GB2095505A - Automatic focusing - Google Patents

Abstract

An automatic focusing system for a zoom lens 1 of a camera in which a beam of infra-red light is directed towards a subject 6 from a light source 4 and a portion of the light beam reflected from the subject 6 back to the camera is sensed by photoelectric means 5. The angle between the emitted and reflected beams is translated by mechanical means into a mechanical movement which is a function of the distance to the point of reflection and the lens 1 is thereby focused. The path of the light beam passes through the lens 1, before reaching the photoelectric means 5. Thus the distance measuring area in the viewfinder is not changed in telephoto and wide-angle modes. <IMAGE>

Description

SPECIFICATION Automatic focusing systems for lenses This invention relates to automatic focusing systems for lenses, and more particularly to automatic focusing systems which are particularly suitable for use with a television camera which includes a zoom lens.

US patent 3442193 describes a camera in which the focusing state is detected and displayed in a viewfinder. It is also known to control the focusing state of a camera automatically by utilizing the above focusing state detecting system.

A known automatic focusing system is illustrated in Fig. 1 of the accompanying drawings. A pick-up or camera lens 1 is associated with a focusing screen 2 and is provided with a focus adjusting mechanism driven by a driving means 3. There is further provided a light source 4 and a light receiving member 5 used to measure a distance between a subject 6 and the body of the camera. The light source 4 emits, for example, an infrared radiation beam towards the optical axis of the lens 1, and the light receiving direction of the light receiving member 5 is changed in a plane which includes the infrared radiation beam emitted from the light source 4 and the optical axis of the lens 1, so that infrared radiation from the light source 4 which has been reflected by the subject 6 is detected thereby.

The detection signal from the light receiving member 5 is supplied to a signal processing circuit 7.

To measure the distance, the light source 4 is energized by a signal derived from the signal processing circuit 7 and at the same time the light receiving direction of the light receiving member 5 is gradually changed from the direction parallel to the optical axis of the lens 1 in the inward direction as shown by an arrow P in Fig. 1. When the reflected light beam from the subject 6 is detected by the light receiving member 5, a detection signal dependent on the angle of the light receiving direction of the light receiving member 5 is supplied to the signal processing circuit 7 to measure the distance from the camera body to the subject 6.

A signal dependent on the measured distance is then supplied from the signal processing circuit 7 to the driving means 3 to adjust the focus of the lens 1.

As shown in Fig. 1, the lens 1 includes a zoom system 8. In this example, the zoom system 8 comprises a concave lens 8a and a convex lens 8b and is used in telephoto mode by locating the concave lens 8a near the convex lens 8b, and in wide-angle mode by locatig the concave lens 8a away from the convex lens 8b. The focus adjustment of the lens 1 is carried out by moving a convex lens 1 a, another convex lens 1 b remaining fixed.

As shown in Fig. 1, luminous flux over an extent W is received from the subject 6 in the wide-angle mode and over an extent T is received in the telephoto mode.

In this automatic focusing system, owing to the fact that the lens 1 and the optical system comprising the light source 4 and the light receiving member 5 for the distance measur ing are all independently constructed, an area S irradiated by luminous flux emitted from the light source 4 is not altered, even if the optical field of the lens 1 is changed by the zoom action.

This will be described more specifically with reference to Figs. 2A to 2C of the accompany ing drawings. Fig. 2A shows a case where the zoom lens is utililized in a standard focal length mode. In this case, the subject 6, which will be displayed in a viewfinder F of the camera, is seen therein as shown in the figure. It is assumed that the area S of the irradiated luminous flux at this time will be as represented by a broken line D. On the other hand, Fig. 2B shows a case in which the zoom lens is used ina telephoto mode and therefore the subject 6 is enlarged as illus trated in the figure.At this time, although the area S of the irradiated luminous flux remains fixed, the distance measuring area is forced to be enlarged to cover almost all the area of the field as shown by a one-dot chain line D' because the subject 6 displayed within the viewfinder F is also enlarged. Fig. 2C shows a case in which the zoom lens is used in a wide angle anglo mode and the subject 6 as displayed in the viewfinder F is reduced as illustrated in the figure and the distance measuring area (shown by a two-dots chain line D" in Fig.

2C) becomes small.

As described above, in the case of the zoom lens, the apparent distance measuring area relative to the frame of the viewfinder F fluctuates. But in these cases, although the distance measuring areas D' and D" relative to the frame of the viewfinder F are increased or reduced, such increase or reduction is not perceivable in the viewfinder F, so it cannot be seen whether the subject 6 is precisely focused.

In other words, in such cases, the user is apt to handle the camera system as if the distance measuring area were fixed (as shown by the broken line D). As a result, when the zoom lens is used, for example in a telephoto mocge, there is a risk that if another object is present between the distance measuring areas D (shown by the broken line) and D' (shown by one-dot chain line) which is in front of or behind the subject 6, the focus adjustment will be set in front of or behind the subject 6, even when the subject 6 is within the broken line D in the viewfinder F in Fig. 2B.This results in the defect that when a picture of a rounded subject 6 such as a human figure is taken, the focus adjustment is liable to be performed around (rather than on) the part of the subject 6 to be picked up, so that the focus is not correctly made coincident with the intended part of the subject 6. When the zoom lens is used in a wide-angle mode, even if the subject 6 is caught within the broken line D in the viewfinder F as seen in Fig. 2C, there is also a risk that the focus adjustment will not be performed when the subject 6 is outside the two-dots chain line D".

In this case, in the telephone mode, since the depth of field is small, it is necessary to adjust the focus correctly. On the other hand, in the wide-angle mode, the focus is less critical because the depth of field is large.

Thus with the auto focus system described above, the precision of distance measurement is lost particularly when the zoom lens is used in a telephoto mode, which is when high precision in focus adjustment is particularly required.

According to the present invention there is provided an automatic focusing system for a iens including a zoom system and a focus adjusting mechanism, the automatic focusing system comprising: a distance measuring light source; and a distance measuring light receiving member having means for detecting the angle of the reflected light beam emitted from said light source, reflected by a subject and received by said light receiving member; said focus adjusting mechanism being driven in dependence on a distance measurement output from said light receiving member; and one of said light source and said light receiving member being disposed behind said zoom system within said lens.

According to the present invention there is also provided an automatic focusing system for a camera wherein a beam of light is directed towards a subject from a light source within the camera, a portion of said light beam being reflected from the subject back to the camera, the path of said beam passing through a lens of the camera, and sensing means is provided in said camera by which the angle between the emitted and the reflected beam is translated by mechanical means into a mechanical movement which is a function of the distance to the point of reflection on the subject, said mechanical movement being used to focus said lens on the subject.

According to the present invention there is also provided in a photographic camera having a zoom lens, said zoom lens having at least one lens component mounted for movement along the optical axis of said lens for providing a change in the effective focal length of said lens, an auto focusing means for automatically adjusting the effective focal length of said zoom lens to bring said lens into sharp focus.

The invention will now be described by way of example with reference to the accompany ing drawings, throughout which like references designate like elements, and in which: Figure 1 is a diagram showing the form of known automatic focusing system; Figures 2A to 2C are diagrams used to explain the automatic focusing system of Fig.

1; Figure 3 is a diagram showing an embodiment of automatic focusing system according to the invention; and Figures 4A and 4B are explanatory diagrams useful for showing the effect which the embodiment of Fig. 3 can achieve.

As shown in Fig. 3, the embodiment of automatic focusing system to be described comprises a half-mirror 9 disposed at an angle of 45 relative to the optical axis of a lens 1 between a zoom system 8 and a convex lens 1 b of the lens 1. A light receiving member 5 such as a photoelectric means for measuring the distance is located at the position where the optical axis of the lens system 1 is reflected by the half-mirror 9. A light source 4 is provided at the position of the light receiving member 5 as was previously illustrated in Fig. 1. The irradiation direction of the beam from the light source 4 is arranged to change in a plane which includes the optical axis of the lens 1.

The distance measurement is carried out as follows. When it is desired to perform the distance measurement, the light source 4 is energized by the signal derived from a signal processing circuit 7 and the irradiation direction of the light beam therefrom is gradually changed inwardly from the direction parallel to the optical axis of the lens 1, as indicated by an arrow P. When the reflected light beam from a subject 6 enters a convex lens 1 a, it is passed through the zoom system 8 and is relfected by the half-mirror 9 so as to fall on the light receiving member 5. A detecting signal dependent on the angle of the irradiation direction of the beam from the light source 4 is supplied from the light receiving member 5 to the signal processing circuit 7, and so the distance from the camera body to the subject 6 is measured.

Therefore, with this system, since the light receiving member 5 is located behind the zoom system 8, the distance measuring area of the light received by the light receiving member 5 always follows the field enlargement or reduction due to the zoom system 8 and the distance measuring ranges in the telephoto and wide-angle modes have constant values relative to the frame of the viewfinder F. In other words, as shown in Fig.

3, in a telephoto mode, the distance measurement is carried out over an extent shown by a one-dot chain line T, and in the wide-angle case, the distance measurement is carried out over an extent shown by a two-dots chain line W. In a telephoto mode, the distance mea surement is carried out with high accuracy higher than in a wide-angle mode. This is because in a wide-angled mode, the focal position of the reflected light for all areas of the subject 6 may be anywhere within the range Lw in Fig. 3, thus giving rise to scattering of the focal position. On the contrary, in a telephoto mode, the area of the reflected light is quite narrow, so that the focal position is only moved within the range LT in Fig. 3.

Accordingly, the precision in the focus adjustment is improved in a telephoto mode as compared with a wide-angle mode.

In this way, the distance is measured and the focus of the lens 1 is adjusted. With the embodiment, since the distance measuring area in the viewfinder F is not changed in telephoto and wide-angle modes, there is no possibility that the user may misoperate the automatic focusing system. In addition, in a telephoto mode, more accurate distance measurement is carried out and therefore good focus adjustment can always be obtained.

Moreover, although in the known automatic focusing system of Fig. 1 there is a greater risk of error in measuring short distances because the beam from the light source 4 is spaced from the optical axis of the lens 1, with the embodiment of automatic focusing system, the light receiving member 5 is so arranged that it receives light passing along the optical axis of the lens 1, so that there is no risk of such error. This will be described with reference to Figs. 4A and 4B which respectively illustrate the presence or absence of paralax in measuring the distance in case of the prior known automatic focusing system and the embodiment. There figures additionally show a camera body 10.

As shown in Fig. 4A, due to the fact that the irradiation direction of the infrared light emitted from the light source 4 is fixed in the known system, areas Sx, S,, Sz of subjects X, Y and Z irradiated by the light source 4 are displaced in dependence on the distance of the subjects X, Y and Z from the camera body 10. Accordingly, the value of the luminous flux to be detected by the light receiving member 5 is changed by the distance between the camera body 10 and the subjects X, Y and Z, thereby giving rise to the error in measuring the distance.

Fig. 4B illustrates the case of the embodiment in which the light source and the light receiving member are interchanged compared with the Fig. 3 arrangment. Thus, the infrared light is emitted from a light source 1 2 built into the camera body 10 to irradiate the respective subjects X, Y and Z by way of a half-mirror 13, and the light is received on a light receiving member 11. In the irradiated areas Sx, 5y and Sz are always coincident with the respective subjects X, Y and Z irrespective of the distances between the camera body 10 and the subjects X, Y and Z, so no parallax occurs.

In case of the known automatic focusing system as shown in Fig. 1, since the light source 4 and the light receiving member 5 are provided in addition to the lens 1, this greatly restricts the design of the system.

However, with the embodiment, since only the light source 4 or 1 2 is required in addition to the lens 1, there is more design freedom.

In the embodiment, the half-mirror 9 may be a dichroic mirror which reflects the infrared rays to the light receiving member 5 or 11 and passes visible light to a target screen (not shown) of the pick-up tube. Thus, no cut filter for infrared rays is required in front of the pick-up tube.

While in the described embodiment either one of the light source and the light receiving member is rotated to measure the distance of the subject from the camera body, it is possible for both the light source and the light receiving member to be fixed and instead, there are simply provided two light receiving members and the output ratio of the signals from these two light receiving members is used to detect the focusing distance. In this case, it is also possible for the light source and the light receiving member to be interchanged in position as shown in Fig. 4B, so that the light source is located within the lens.

Although described in relation to a television camera having a pick-up tube, the invention is also applicable to a. photographic camera using photographic film or magnetic video-type image recording.

Claims (7)

1. An automatic focusing system for a lens including a zoom system and a focus adjusting mechanism, the automatic focusing system comprising: a distance measuring light source; and a distance measuring light receiving member having means for detecting the angle of the reflected light beam emitted from said light source, reflected by a subject and received by said light receiving member; said focus adjusting mechanism being driven in dependence on a distance measurement output from said light receiving member; and one of said light source and said light receiving member being disposed behind said zoom system within said lens.

2. An automatic focusing system according to claim 2 wherein said means for detecting the angle of the reflected light beam comprises means for relatively rotating one only of said light source and said light receiving member.

3. An automatic focusing system according to claim 2 wherein said light source emits infrared light and said lens includes a mirror for reflecting a portion of said infrared rays.

4. An automatic focusing system for a camera wherein a beam of light is directed towards a subject from a light souce within the camera, a portion of said light beam being reflected from the subject back to the camera, the path of said beam passing through a lens of the camera, and sensing means is provided in said camera by which the angle between the emitted and the reflected beam is translated by mechanical means into a mechanical movement which is a function of the distance to the point of reflection on the subject, said mechanical movement being used to focus said lens on the subject.

5. In a photographic camera having a zoom lens, said zoom lens having at least one lens component mounted for movement along the optical axis of said lens for providing a change in the effective focal length of said lens, an auto focusing means for automatically adjusting the effective focal length of said zoom lens to bring said lens into sharp focus.

6. An automatic focusing system substantially as hereinbefore described with reference to Fig. 3 of the accompanying drawings.

7. An automatic focusing system substantially as hereinbefore described with reference to Fig. 3 as modified by Fig. 4B of the accompanying drawings.