GB2030415A - Focus detection device - Google Patents

Abstract

A focus detection device for a photographic camera and of the contrast-sensing type includes a photoelectric element (15) of the self- scanning type driven by a driver circuit (16). The element produces for each scanning cycle a train of time- discontinuous pulses which are amplified (17) and applied alternately to first and second sample holding circuits (18, 19). A difference amplifier (20) provides a signal representing the difference between successive pairs of adjacent pulses in the pulse train. The absolute value (21) of the difference signal is taken, and its peak for each scanning cycle is detected (22). This peak will take a maximum value at the in-focus position. High spatial frequency components are not lost by filtering, and the need for differentiation is avoided, while the accumulation effect of the self- scanning element is employed to advantage. <IMAGE>

Description

SPECIFICATION Focus detection device The present invention relates to a focus detection device operating in accordance with the principle that an image of an object exhibits the maximum contrast when the image is precisely focussed.

An optical image of an object to be photographed which is projected by an objective onto the image plane exhibits the maximum luminous difference or contrast in the image area when the image is precisely focussed. Particularly with respect to the edge regions of the brightness distribution of the image, it is observed that the brightness variation curve exhibits the steepest slope when the image is precisely focussed. This phenomenon can be explained by the fact that the light intensity (power spectrum) of the image with respect to each spatial frequency takes the maximum value when the image is precisely focussed, and there have already been proposed various automatic focus detecting devices based on such phenomenon.

These known devices can be generally classified into the following two types depending on the method of detection used in the device. in the first type of device, a plurality of microphotoelectric elements are arranged in the image plane and an output difference between a pair of adjacent ones of these microphotoelectric elements is detected as a contrast signal, while, in the second type, the object to be photographed is mechanically or electrically scanned to obtain a photoelectrically converted waveform having a series of time-discontinuous peaks, and an output is obtained by differentiation of this waveform.

This output corresponds to the slope of the image brightness distribution, and is used as a contrast signal.

As an example of the latter, a compact device utilizing a self-scanning photoelectric element as the scanning means has recently been proposed.

This self-scanning photoelectric element comprises a plurality of microphotoelectric elements and a scanner circuit, and accumulates a quantity of light to which the chain of microphotoelectric elements are exposed within a single scanning cycle, generating a waveform having a series of time-discontinuous peaks. In this device, the output waveform having a series of time-discontinuous peaks from the selfscanning photoelectric element is converted by using a sample holding circuit and a smoother to the corresponding analog waveform, which is then subjected to the action of a differentiator to extract the differentiated value from the waveform. This is, in turn, converted by the absolute value circuit to the corresponding absolute value waveform, and thereafter the peak value of this output wave form is detected and held for every scanning cycle.This peak value output is used as the contrast signal with which the focus is indicated or the objective is driven. The self-scanning photoelectric element is advantageous in that an adequate output is obtained even with respect to weak luminance, since an element of this nature can accumulate a quantity of light within a single scanning cycle.

The known device of the first type is certainly convenient in that no movable part is necessary, but it is difficult for this type to detect weak light The known device of the second type, in which the self-scanning photoelectric element is used and the brightness distribution of the image is differentiated, in spite of the advantage such that even weak light can be detected under the accumulating action of said element, has disadvantages that it is difficult for this type to hold the output and the circuit arrangement is necessarily complicated.

The present invention is defined in the appended claims, to which reference should now be made.

The invention will be described in more detail, by way of example, with reference to the drawings, in which: Fig. 1 diagrammatically illustrates the principle on which a focus detection device embodying the present invention operates, showing a typical brightness distribution of an object to be photographed in focussed and non-focussed states; Fig. 2 is a circuit diagram of a known focus detecting device utilizing a self-scanning photoelectric element; Fig. 3 diagrammatically shows the manner in which the waveform varies in the known focus detecting device of Fig. 2, in which Fig. 3A shows the output waveform from the self-scanning photoelectric element, Fig. 3B shows a sample hold waveform, Fig. 3C shows a smoothed waveform, Fig. 3D shows a differentiated waveform, Fig. 3E shows an absolute value waveform and Fig. 3F shows the waveform with the peak value being held.

Fig. 4. diagrammatically illustrates a distribution of the peak value output serving as the contrast signal of the object to be photographed at focussed and non-focussed states; Fig. 5 is a circuit diagram illustrating a focus detection device embodying the present invention; Fig. 6 diagrammatically shows the manner in which the waveform varies in the focus detecting device of Fig. 5, in which Fig. 6G shows an output waveform from the self-scanning photoelectric element, Fig. 6H shows a first sample hold waveform, Fig. 61 shows a second sample hold waveform, Fig. 6J shows a different waveform derived from two sample hold waveforms, and Fig.

6K shows a waveform with the peak value being held; Fig. 7 is a circuit diagram illustrating in more detail a focus detecting device embodying the present invention; and Fig. 8 is a timing chart of driving pulses for the self-scanning photoelectric element as well as of control pulses for four analog switches employed in the focus detecting device of Fig. 7.

Fig. 1 diagrammatically shows characteristic brightness distribution curves for an image of an object exhibited when the image is focussed and not focussed (out of focus) respectively. It is seen from the diagram that the slope of these brightness distribution curves becomes more gentle as the image deviates from the focussed condition. This slope can be expressed as a brightness difference between twQ points on the image area, which takes a maximum value when the object image is focussed.A brightness difference between, for example, points a and b takes a value A, when the image is focussed and a value A2 when the image is not focussed, and it will be apparent from the diagram that the value A is larger than the value 2. Thus, focus detection can be achieved on the basis of the fact that the brightness difference (or contrast signal) between two points on the image area, particularly within a range corresponding to the slope of the brightness difference curve, takes a maximum value when the image is focussed.

There has already been proposed an automatic focus detecting device based on the focus detection principle as described above, which may be executed, for example, as illustrated by Fig. 2.

The device illustrated uses a photoelectric element of self-scanning type as the scanning means for the image. An object to be photographed, designated by reference numeral 3, is projected by an objective 4 on the photoelectric element 5 of self-scanning type which is driven by a driver circuit 6 and generates a waveform defined by a series of time-discontinuous peak values in accordance with the image brightness. This waveform is amplified by an amplifier 7, and this converted by a sample holder 8 to the corresponding analog waveform. This analog waveform is filtered to remove undesirable high frequency components, if any, in a smoother 9 so that the waveform may be converted to a waveform corresponding to the original brightness distribution of the image.The output waveform is then differentiated by a differentiator 10, and a signal representative of the slope of the brightness distribution is extracted. The signal is then converted by an absolute value circuit 11 to a positive or negative voltage waveform, of which the peak value within a single scanning cycle is then detected by a peak value detector 12. This peakvalue is held until the next peak value appears. The peak value output obtained in the manner as just described corresponds to a contrast signal, serving, for example, to control a drive motor for the objective in a photographic camera of automatic focussing type.

Fig. 3 diagrammatically shows the waveform variation in the previously described device, in which V represents voltage values and t represents time. Fig. 3A shows an output waveform of the automatic scanning photoelectric element 5, and a brightness distribution of the image is illustrated by the broken line. It should be noted that, in this diagram, the waveform is illustrated with respect only to two continuous scanning cycles T1 and T2, and the waveform within the cycle T2 represents a more precisely focussed state that the waveform within the cycle T. Fig. 38 shows the output from the sample holder 8, which exhibits, as shown, a stepped analog waveform. Fig. 3C shows the waveform obtained as a result of smoothing the analog waveform by the smoother 9.Fig. 3D shows the waveform obtained as a result of differentiation of the smoothed waveform by the differentiator 10. Fig.

3E shows the absolute value waveform obtained from processing the differentiated waveform by the absolute value circuit 11. The peak value of the absolute value waveform within the cycle T2 is larger than the peak value of the absolute value waveform within the cycle T,, since the waveform of the cycle T2 represents a more precisely focussed state than the waveform of the cycles1, as previously mentioned. Finally, Fig. 3F shows the manner in which the peak values are held by the peak value detector 12.

The peak value output varies relative to the position to which the objective has been driven and takes the maximum value at the in-focus position, as shown by Fig. 4. Although the known device as has been described hereinabove can achieve automatic focus detection, there are some problems remaining unsolved. One of these problems lies in that the waveform smoothed by the smoother 9 can not be precisely analogous to the brightness distribution of the original object image, as seen from Fig. 3C. More specifically, there occurs sometimes loss of high spatial frequency components due to a low frequency filter included in the smoother 9, but attempts to prevent such loss would result in excessive high frequency components remaining in the stepped waveform output from the sample holder 8, which excessive high frequency components function as noise for the differentiated waveform.Another serious problem lies in that, when the peak value is detected by the peak value detector 1 2 from the absolute value waveform of the differentiated output, the differentiated waveform is too sharp for the peak value to be held accurately. This makes it difficult to discriminate the in-focus position, reducing the accuracy with which the focus can be detected. For these reasons, in practice it has been difficu It to manufacture and use the known device.

The focus detecting device shown in Fig. 5 makes the most of advantageous properties of the self-scanning photoelectric element while compensating for the drawbacks as mentioned above to obtain a precise contrast signal. Fig. 6 diagrammatically shows the manner is which the waveforms vary with the device of Fig. 5. An object 13 to be photographed is imaged by an objective 14 onio a self-scanning photoelectric element 15, which is driven by a driver circuit 1 6 and is adapted to generate a signal having a series of time-discontinuous peaks as illustrated by Fig.

6G. It should be noted that, in Fig. 6G, the waveform is illustrated with respect only to two successive scanning cycles T3 and T4, and the waveform within the cycle T4 represents a more precisely focussed state than the waveform within the cycle T3. The broken line in Fig. 6G represents the brightness distribution of the image. The signal having a series of time-discontinuous peaks from the self-scanning photoelectric element 1 5 is amplified by an amplifier 17 and simultaneously applied to two sample holders 18, 1 9.The first sample holder 18 holds, as seen in Fig. 6H, timediscontinuous output peaks from alternate ones of the microphotoelectric elements of the photoelectric element 1 5. The second sample holder 19 similarly holds, as seen in Fig. 61, timediscontinuous output peaks from the other alternate microphotoelectric elements, being those shifted from the output waveform of the first sample holder 18 by one photoelectric element driving pulse. Each pair of output signals from the two sample holders 18, 19 are converted by a differential circuit 20 to a signal having a waveform corresponding to the difference between these two outputs. The difference waveform is shown by Fig. 6J. This difference waveform is converted by an absolute value circuit 21 to a positive or negative absolute value waveform as seen in Fig. 6K.The peak value of the absolute value waveform within a single scanning cycle is held by a peak value detecLor 22, until the peak value within the next cycle of scanning appears. Fig. 6L shows the waveform of the peak value output thus held. This peak value signal corresponds to an image contrast signal and takes a maximum value at the in-focus position as seen in Fig. 4.

A more detailed circuit arrangement of the device is illustrated by way of example in Fig. 7. A self-scanning photoelectric element 23 is driven by a driving circuit 24 including a pulse oscillator, and provides an output signal having a series of time-discontinuous peaks, which is amplified by an operational amplifier A,. In Fig. 7, the references R1 to R,6 designate resistors. The amplified signal having time-discontinuous peaks is divided by two analog switches S2, S3 into two halves which are respectively converted by two C tothec capacitors C,, 2 to the corresponding analog signals. The two analog signals are applied through buffer amplifiers A2, A3 to an operational amplifier A4to extract a difference output derived from these two signals.Control pulses for the analog switches S2 and S3 have a period twice that of the pulses serving to drive the self-scanning photoelement 23, and are shifted in phase with respect to the latter by one pulse. The analog switch S, is arranged to reset the output for every driving pulse and thereby to hold the precise output of the photoelectric element.

The difference output is converted by an operational amplifier As and diodes Dr, D2to a positive absolute value waveform. This absolute value waveform is then amplified by an operational amplifier A6, and the peak value of the absolute value waveform within a single scanning cycle is held by an operational amplifier A7, a diode D3 and a capacitor C3. An analog switch S5 is provided to discharge the capacitor C3 for every scanning cycle.The peak value output thus obtained is processed by a buffer amplifier A8 to produce a focus control signal Vout Fig. 8 is a timing chart of control pulses P1, P2, P3, P4 respectivelyforthe analog switches S" S2, S3, S4, as well as of the driving or clock pulses for self-scanning photoelectric element. T designates a single scanning cycle.

With the device illustrated and described above, there occurs no loss of high spatial frequency components necessary for detection of the image contrast, since it is not required to smooth the sample hold waveform of the photoelectrically converted output having timediscontinuous peaks as in the usual method of contrast detection. Accordingly, more precise focus detection can be achieved. Furthermore, the absolute value waveform of the differential signal representative of the image contrast is not so sharp at the differentiated waveform and, therefore, it is easy to hold the peak value thereof and thereby to extract the precise peak value.

Moreover, the circuit arrangement can be simplified, permitting the device to be compact.

Finally, the device utilizes the accumulation effect of the self-scanning photoelectric element, so that the contrast detection is possible even with respect to a dark object to be photographed. The device can be compact while still providing high precision of focus detection.

Claims (5)

1. A focus detection device comprising a selfscanning photoelectric sensor having a plurality of microphotoelectric elements, for providing a pulse train representative of incident illumination, means for generating the difference between successive pairs of adjacent pulses in the pulse train, and means for detecting for each scanning cycle of the sensor the peak value of the absolute magnitude of the output of the difference generating means.

2. A device according to claim 1, in which the difference generating means comprises two sample holding circuits connected to the sensor to receive alternate pulses of the pulse train, and a difference circuit connected to the two sample holding circuits.

3. A device according to claim 2, in which the output of the two sample holding circuits are converted into analog signals.

4. A focus detection device substantiaily as herein described with reference to Figs. 5 to 8 of the drawings.

5. A photographic camera provided with a focus detection device in accordance with any preceding claim.