown in FIG. 5, the time t1 when Sa + Sb reaches the threshold value V1
Is detected, and the level of | Sa−Sb | at this time t1 is determined. Then, if | Sa−Sb |> V2, it is out of focus, and | S
When a−Sb | <V2, it is determined to be in focus.

However, such a conventional active type two-element output differential type automatic focus adjustment system has a major drawback described below. That is, in the conventional method, when the distance to a subject having a high reflectance and a short distance is measured, the outputs of the light receiving elements 4a and 4b become very large, so that the amplification characteristics of the amplifiers 5a and 5b are saturated, and Two integrators 7a, despite being in focus,
There is a problem that the output of 7b becomes equal and the motor that moves the projection lens stops. In order to solve this problem, the amplification gain of amplifiers 5a and 5b is made variable,
A method of lowering the gain has been proposed for a subject having high reflectance. However, when switching the gain, 2
The gain balance of the two amplifiers 5a and 5b is slightly shifted due to the constant error of the circuit. Further, since the gain changes and the input levels of the integrators 7a and 7b change and the integration time changes, the S / N ratio of the outputs of the integrators 7a and 7b changes. For these reasons, there is a drawback that the motor control becomes unstable at the time of switching the gain, and the photographing lens moves slightly.

A switching diagram of gain switching according to the conventional example is shown in FIG. 6, and the vertical axis represents the number of light emission pulses when the integrated value reaches a predetermined output. The received light amount R detected by the light receiving elements 4a and 4b is R
In R1, the gain is set to 64 times, in R1 <R <R2, it is set to 8 times, and in R2 <R, it is set to 1 time. Generally, the light receiving element
Noise is superimposed on the output signals obtained by 4a and 4b, and even if the distance and the reflectance of the subject are constant, the received light amount R slightly changes with time. For this reason, if the received light amount slightly changes for a subject under the gain switching condition, the gain is complicatedly switched, and the motor control becomes unstable due to the change of the gain balance and the change of the integration time.

[Object of the Invention] It is an object of the present invention to provide an automatic focus detection method which makes variable the amplifier gain of the light receiving element output and has a hysteresis characteristic in the switching level at the time of switching to stabilize the motor control. is there.

[Summary of the Invention] The gist of the present invention for achieving the above-mentioned object is to provide a light projecting means for projecting a spot light toward a subject and a pair of light receiving means for receiving a reflected light of the spot light from the subject. And an integrating means for integrating the signals from the pair of light receiving means respectively, and in the detection method for performing automatic focus detection based on the output signals from the pair of light receiving means, When the output signals are amplified by amplifiers that can switch gains, the gains of these amplifiers are switched according to the amount of received light of the reflected light from the subject, and the switching level is different between when the amount of received light is increasing and when it is decreasing. It is an automatic focus detection method characterized by having a hysteresis characteristic of

[Embodiment of the Invention] A configuration for realizing the method according to the present invention will be described in detail based on an embodiment shown in FIGS. 1 and 2. In addition,
The same reference numerals in FIG. 3 indicate the same circuit elements, and duplicate description will be omitted.

The outputs of the light receiving elements 4a and 4b are input to the adder 8 and the subtractor 9 via the amplifiers 20a and 20b, the synchronous detection circuits 6a and 6b, and the integrators 7a and 7b, respectively. The outputs of the adder 8 and the subtractor 9 are connected to the arithmetic processing circuit 12 via comparators 10 and 11, respectively. From the arithmetic processing circuit 12 to the counting circuit 21 for time measurement
The output of the counting circuit 21 is connected to the gain comparison operation circuit 23 via the threshold circuit 22. The output of the gain comparison calculation circuit 23 is connected to the amplifiers 20a and 20b via the gain setting circuits 24a and 24b, respectively. Also,
The switches 25a and 25b for controlling the operations of the integrators 7a and 7b are activated by the output of the arithmetic processing circuit 12, and at the same time, the output is sent from the arithmetic processing circuit 12 to the start terminal S of the counting circuit 21. There is. The output of the comparator 10 is input to the reset terminal r of the counting circuit 21, and the arithmetic processing circuit 12
An automatic focusing start switch 26 is connected to the control unit 26, and a signal is output from the arithmetic processing circuit 12 to the gain setting circuits 24a and 24b.

Amplifiers 20a, 20b for amplifying outputs from the light receiving elements 4a, 4b
Is an amplifier whose amplification gain can be switched. Then, these gains are variably controlled by a command from the arithmetic processing circuit 12 via the gain setting circuits 24a and 24b while holding the same values. On the other hand, the integrators 7a and 7b
The integration operation of is controlled by a signal from the arithmetic processing circuit 12, and starts when the switches 25a and 25b are turned off, and at the same time, the counting circuit 21 starts.

The output of the comparator 10 is H level, that is, the sum of integrated values Sa +
When the output of Sb exceeds a predetermined threshold value V1, that is, at time t1, an H level signal is output, and the counting circuit 21 stops counting by this signal. The counting circuit 21 measures the time t1 and compares the time t1 with predetermined timing times ta and tb (ta <tb). In the threshold circuit 22 in the next stage, when the time t1 is between 0 and ta, the mode I , Mode II when the time t1 is between ta and tb,
When the time t1 is tb or more, it is determined to be mode III. The output of the threshold circuit 22 is input to the gain comparison calculation circuit 23, and the gains of the amplifiers 20a and 20b are adjusted. Table 1 shows the operation categories of the gain comparison calculation circuit 23.

That is, in this Table 1, when the time t1 is the mode I, the gain setting circuits 24a and 24b output the gain at the previous measurement, for example, at 1 time, and the gain is kept at 1 time. If it is doubled, it will be decreased by 1 step. If it was 64 times the previous time, it will be decreased by 8 steps. Also, in mode II, the previous gain is used as it is, in mode III, when the previous gain is 1 time, it is increased to 8 times, and when the previous gain is 8 time, it is increased to 64 times. , If the previous gain was 64 times, keep the same gain.

As described above, according to the present invention, the integral value Sa + until reaching the threshold value V1
The long integration time of Sb means that the output signals of the light receiving elements 4a and 4b are small and the S / N ratio is large, so if the amplification gain of the amplifiers 20a and 20b has a margin, increase it. To control. Further, the fact that the integration time to reach the threshold value V1 is short means that the output signals of the light receiving elements 4a and 4b are sufficiently large, and the amplifiers 20a and 2a have already been output.
When the gain of 0b is increased and used, it is decreased step by step.

However, as described above, if the amplification gain is increased or decreased with the same value as the switching level when the amount of received light is increased or decreased, the motor control becomes unstable when the gain is switched.

In the present invention, in order to solve this, a command from the arithmetic processing circuit 12 to the gain setting circuits 24a and 24b gives a hysteresis characteristic to the switching level of the amplification gain as shown in FIG. 2 to stabilize the system. It is a thing. That is, the switching level when the gain is decreased is performed at a stage where the received light amount is slightly large, and the switching level when the gain is increased is performed at a stage where the received light amount is slightly reduced.

In FIG. 2, if the received light amount R is R <R12 at the start of distance measurement, the gain is set to 64 times, and the subject condition changes from this state, and when R12 <R <R22, the gain is changed to 8 times. It At this time, since R11 and R12 are set so that R <R11 does not hold even if R slightly changes, the gain is stably held at 8 times. The subject condition is R <R11
The gain is returned to 64 times only when becomes.

Note that the gain is switched from 64 times to 8 times when the number of light emission pulses is N <16, and the amount of received light at this time is R12.
Is. On the contrary, the gain changes from 8 times to 64 times,
When the number of light emission pulses is N> 256, the amount of received light at this time is R11. It has the same hysteresis characteristic when the gain is switched between 8 times and 1 time.

Even if the way of passing through the hysteresis loop is reversed, that is, when switching the gain down, the light receiving amount is small, and when switching up the gain is large. Good.

[Effects of the Invention] As described above, according to the automatic focus detection method of the present invention, it is possible to perform a stable driving operation of the taking lens by providing the switching level with hysteresis when the gain is switched.

[Brief description of drawings]

FIG. 1 shows an embodiment for carrying out the automatic focus detection method according to the present invention. FIG. 1 is a block circuit configuration diagram thereof, FIG. 2 is a gain switching diagram, and FIG. FIG. 4 is a block circuit diagram of the signal processing circuit of FIG. 4, FIG. 4 and FIG. 5 are operation explanatory diagrams thereof, and FIG. 6 is a conventional gain switching diagram. Reference numeral 2 is a light emitting element, 4a and 4b are light receiving elements, 6a and 6b are synchronous detection circuits, 7a and 7b are integrating circuits, 8 is an adder, 9 is a subtractor,
Reference numerals 10 and 11 are comparators, 12 is an arithmetic processing circuit, 20a and 20b are amplifiers, 21 is a time counting circuit, 22 is a threshold circuit, 23 is a gain comparison arithmetic circuit, 24a and 24b are gain setting circuits, 25
a, 25b and 26 are switches.

Claims (1)

[Claims] 1. A light projecting means for projecting spot light toward a subject, a pair of light receiving means for receiving reflected light of the spot light from the subject, and signals from the pair of light receiving means are respectively integrated. In a detection method having an integrating means and performing automatic focus detection based on output signals from the pair of light receiving means, when each output signal from the pair of light receiving means is amplified by a gain switchable amplifier. The gain of these amplifiers is switched according to the amount of received light of the reflected light from the subject, and the switching level has a hysteresis characteristic that takes different values when the amount of received light increases and when the amount of received light decreases. Focus detection method.