GB2047036A

GB2047036A - Focus detection device

Info

Publication number: GB2047036A
Application number: GB8004233A
Authority: GB
Original assignee: Asahi Kogaku Kogyo Co Ltd
Current assignee: Pentax Corp
Priority date: 1979-02-09
Filing date: 1980-02-08
Publication date: 1980-11-19
Also published as: DE3050691C2; DE3004147A1; GB2047036B; FR2448732A1; FR2448732B1; DE3004147C2

Abstract

A focus detection device of the kind in which an array 8, Figure 4, of photo-electric sensors are scanned 13 at a rate dependent upon the average brightness of the object 6 as detected by an adjacent cell 9, is characterised in that before being used to determine the scanning rate the average brightness signal is smoothed, either optically or electrically, so that any high frequency (i.e., greater than a few tens of cycles per second) variations, caused, for example, by movement of the object, are eliminated. A suitable circuit (Figure 8, not shown) for smoothing the output signal from the average brightness detector 9, and two suitable optical arrangements (Figures 9 and 10, not shown) for smoothing the light before it reaches the detector 9, are described. <IMAGE>

Description

SPECIFICATION Focus detection device This invention relates to a device for detecting the focus condition of an optical image, and operates on the principle that the contrast of the image reaches a maximum when the image is properly in focus.

A variety of focus detection devices operating on this principle have been proposed. In one such device, the brightness distribution of the image is electrically scanned by means of a self-scanning type photoelectric conversion unit which produces a time-series waveform representing said brightness distribution. This waveform is processed to produce a signal indicative of the contrast of the image.

The self-scanning type photoelectric conversion unit includes an array of microphotoelectric elements and a scanning circuit for scanning the photoelectric outputs of the elements. The unit can be of the MOS-FET type or of the CCD type (both of which are commercially available) depending on the arrangement of the scanning circuit employed. Each microphotoelectric element is capable of accumulating an electric charge in dependence upon the amount of light received thereby during each scanning cycle. If the scanning rate of the photoelectric conversion unit is maintained constant, then the amount of charge accumulated will change with the intensity of the incident light, i.e. the brightness of the image. As the scanning rate is made slower, the amount of accumulated charge increases.Therefore, even if the brightness of the image is low, the photoelectric conversion unit can provide a sufficiently high output to ensure accurate focus detection.

Explaining this in more detail, typically a line or edge separating areas of high contrast in the image is applied to the photoelectric conversion unit, and the difference between the photoelectric outputs of two microphotoelectric elements adjacent to the edge or of two microphotoelectric elements on either side of the edge is evaluated. The focus condition of the image is then determined from increases and decreases in this output difference. Even if the contrast of the image remains constant, the output difference thus evaluated will change as the brightness of the image varies.If the intensity of light received by the two microphotoelectric elements is represented by E1 and E2 respectively, then the difference in their photoelectric outputs will be: (E,E2) E2)TcSp C where To is the scanning period of the photoelectric conversion unit, Sp is the optical sensitivity of the microphotoelectric elements and Cj is their junction capacitance. Under this condition, if the brightness of the image changes by a factor a ,the difference between the photoelectric outputs of the two microphotoelectric elements will become ZSV' = ccnV. It therefore becomes impossible to detect with accuracy when the image is properly focussed if its brightness changes.In addition, the dynamic range of the output waveform of the photoelectric conversion element with respect to luminous flux is not as great as might be desired, and accordingly for a constant scanning rate the photoelectric outputs of the individual elements may become saturated or may fall to a noise level, resulting in the evaluated difference between the photoelectric output becoming inaccurate.

Atechnique for eliminating the above-described difficulty has been proposed in which the average brightness of the image is determined by means of a photodetector disposed adjacent to the array of microphotoelectric elements, and the scanning rate of the photoelectric conversion element is varied in accordance with the brightness thus determined. If the average brightness of the image changes by a factor a, the intensity of the light received by the two microphotoelectric elements will change from E1 to a E1 and from E2 to a E2 respectively.From the above equation it will be appreciated that the difference AV in the photoelectric outputs of the two elements will remain constant if the scanning time is changed to To la. Thus, even where the average brightness of the image changes, the difference in the photoelectric outputs of the microphotoelectric elements remains unchanged and the dynamic range of the output signal of the photoelectric conversion device with respect to luminous flux is increased. Accordingly, focus detection can be performed even in low light levels.

However, inaccuracies can still arise in focus detection due to movement of the image during the scanning cycle of the photoelectric conversion element or due to camera shake where the focus detection device is incorporated into a camera. Such relative movement between the image and the photoelectric conversion unit causes fluctuations in the output differences between the individual microphotoelectric elements.

It is an object of the present invention to prevent such inaccuracies from arising.

According to the present invention there is provided a device for detecting the focus condition of an optical image, comprising an array of light-sensitive elements which receive said image thereon and which each produce a respective photoelectric output, scanning means arranged to produce a contrast signal representative of the contrast of said image by scanning the photoelectric outputs of said array of light-sensitive elements, a light-sensitive detector which produces an output signal representative of the average brightness of said image, variations in said output signal due to movement of said image relative to the light-sensitive detector being smoothed out, and a control which controls the rate at which the scanning means scans said photoelectric outputs in dependence upon the output signal from the light-sensitive detector.

The contrast signal is thus unaffected by changes in the brightness of the image and fluctuations caused by movement of the image during a scanning cycle or (where the device is incorporated in a camera) by camera shake. Accordingly, accurate focus detection can be performed even at low light levels.

The variations in the output signal of the light-sensitive detector can be smoothed out either by removing a high frequency component of the output signal itself, for example by means of a low-pass filter, or by removing a high spatial frequency componentof the image before it reaches the detector, for example by diffusing or defocussing the image. Where the device is incorporated into a single lens reflex camera, such diffusion of the image can be performed by means of a focussing screen which forms part of the camera's viewfinder optical system. In this case, the light-sensitive detector responds to the average brightness of the image projected onto the focussing screen in a conventional manner by the camera optics.

The present invention will be further explained, by way of example, with reference to the accompanying drawings in which: Figure 1 is a graph illustrating the manner in which the photoelectric outputs produced by an array of light-sensitive elements vary as the focus condition of an image received thereby changes; Figure 2 is a graph illustrating the manner in which a contrast signal derived from said photoelectric outputs varies in accordance with the focus condition of the image; Figure 3 is a diagram illustrating the illuminance vs exposure characteristic of a typical self-scanning photoelectric conversion unit; Figure 4 is a diagram, partly in block form, of a conventional focus detection device including such a unit; Figure 5 is a diagram of part of the device shown in Figure 4;; Figure 6 is a diagram illustrating movement of the image with respect to the self-scanning type photoelectric conversion unit in the device of Figure 5; Figure 7 is a graph indicating the fluctuations which can occur in the output of a photoelectric detector in the conventional device due to such movement of the image; Figure 8 is a circuit diagram of part of one embodiment of a focus detection device according to the present invention; Figure 9 is a diagram, partly in block form, of a second embodiment of a focus detection device according to the present invention; Figure 10 is a diagram of a third embodiment of a focus detection device according to the present invention; and Figure ii is a circuit diagram of part of the focus detection device shown in Figure 9 or Figure 10.

Figure 1 is a graph illustrating the photoelectric outputs produced by six microphotoelectric elements disposed at positions P1 to P6 respectively when a line or edge between two areas of high contrast in an optical image is applied thereto. The profile of the line or edge when the image is in focus is indicated by broken line 1, and under this condition the two microphotoelectric elements disposed at positions P3 and P4 produce photoelectric outputs l(P3) and l(P4) respectively as indicated by solid lines.When the image is out of focus, the profile of the line or edge becomes as indicated in chain-dotted line at 2, and the photoelectric outputs of the two elements become l(P3') and l(P4') respectively, as indicated in broken lines at P3 and P4'.

The absolute value C = /I(P3) - I(P4)/ of the difference between the photoelectric outputs of the two elements when the image is in focus is greater than the corresponding absolute value C' = /I(P3') - I(P4')/ when the image is out of focus. That is, the absolute value becomes a maximum when the image is properly focussed: the absolute value is referred to as a contrast output since it is indicative of the contrast of the image. The manner in which the contrast output varies as the focus condition of the image changes is shown in Figure 2, from which it can be seen that the contrast output reaches a maximum value when the image is in focus. in Figure 2, the abcissa represents the amount of extension of a lens which projects the image onto the microphotoelectric elements.

The contrast output can be evaluated using a self-scanning type photoelectric conversion unit which comprises a plurality of microphotoelectric elements and a scanning circuit for scanning the photoelectric outputs of the elements. Figure 3 is a graph illustrating an ordinary illuminance (E) vs. exposure (Q) characteristic of such a unit. With respect to scanning times T1, T2 and T3 (T < T2 < T3) in a single scanning period, the characteristic varies as indicated by lines 3, 4, and 5 respectively. A dynamic range over which the exposure is not saturated is indicated by D. If the scanning rate is constant, then the photoelectric output varies linearly with the magnitude of illumination only in the range D.If, however, the scanning rate is varied in dependence upon the brightness of the image (so that the scanning rate increases as the image brightness increases), the unit will always operate in the range D. The dynamic range of the unit is therefore increased, and the contrast output is unaffected by changes in the image brightness.

A conventional focus detection device which makes use of this technique is shown in Figure 4. A lens 7 projects an image of an object 6 onto a self-scanning type photoelectric conversion unit 8. A photodetector 9 arranged to measure the average brightness of the image is disposed in the same plane as the unit 8, and produces a photocurrent whose magnitude is dependent upon the average brightness thus measured. This photocurrent is converted into a corresponding voltage by a current-to-voltage conversion circuit 10, and this voltage is supplied to a frequency conversion circuit 11 which produces a periodic signal whose frequency is dependent upon said voltage.The periodic signal is in turn supplied to a clock pulse generator 12 which produces clock pulses of the same frequency as the periodic signal and the clock pulses are applied to a scanning circuit 13 which drives the self-scanning type photoelectric conversion unit 8 at a scanning rate dependent upon the clock pulse frequency. The unit 8 scans the image and produces a time-series output waveform representative of its brightness distribution. This waveform is converted by a contrast detecting circuit 14 into a contrast signal representing the contrast of the image.

If the average brightness of the image increases, this is detected by the photodetector 9 and the scanning rate of the unit 8 is increased to compensate. Similarly, if the average brightness of the image decreases, the scanning rate of the unit 8 is decreased in correspondence thereto. Thus, the contrast signal is independent of variations in the image brightness.

Figure 5 shows the self-scanning type photoelectric conversion unit 8 and photodetector 9 in greater detail. A plurality of microphotoelectric elements 8a to 8zare arranged on a substrate 15, and a scanning circuit 16 for scanning the photoelectric outputs of the elements and the photodetector 9 are provided on the substrate adjacent thereto. The elements 8a to 8z and the circuit 16 together form the self-scanning type photoelectric conversion unit 8. It is not always required to dispose the photodetector 9 on the substrate 15, in which case the photodetector can be provided separately from the unit 8 but at a position optically equivalent thereto.

Although, as mentioned above, the contrast output is unaffected by changes in image brightness, problems can arise due to movement of the image during a scanning cycle, for example due to movement of the object 6 itself or (where the focus detection device is incorporated into a camera) due to camera shake.

As illustrated in Figure 6, relative movement between the image and the focus detection device causes the line or edge 17 between high contrast areas in the image to move through a distance Wto a position 18 or 19 on the photodetector 9. This causes the photocurrent produced by the photodetector 9 to fluctuate even though the contrast of the image remains constant, causing the scanning speed to vary and the contrast output to change. Figure 7 is a graph of the output of the photodetector 9 as a function of time and solid line 21 indicates the manner in which the output can fluctuate due to this effect. Broken line 20 indicates the output which would be obtained if the image were to remain stationary.

In order to eliminate this problem, the present invention smooths out these fluctuations to obtain an output as indicated by chain-dotted line 22 in Figure 7. This is done by removing a high frequency component of the output of the photodetector 9, or by removing a high spatial frequency component of the image before it reaches the photodetector. In general fluctuations in the photodetector output due to camera shake have a frequency of the order of tens of Hertz. Accordingly, if frequencies higher than several Hertz are removed from the photodetector output, then the effects of camera shake can be practically disregarded.

In one embodiment of the present invention, the focus detection device shown in Figure 4 is modified by means of the circuit depicted in Figure 8. The photocurrent produced by a photodiode D, (corresponding to the photodetector 9) is converted into a corresponding voltage by an operational amplifier A1 having a resistor R1 connected in parallel therewith. The voltage thus produced is smoothed (i.e. the high frequency components thereof are removed) by a low-pass filter comprising a resistor R2 connected to the output of the amplifier As and a capacitor C1 connected between a terminal of the resistor R2 remote from the amplifier and ground.The smoothed voltage is applied through a buffer amplifier or an operational amplifier A2 and a resistor R3 to a capacitor C2 connected in parallel with an operational amplifier A3, so that the capacitor C2 is charged at a rate dependent upon the smoothed voltage. The voltage across the capacitor C2 is compared with a reference voltage pre-set by means of resistors R4 and R6 by a comparator which includes an operational amplifier A4. When the voltage across the capacitor C3 exceeds the reference voltage, an output signal from the comparator renders an analogue switch S, connected in parallel with the capacitor C2 conductive, so that the capacitor is quickly discharged. At the same time, the output signal of the comparator changes thereby rendering the analogue switch S1 non-conductive again.The capacitor C2 is repeatedly charged and discharged in this manner, as a result of which a periodic signal whose frequency is dependent upon the smoothed voltage appears at the terminal marked OUT. The periodic signal is applied to the scanning circuit of the self-scanning type photoelectric conversion unit to control the scanning operation.

Shown in Figure 9 is a second embodiment of a focus detection device according to the present invention which can be used in a single lens reflex camera. Those components which are common to this embodiment and the arrangement of Figure 4 have been accorded the same reference numerals. A semi-transparent mirror 23 is now placed behind the lens 7 so as to divide the image into a reflected part which is received by the self-scanning type photoelectric conversion unit 8 and a transmitted part which is received by the photodetector 9. The photodetector is positioned so that it does not lie in a focal plane of the lens 7 when an in-focus image is produced on the unit 8. In the arrangement illustrated, the photodetector lies behind the focal plane: however it can equally well be positioned in front of the focal plane.Such positioning of the photodetector 9 ensures that it receives a de-focussed image thereon, which means that the high spatial frequency components have been removed from the image before it reaches the photodetector. The output of the photodetector therefore varies very little due to movement of the object or camera shake, and hence the contrast output produced by the circuit 14 is substantially independent of these effects.

The arrangement shown in Figure 9 can be modified so that the unit 8 receives the transmitted image part and the photodetector 9 receives the reflected image part. In either case, the unit 8 and the photodetector 9 receive their respective image parts in mutually perpendicular planes.

A third embodiment of the present invention is illustrated diagrammatically in Figure 10, in which the high spatial frequency component of the image is removed by means of a diffuser plate 24. More particularly, the part of the image reflected by the semi-transparent mirror 23 is projected onto the diffuser plate 24 (the position of the latter being optically equivalent to that of the unit 8), and the image formed on the plate is projected onto the photodetector 9 by a lens 25. Rather than relaying on diffusing alone to remove the high spatial frequency component of the image, the lens 25 can be arranged to project a defocussed image onto the photodetector 9 so that a more effective result can be obtained.

This particular arrangement is very well adapted for incorporation into a single lens reflex camera, since the diffuser plate 24 can be constituted by a light-diffusing focussing screen which forms part of the viewfinder optical system of the camera.

Figure 11 illustrates a circuit which can be used in either of the embodiments of Figures 9 and 10. In the circuit, the photocurrent produced by a photodiode PD (corresponding to the photodetector 9) is converted into a corresponding voltage by an operational amplifier A1 having a resistor R1 connected in parallel therewith. A capacitor C connected in parallel with another operational amplifier A3 is charged by this voltage. The voltage across the capacitor C thus charged is compared with a reference voltage determined by resistors R4 and R5 by a comparator which includes an operational amplifier A4. When the voltage across the capacitor C exceeds the reference value, an analogue switch S, is turned on as a result of which the capacitor C is quickly discharged to its initial condition.At the same time, the state of the output of the comparator changes and, accordingly, the analogue switch S, is turned off again. The capacitor C is repeatedly charged and discharged in this manner, producing at a terminal marked OUT a periodic signal whose frequency is dependent upon the output of the photodiode PD. The periodic signal is applied to the scanning circuit of the self-scanning type photoelectric conversion unit 8 to control the scanning operation.

As is apparent from the above description, in a focus detection device according to the present invention the contrast output is unaffected by changes in image brightness. In addition the dynamic range of the photoelectric output signal with respect to the luminous flux is correspondingly increased. Accordingly, the contrast output is accurately provided regardless of the image brightness and is substantially free from effects caused by movement of the object or shaking of the camera. Thus, the focus detection device can be made high in reliability and accuracy, and where it is incorporated into a single lens reflex camera, the camera can be made compact in size and economical to manufacture.

Claims

1. A device for detecting the focus condition of an optical image, comprising an array of light-sensitive elements which receive said image thereon and which each produce a respective photoelectric output, scanning means arranged to produce a contrast signal representative of the contrast of said image by scanning the photoelectric outputs of said array of light-sensitive elements, a light-sensitive detector which produces an output signal representative of the average brightness of said image, variations in said output signal due to movement of said image relative to the light-sensitive detector being smoothed out, and a control which controls the rate at which the scanning means scans said photoelectric outputs in dependence upon the output signal from the light-sensitive detector.

2. A device as claimed in claim 1, wherein the light-sensitive detector is disposed at a position substantially optically equivalent to that of said array of light-sensitive elements, and the output signal of the light-sensitive detector is smoothed by removing a high frequency component thereof.

3. A device as claimed in claim 2, wherein a low-pass filter is coupled to an output of the light-sensitive detector, the control means being responsive to an output of the low-pass filter.

4. A device as claimed in claim 3, wherein the low-pass filter includes an amplifier to an input of which the output signal of the light-sensitive detector is applied, a resistor connected to an output of the amplifier and a capacitor connected between a terminal of the resistor remote from the amplifier and ground.

5. A device as claimed in claim 1, wherein the output signal of the light-sensitive detector is smoothed by removing a high spatial frequency component of the image before it reaches the detector.

6. A device as claimed in claim 5, further comprising a beam-splitter arranged to divide said image into reflected and transmitted parts, one of the parts being received by said array of light-sensitive elements, the other image part being received by the light-sensitive detector after having said high spatial frequency component removed therefrom.

7. A device as claimed in claim 6, wherein the array of light-sensitive elements and the light-sensitive detector receive their respective image parts in planes which are substantially mutually perpendicular.

8. A device as claimed in claim 5,6 or 7, wherein the high spatial frequency component of the image is removed by de-focussing or diffusing the image before it reaches the light-sensitive detector.

9. A device as claimed in claim 8, further comprising a lens which projects the image onto said array of light-emitting elements, the light-sensitive detector being disposed out of a focal plane of the lens when said image is in focus.

10. A device as claimed in claim 8, further comprising an optical diffuser which receives said image and a lens which projects the diffused image onto the light-sensitive detector.

11. A device as claimed in any preceding claim, wherein the control includes a pulse generator which produces pulses at a rate dependent upon the output signal of the light-sensitive detector, the rate of which the scanning means scans said photoelectric outputs being dependent upon the rate at which said pulses are produced.

12. A device as claimed in claim 11, wherein said array of light-sensitive elements form part of a self-scanning type photoelectric conversion unit which also includes a scanning circuit for scanning said photoelectric outputs, the scanning circuit being driven by said pulses.

13. A device as claimed in claim 11 or 12, wherein the pulse generator includes a capacitor which is charged at a rate dependent upon the output signal of the light-sensitive detector, and switch means arranged to discharge the capacitor each time the charge thereon reaches a predetermined level.

14. A device as claimed in claim 13, wherein a comparator produces a signal when the voltage across the capacitor reaches a predetermined voltage, and the switch means discharges the capacitor in response to said signal.

15. A device as claimed in claim 14, wherein the pulse generator also includes an operational amplifier to an input of which is applied a signal dependent upon the output signal of the light-sensitive detector, the comparator has an inverting input to which an output of the operational amplifier is applied and a non-inverting input to which said reference voltage is applied, the capacitor is connected between the input and output of the operational amplifier, and the switch means is an analogue switch whose main conduction path is connected in parallel with the capacitor and whose control terminal is connected to an output of the comparator.

16. A device as claimed in claim 1, wherein the control comprises a current-to-voltage converter arranged to convert a photocurrent output of the light-sensitive detector into a corresponding voltage, a voltage-to-frequency converter arranged to produce a periodic signal whose frequency is dependent upon said corresponding voltage and a clock pulse generator arranged to drive the scanning means at a rate dependent upon the frequency of said periodic signal.

17. A device for detecting the focus condition of an optical image, substantially as hereinbefore described with reference to any one of Figures 8 to 11 of the accompanying drawings.

18. A single lens reflex camera including a body having a light entrance aperture therein, a mirror movable between a first position in which it reflects light entering the body through the light entrance aperture towards a viewfinder optical system and a second position in which it allows said light to pass to a film plane, and a device as claimed in claim 1 for detecting the focus condition of an image of an object to be photographed, said array of light-sensitive elements being disposed at a position optically equivalent to that of said film plane.

19. A single lens reflex camera as claimed in claim 18, wherein at least a portion of the mirror is semi-transparent so that when in its first position the mirror splits said image into a transmitted part which is received by said array of light-sensitive detectors and a reflected part which is reflected towards said viewfinder optical system, and the viewfinder optical system includes a light diffusing focussing screen, the light-sensitive detector being responsive to the image formed on said screen.