GB1576305A

GB1576305A - Focus detecting system

Info

Publication number: GB1576305A
Application number: GB668278A
Authority: GB
Original assignee: Asahi Kogaku Kogyo Co Ltd
Current assignee: Pentax Corp
Priority date: 1977-02-19
Filing date: 1978-02-20
Publication date: 1980-10-08
Also published as: GB1576306A; DE2806868C3; DE2806868B2; DE2806868A1

Description

(54) FOCUS DETECTING SYSTEM (71) We, ASAHI KOGAKU KOGYO KABUSHIKI KAISHA, a corporation organised and existing under the laws of Japan, of No. 36-9, Maeno-cho, 2-chome, Itabashi-ku, Tokyo-to, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a focus detecting system.

There are many types of image-forming optical systems which need focussing, these including, for example, a photographic still camera a cine camera, an enlarger, a slide projector, and a cine projector. In some of these there is an overall magnification over the original image or scene size and in others the size is decreased. However, they all need to be focussed.

Automatic focussing systems have been proposed, based on the phenomenon that the output of a photosensitive element occupying a finite area in the focussing plane will take a limit, or peak, value at the in-focus position, and will progressively move away from this limit value as the image moves out of focus.

Known systems using this phenomenon are broadly of two types. In a first type a single element such as a photoconductor is mechanically oscillated over a region of the image plane. This oscillation provides a wider effective area sensed by the element, and the output thereof will be a varying signal generated during the scanning. The in-focus position is detected by searching for the maximum value of the alternating component of this signal, or by adjusting the focus until this component is a maximum.

The maximum value arises when the light and dark portions of the image are correctly differentiated, i.e. at the in-focus position.

In the second type a plurality of e.g. photoconductive elements are located at different positions in the image forming plane, and the magnitudes of the resultant signals are compared as the focus is changed so as to search for the in-focus position.

In the former, oscillatory system, the element must be oscillated at essentially constant amplitude to obtain consistent results, and this is not easy. The oscillation of the element can also affect its reliability over a period of years. Theoretically the oscillating-element system is better for use in optical apparatus which must be compact and light in weight. However problems arise in constructing the device. If the element is oscillated on an arm, it may move over an arc away from the image forming plane. Alternatively the element can be stationary in the image forming plane. A pin-hole or slit can be oscillated across the path of light to the element, but this has the disadvantage that diffusion around the pin-hole or slit reduces the accuracy of the device. Also the light detection is performed at different points over the element, so that elements having any local unevennesses in their characteristics cannot be used. Other proposals require the addition of intermediate imaging systems in the light path to the detecting element. In practice such systems are very difficult to construct. None of these prior arrangements lend themselves to precision operation. They have not therefore been used in practice to any extent.

With the multiple-element type of system, the apparatus becomes bulky and the circuitry complicated. This is particularly undesirable for portable and hand-held systems.

According to this invention there is provided a focus detecting system including a light detecting device and a detection circuit, the light detecting device comprising means defining an optical path extending from an input path portion to an output path portion, said path-defining means being mounted for rotation about an axis which is parallel to but transversely spaced from the input path portion, drive means for rotating the pathdefining means about its rotary axis, and a photo-sensitive element positioned in the optical path of light which passes along the said output path portion so as to receive light transmitted by the path-defining means as it is rotated by the drive means around its rotary axis, and the detection circuit being connected to the photo-sensitive element for enabling the detection of a limit value in the magnitude of the output of the photosensitive element.

The invention will now be described in more detail, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a partially broken away perspective view of a light detecting device of a focus detecting system embodying this invention; Fig. 2 is an axial sectional view of the device of Fig. 1; Fig. 3 is a perspective view of another device with a modified drive; Fig. 4 is a perspective view illustrating another possible modification of the device; Fig. 5 is a circuit diagram of a focus detecting system which uses the light detecting device of Fig. 1; Fig. 6 is a waveform diagram showing the variation with time of the output of the light detecting device; Fig. 7 is a perspective view of another light detecting device incorporating a prism; Fig. 8 is an axial sectional view taken on the line VIII-VIII of Fig. 7; Fig. 9 is a perspective view of a light detection device incorporating two mirrors; Fig. 10 is an axial sectional view taken on the line X-X of Fig. 9; Fig. 11 is a perspective view of a light detecting device using a single mirror.

The light detecting device shown in Figs. 1 and 2 comprises an optical fibre 1 the upper end la of which as seen in the drawings constitutes an entry surface for receiving incident light on an input optical path L1, and the lower end 16 of which constitutes an exit surface by which light leaves on an output optical path L2. The optical fibre is mounted for rotation about an axis c, and the input path L1 is parallel to but transversely spaced by a distance r from the axis C. As seen in Figs. 1 and 2, the output optical path L2 coincides with the rotary axis C. A photosensitive element 4, which may conveniently be an electromotive element such as a photocell or may be a photoconductive element such as a photo-sensitive resistor, in which case it is connected to a separate power source, is positioned to receive light transmitted by the optical fibre as it is rotated around its rotary axis C. To this end the photo-sensitive element is positioned on the axis C with its working surface 14 just beneath the exit surface 1b of the optical fibre 1, and provides an output between terminals 15 and 16.

The optical fibre 1 has a portion ic on the coinciding axis C and optical path L2 which is closely surrounded by a so-called mantle tube 5 which has the dual functions of inhibiting light leakage from the portion ic of the optical fibre and also supporting the optical fibre for rotation about the axis C. The tube 5 is journalled in bearing plates 7 and 8, and carries a pulley 6 which is driven by a belt 11 from a smaller pulley 10 mounted on the drive shaft of an electric motor 9.

Thus the entry surface la of the optical fibre 1 always lies in a predetermined plane 13 which coincides with the image forming plane of the optical image-forming system (not shown) which is to be focussed. The entry surface la of the optical fibre will travel along a circular path defined by the radius r about the axis C. The optical fibre 1 thus includes a bent arm portion 1d extending between the input surface la and the straight portion ic, and by virtue of its multiplyreflective characteristics will direct light received on the path L1 onto the path L2 and hence to the photo-sensitive element 4.

A shield 2 has a small circular hole 3 which is slightly larger than the circle defined by the radius r about axis C and includes the path traversed by the entry surface la of the optical fibre, and the undersurface of the shield 2 is as shown closely spaced above the image plane 13, although this is not absolutely necessary. The periphery of the hole 3 is rendered so as not to diffuse light passing through it. The distance r is considerably exaggerated in the drawings, and is typically of the order of 0.2 mm to 1.5 mm, preferably about 0.6 mm, for example, in a photographic camera having a reducing optical image-forming system, and being substantially larger for an enlarging projecting optical system.

The optical fibre is rotated at a speed of the order of hundreds to thousands of rotations per minute, and preferably about 4,000 r.p.m. This high velocity can advantageously be achieved using the alternative drive shown in Fig. 3. In the embodiment of Fig. 3, the motor 9 is mounted on a support block 12 concentrically about the straight portion ic of the optical fibre. The tube 5 forms the motor armature. In this embodiment the tube 5 does not suffer a sideways deflecting force such as is imparted by the tension of the belt 11 in Figs. 1 and 2. It is not now possible, however, to incorporate a mechanical speed reduction mechanism between the rotary drive source and the optical fibre 1, since the fibre 1 is rotated in unison with the motor armature. This means that the r.p.m. of the motor 9 must be pre-adjusted to the required value, and such adjustment can only be achieved electrically.

Fig. 4 shows an embodiment based on that of Fig. 1 or Fig. 3 in which there are three optical fibres the light entry surfaces of which are differently spaced by distances rl, r2 and r3 from the rotary axis C. This arrangement improves the light collection. The drive for the optical fibres is not shown but may be similar to that shown in Fig. 1 or Fig. 3.

Fig. 5 illustrates by way of example an electric focus detecting circuit which can be used with the light-sensitive devices of the preceding figures. The circuit shown in Fig. 5 converts the light intensity to which the photo-sensitive element 4 is exposed via the optical fibre 1 into a corresponding electric signal and then processes this signal. As shown, one terminal 15 of the element 4, illustrated as a photodiode, is grounded and the other terminal is connected through a capacitor 17 to a level converter 30, shown with the dashed line. The level converter 30 comprises essentially an amplifier 18, a transformer 19 and an automatic level or gain control (A.G.C.) circuit 29. The capacitor 17 is connected through the amplifier 18 to the primary winding of the transformer 19, the secondary of which has three terminals, namely an intermediate tap 20 which is grounded, one end terminal 21 which serves together with the tap 20 as output terminals for earphones 23, and another end terminal 22. The anode of a diode 24 and the cathode of a diode 25 are both connected to the terminal 22. The cathode of diode 24 is connected through a manually-operable switch 26 to a servo mechanism 27 associated with a focus adjusting mechanism (not shown) of the optical system. A display 28, for instance a voltmeter, is connected between the switch 26 and ground so as to indicate the focus condition. The anode of diode 25 is connected to a capacitor 31, the other side of which is grounded, and to the input of the A.G.C. circuit 29.

The focussing detection system thus constituted operates as follows. Light which passes through the hole 3 in shield 2, which it is desired to focus in the plane 13, is intercepted by the entry surface la of the optical fibre 1 as it is rotated by the motor 9. The entry surface la thus scans a circumferential or circular path. The light which enters the entry surface la passes through the bent arm portion 1d and the straight portion lc, exits from the optical fibre 1 at the light exit surface lb, and is incident upon the working surface 14 of the photo-sensitive element 4.

In fact, only part of working surface 14 is used, namely the area corresponding to the light exit surface 1b of the fibre, in the case of the devices of Figs. 1 and 3, or only the area corresponding to the total area of the respective light exit surfaces of the three fibres in the device of Fig. 4. The rest of the working surface 14 is not used.

When a contrast occurs between light and dark in a projected image within the circumferential region of the image forming plane 13 along which the light entry surface la of the optical fibre 1 scans, the light intensity detected will vary with time as the surface la completes each full traverse along the circular path. As a result, the photo-sensitive element 4 provides a cyclically varying voltage between the terminals 15 and 16. Such voltage variation repeats for each cycle of the light entry surface la, as shown in Fig. 6, in which the abscissa indicates elapsed time and the ordinate the voltage output of element 4.

The time AT corresponds to one rotation or cycle. The waveform will differ in dependence upon the nature of the image being focussed, and the waveform of Fig. 6 is purely one example, but so long as the image on the element contains a contrast of light and darkness, an A.C. signal will be generated in a continuous waveform while the circumferential scanning takes place.

The A.C. portion of the signal, that is the difference VH between the maximum and minimum of the waveform, will take a maximum value when the image is precisely focussed in the image forming plane 13.

Conversely, an imprecise focussing of the image results in poor contrast between light and dark and therefore a relatively flat waveform with a small-amplitude A.C. component. As the focussing point is approached, the amplitude of the A.C. component increases.

The output from the element 4 is applied to the capacitor 17 which blocks the D.C.

component and passes only the A.C. component of the signal, and applies this through the amplifier 18 to transformer 19. In consequence, sound corresponding to the amplified A.C. signal can be heard in the earphones 23, if these are connected to terminals 20 and 21. This sound can be heard as a continuous pulsing sound when the optical fibre is rotating at high speed. The sound takes its maximum amplitude when the image is precisely focussed, and progressively reduces as it moves away from the focussed condition. When the contrast of light and dark in the image scanned by the light entry surface la is finely distributed, the pulse frequency is high and the sound is of a high-pitched tone. Thus the in-focus condition can be obtained by adjusting the focussing member of the image-forming optical system and by stopping the said member at a position corresponding to the maximum sound volume. By using such a method, even a dark object illuminated only by infra-red radiation can be focussed by relying upon the sound volume. Also focussing of visual images can be achieved even by persons having poor sight, e.g. those who suffer from hypermetropia or astigmatism.

Focussing can be achieved without relying upon the oscillating sound volume as follows.

The A. C. signal from the transformer terminal 22 is rectified by the diode, and upon closure of switch 26 is indicated on the display 28. The rectified signal displayed varies in direct proportion to the magnitude of the A.C. signal. Accordingly, the focus adjusting member of the optical system can be moved manually until the displayed voltage is at a maximum value, in which position the image will be in focus in the plane 13.

Alternatively automatic focus adjustment may be achieved by applying the signal from diode 24 to a servo mechanism 27, which operates the focus adjusting member to bring the A.C. voltage VH appearing at its input to a maximum value. For further details of a suitable servo mechanism reference should be made to our British Patent Application No. 6684/78 (Serial No. 1576306).

In actual practice, the brightness of the scanned area of the image-forming plane 13 usually varies within a very wide range from a high intensity to a low intensity depending upon the object to be photographed, and an abrupt variation in brightness is often encountered in focussing. The output signal from the terminal 4 of the photosensitive element then has a substantial variation in its level. To reduce this wide range somewhat, it is preferred to include the A.G.C. circuit 29 between the terminal 22 and the amplifier 28, so that the amplifier output signal is controlled by the A.G.C. circuit 29, avoiding a condition in which the value indicated by the display 28 or received by the servo mechanism 27 exceeds a predetermined range. The capacitor 31 averages the signal over a time period in excess of one cycle of the waveform of Fig. 6. The focus detection can thus be achieved independently of the brightness of the scanned area in the image-forming plane.

In an alternative arrangement, the A.G.C.

loop can operate on the signal before the D.C. component is blocked (as by capacitor 17). In many applications, where very wide intensity variations are not encountered, it will not be necessary to include the A.G.C.

loop.

In use of the device illustrated in a photographic camera, the area to be scanned in the image-forming plane 13 is in practice arranged on the optical axis of the camera lens system. The servo mechanism 27 permits the focus adjusting member to be moved to the in-focus position merely by directing the optical axis of the camera lens to the object to be photographed. However, it is often required to focus on an object which does not lie precisely on the optical axis of the camera objective. To overcome this problem, all that is necessary is for the photographer first to direct the camera optical axis to the principal object to be focussed and close the swtich to initiate the automatic focus adjustment, and then to open the switch 26, whereupon the camera can be directed to photograph precisely the desired scene. The provision of the manuallyoperable switch 26 is extremely useful for this purpose.

When the circuit of Fig. 5 is used with the light detecting device of Fig. 1 or Fig. 3, having a single optical fibre, the area of the image-forming plane scanned is a single annular area, and when the contrast between light and darkness of the image is low the waveform shown in Fig. 6 will be relatively flat, i.e. the A.C. component VH will be of small amplitude, so that its detection is difficult. With a device having several optical fibres, spaced different distances from the common rotary axis, as shown in Fig. 4, the total area scanned by the respective light entry surfaces of the fibres is accordingly increased, and the output from the photosensitive element 4 corresponds to the sum of the outputs derived from the illumination coming through the individual light entry surfaces. The scanned area of the imageforming plane 13 is thus enlarged by employing a plurality of optical fibres. If there is a high contrast of the image in any annular scanning area of any light entry surface, the A.C. output voltage VH will be of substantial amplitude and the gain of the signal, particularly in the out-of-focus condition, is improved.

Reference will now be made to Figs. 7 to 11 showing alternative light detecting devices which can be used in place of the devices of Figs. 1 to 4. Only the differences from the devices of Figs. 1 to 4 will be described.

In the device of Figs. 7 and 8 the optical fibre is replaced by an alternative form of rotatable light reflecting system 101. The light entry surface 101a of the system is spaced a distance r from a rotary axis C. A prism 110 totally internally reflects on surfaces 111 and 112 incident light on a light path L1 onto a light path L2 coincident with the rotary axis, as best seen in Fig. 8. The system 101 has a straight portion 101c terminating in a light exit surface 101b from which light passes to the photo-sensitive element 4.

In the device of Figs. 9 and 10 the prism 110 is replaced by two mirrors 211 and 212 of equivalent function.

In the device of Fig. 11, a single mirror 311 directs light incident on a path L1 at an entry surface 301a of the reflective system 301 to the photosensitive element 4 on a path L2'.

In this embodiment the light leaving the sys tem 301 on the path L2' does not travel along the optical axis C but rather intersects it.

Thus the element 4 must be positioned at the particular axial position shown; unlike the previous examples, in which the light ot the element 4 can if need be be further reflected after leaving the optical fibre or other reflective system.

With the devices illustrated accurate detection of the in-focus position can be achieved. The scanning rate is high and the operation is stable. There are no moving contacts, such as with an oscillated photosensitive element. The detection relies on the determination of variations in an A.C. signal, and the absolute magnitude of the signal is unimportant, so that long-term variations in characteristics do not adversely affect the operation of the system. Preferably, indeed, the A.G.C. loop automatically provides a degree of compensation for overall changes in brightness.

The option of an audible output is particularly valuable for those who are unable to focus well with their own eyes, and for use at night or under infrared radiation.

WHAT WE CLAIM IS: 1. A focus detecting system including a light detecting device and a detection circuit, the light detecting device comprising means defining an optical path extending from an input path portion to an output path portion, said path-defining means being mounted for rotation about an axis which is parallel to but transversely spaced from the input path portion, drive means for rotating the pathdefining means about its rotary axis, and a photo-sensitive element positioned in the optical path of light which passes along the said output path portion so as to receive light transmitted by the path-defining means as it is rotated by the drive means around its rotary axis, and the detection circuit being connected to the photo-sensitive element for enabling the detection of a limit value in the magnitude of the output of the photosensitive element.

2. A system according to claim 1, wherein the said output path portion lies substantially on the rotary axis.

3. A system according to claim 1 or 2, wherein the path-defining means comprises an optical fibre.

4. A system according to claim 1 or 2, wherein the path-defining means comprises two or more optical fibres each defining an input path portion, the input path portions being spaced by differing amounts from the rotary axis.

5. A system according to claim 1 or 2, wherein the path-defining means comprises means for reflecting incident light twice between the input and output path portions.

6. A system according to claim 5, wherein the path-defining means comprises a prism with two totally internally reflecting faces.

7. A system according to claim 5, wherein the path-defining means comprises two mirrors.

8. A system according to claim 1 or 2, wherein the path-defining means comprises a single reflecting surface.

9. A system according to any preceding claim, wherein the drive means comprises an electric motor the rotor of which is mounted on the path-defining means about the said axis.

10. A system according to any preceding claim, wherein the detection circuit comprises a blocking member connected to the element output for blocking the D.C. portion thereof but passing the A.C. component.

11. A system according to claim 10, wherein the blocking member comprises a capacitor.

12. A system according to any preceding claim, wherein the detection circuit comprises an A.G.C. loop.

13. A system according to any preceding claim, wherein the detection circuit comprises a servo mechanism for automatically adjusting a focus adjusting member until the signal applied to the servo mechanism reaches a limit value.

14. A system according to claim 13, wherein the detection circuit comprises a manually-operable switch connected prior to the servo mechanism.

15. A system according to claim 13 or 14, wherein the detection circuit comprises a rectifier connected prior to the servo mechanism.

16. system according to any preceding claim, wherein the detection circuit comprises a visual display for indicating a limit value at the in-focus position.

17. A system according to any preceding claim, wherein the detection circuit comprises means for providing an audible indication of the magnitude of the said A.C. component.

18. A focus detecting system substantially as herein described with reference to Figs. 1 to 5 of the accompanying drawings.

19. A focus detecting system substantially as herein described with reference to Figs. 7 to 11 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. tem 301 on the path L2' does not travel along the optical axis C but rather intersects it. Thus the element 4 must be positioned at the particular axial position shown; unlike the previous examples, in which the light ot the element 4 can if need be be further reflected after leaving the optical fibre or other reflective system. With the devices illustrated accurate detection of the in-focus position can be achieved. The scanning rate is high and the operation is stable. There are no moving contacts, such as with an oscillated photosensitive element. The detection relies on the determination of variations in an A.C. signal, and the absolute magnitude of the signal is unimportant, so that long-term variations in characteristics do not adversely affect the operation of the system. Preferably, indeed, the A.G.C. loop automatically provides a degree of compensation for overall changes in brightness. The option of an audible output is particularly valuable for those who are unable to focus well with their own eyes, and for use at night or under infrared radiation. WHAT WE CLAIM IS:

1. A focus detecting system including a light detecting device and a detection circuit, the light detecting device comprising means defining an optical path extending from an input path portion to an output path portion, said path-defining means being mounted for rotation about an axis which is parallel to but transversely spaced from the input path portion, drive means for rotating the pathdefining means about its rotary axis, and a photo-sensitive element positioned in the optical path of light which passes along the said output path portion so as to receive light transmitted by the path-defining means as it is rotated by the drive means around its rotary axis, and the detection circuit being connected to the photo-sensitive element for enabling the detection of a limit value in the magnitude of the output of the photosensitive element.