Focus Detector Having First and Second Non-Overlapping Sensor Series

1. Field of the Invention

This invention relates to an optical system for detecting the state of focus, and in particular to such optical system suitable for determining the focusing of a single lens reflex camera such as a camera or a video camera.

2. Description of the Prior Art

The heretofore proposed optical systems for detecting the state of focus of single lens reflex cameras or the like are generally classified into two types, i.e. the image sharpness detection type and the image displacement detection or image correlation detection type.

In the image sharpness detection system such as disclosed in, for example, Japanese Laid-open Patent Application No. 155308/1980, the degrees of sharpness of the object images on three CCD line sensors, disposed on a predetermined image-forming plane of the image-forming optical system and equidistantly forwardly and rearwardly thereof respectively, are compared to detect the state of focus.

On the other hand, the optical system for detecting the state of focus of a camera using the image displacement detection or image correlation detection system is known, for example, from Japanese Laid-open Patent Application No. 143315/1978, Japanese Laid-open Patent Application No. 7323/1979 or U.S. Pat. No. 4,185,191. FIG. 1 of the accompanying drawings schematically show the construction of a device using the system of U.S. Pat. No. 4,185,191. An image forming light beam l from an image-forming lens, not shown, passes through a partly transmitting quick return mirror 1 and then enters a submirror 2 and is downwardly reflected and enters a lenslet array comprising juxtaposed lenslets 3a, 3b, . . . . Respective sets of light-receiving sensors 4a.sub.1, 4a.sub.2 ; 4b.sub.1, 4b.sub.2 ; . . . are disposed rearwardly of the respective lenslets 3a, 3b, . . . . In the case of such image displacement detection or image correlation detection system, the lenslets 3a, 3b, . . . are present at positions conjugate with the predetermined image-forming plane and therefore, during the in-focus state, entirely the same information light enters the sets of sensors 4 a.sub.1, 4a.sub.2 ; 4b.sub.1, 4b.sub.2 ; . . . disposed rearwardly of the respective lenslets 3a, 3b, . . . When the image-forming plane of the image-forming light beam l does not exist on the lenslet array lying on the predetermined image-forming plane, that is, during the out-of-focus state, the sensor array comprising sensors 4a.sub.1 and 4b.sub.1 and the sensor array comprising sensors 4a.sub.2 and 4b.sub.2 receive image information having an amount of image displacement corresponding to the amount of defocus. In FIG. 1, the quick return mirror 1 is not an ordinary beam splitter using a semi-transparent film of dielectric material, but a so-called wavefront division type patterned beam splitter having light-transmitting portions 1a of diameter W. Such a patterned beam splitter performs the role of a low-pass filter, and the size and distribution of the light-transmitting portions 1a are determined in conformity with the arrangement pitch p of each sensor 4a.sub.1, 4b.sub.1 of the sensor array.

When it is supposed that such optical system is incorporated as the image displacement detection or image correlation detection system into a camera, there is, for example, a method whereby the image-forming light beam l is divided into two light beams and two image intensity distributions regarding the object images formed by the divided two light beams are formed and used to detect the state of focus. A roof type submirror 5 as shown, for example, in FIG. 2 of the accompanying drawings is conceivable as a beam splitting element for dividing the image-forming light beam l into two light beams. In FIG. 2, the light beam l from an image-forming lens 6 enters the roof type submirror 5 having two reflecting surfaces 5a and 5b, and then is reflected by the reflecting surfaces 5a and 5b to provide light beams la and lb, which are incident on downwardly positioned line sensors 7a and 7b each comprising, for example, a CCD consisting of a plurality of sensors. The outputs of the line sensors 7a and 7b may then be compared to thereby detect the state of focus. Where the image-forming light beam 1, as shown in FIG. 3 of the accompanying drawings, enters as a circle R intermediate the reflecting surfaces 5a and 5b of the roof type submirror 5, equal quantities of light are directed to the line sensors 7a and 7b, but when use is made of an image-forming lens of great F-number, the off-axis light beam l', for example, enters the roof type submirror 5 as a circle R' and as a result, almost all of the off-axis light beam l' enters the reflecting surface 5b to provide a reflected light 1b' and therefore, there occurs a phenomenon that the reflected light beam enters one line sensor 7b while it hardly enters the other line sensor 7a. In a condition in which imbalance has thus occurred between the quantities of light entering the line sensors 7a and 7b, the distributions of quantity of light regarding the image intensity distributions entering the line sensors 7a and 7b when the object to be photographed is a surface of uniform brightness are expressed as schematically shown by A and B in FIG. 4 of the accompanying drawings. In FIG. 4, the abscissa is coincident with the arrangement direction x of the line sensors 7a and 7b shown in FIG. 2 and 0x is the position whereat the light beams on the optical axes on the respective line sensors 7a and 7b intersect each other. The ordinate represents the relative quantity of light, A indicates the distribution of quantity of light on the line sensor 7a which is rightwardly downward, and B indicates the distribution of quantity of light on the line sensor 7b which is rightwardly upward. Such a phenomenon that the distribution of quantity of light is not uniform even if the surface of uniform brightness is imaged is tentatively called the shading, and for example, in the focus detecting device of FIG. 2, if said shading is present when the distributions of quantity of light A and B of the images created on the line sensors 7a and 7b are to be compared and operationally processed, the state of focus cannot be detected properly.

It is an object of the present invention to provide an optical system for detecting the state of focus using an image displacement detection or image correlation detection system and which can effect accurate detection of the state of focus by the use of a beam splitting element which hardly causes the shading when a light beam from an image-forming optical system is to be divided into a plurality of light beams.

A feature of the present invention is that a patterned beam splitting element in the form of a one-dimensional microprism installed between the image-forming optical system and its predetermined image-forming plane is utilized to optically divide the exit pupil of the image-forming optical system into a plurality of exit pupils and the image-forming light beams from the divided exit pupils are used to form a plurality of image intensity distributions regarding the object image and the relative positional relation between the plurality of image intensity distributions is detected by a sensor unit to thereby discriminate the state of focus of the image-forming optical system.

The invention will become more fully apparent from the following detailed description thereof taken in conjunction with the accompanying drawings.

FIG. 1 shows the construction of a focusing optical system of a camera using the image displacement detection or image correlation detection system according to the prior art.

FIGS. 2 and 3 illustrate a case where a roof type submirror is used as a beam splitting element.

FIG. 4 illustrates the shading of an image intensity distribution created on the surface of a line sensor.

FIGS. 5A and 5B and 6 through 9 show embodiments of the optical system for detecting the state of focus according to the present invention, FIGS. 5A and 5B schematically showing the construction thereof, FIG. 6 being a front view of a multi-surfaced mirror as a beam splitting element, FIGS. 7 and 8 being cross-sectional views of the multi-surfaced mirror, and FIG. 9 showing another construction of the optical system for detecting the state of focus according to the present invention.

The present invention will hereinafter be described with respect to embodiments thereof shown in FIG. 5 et seq.

The gist of the present invention resides in that as compared with FIG. 2, for example, a microprism-shaped multi-surfaced mirror 10 exhibiting an optical action in a one-dimensional direction is employed instead of the roof type submirror 5 and that in FIG. 1, two sensors 4a.sub.1 and 4b.sub.1 are disposed on a predetermined plane rearward of, i.e. to receive light from, a single lens 3a, whereas in the embodiment of the present invention shown in FIG. 5A, a single sensor 12a.sub.1, 12a.sub.2, . . . , 12b.sub.1, 12b.sub.2, . . . is disposed at a first position rearward of individual lens portions or lenslets 11a.sub.1, 11a.sub.2, . . . , 11b.sub.1, 11b.sub.2, . . . .

Before describing FIG. 5A, the function of a multi-surfaced mirror 10 which is a novel optical element will be described by reference to FIG. 6. FIG. 6 is a front view of the multi-surfaced mirror 10 as seen through an image-forming lens 6 in FIG. 5A. In FIG. 6, a circle R is a cross-section of a light beam L imaged near the optical axis O of the image-forming lens 6 when the light beam is incident on the multi-surfaced mirror 10, and a circle R' is likewise a cross-section of a light beam L' imaged at a position spaced apart from the optical axis O. The cross-sectional shape of the polyhedral mirror 10 is as shown, for example, in FIGS. 7 and 8 which will be described later, and mirror surfaces 10a.sub.1, 10a.sub.2, . . . shown in the form of strips and without hatching applied thereto in FIG. 6 and mirror surfaces 10b.sub.1, 10b.sub.2, . . . shown with hatching applied thereto are series having different inclinations with respect to the optical axis O. The series of mirror surfaces 10a.sub.1, 10a.sub.2, . . . correspond to the mirror surface 5a of FIG. 2 and the series of mirror surfaces 10 b.sub.1, 10b.sub.2, . . . correspond to the mirror surface 5b of FIG. 2. Accordingly, as will readily be understood from FIG. 6, the rate of the area of the series of mirror surfaces 10a.sub.1, 10b.sub.2, . . . contained in the light beam L represented by the circle R and the area of the series of mirror surfaces 10b.sub.1, 10b.sub.2, . . . hardly changes even if the light beam L becomes the offaxis beam L' represented by the circle R' and therefore, the shading as shown in FIG. 4 which is caused by the roof type submirror 5 of FIG. 2 does not occur.

Accordingly, the light beam passed through the image-forming lens 6 in FIG. 5 is incident on the multi-surfaced mirror 10 having its surfaces obliquely disposed with respect to the optical axis O and is thereafter separated into two light beams by the two series of mirror surfaces. The respective light beams then pass through a lens element 15 comprising prism surfaces 13a, 13b for correcting the inclination of the image plane from the flat plane and field lenses 14a, 14b formed integrally with the prism surfaces and may form image intensity distributions regarding the object image on the lenslets 11a.sub.1, 11a.sub.2, 11a.sub.3, . . . and lenslets 11b.sub.1, 11b.sub.2, 11b.sub.3, . . . of lenslet arrays disposed in two series. The lenslets 11a.sub.1, 11a.sub.2, . . . and 11b.sub.1, 11b.sub.2, 11b.sub.3, . . . of these two series of lenslet arrays are disposed at positions conjugate with the predetermined image-forming plane of the image-forming lens 6 (in the case of a single lens reflex camera, the film surface), and if the image-forming lens 6 lies at a position which renders the object to be photographed and the film surface conjugate with each other, two sharp image intensity distributions will be formed on each lenslet 11a.sub.1, 11a.sub.2, . . . , 11b.sub.1, 11b.sub.2, . . . of the two lenslet arrays and, if the image-forming lens 6 does not lie at the position which renders the object to be photographed and the film surface conjugate with each other, defocused image intensity distributions will be formed, and further, the spacing between these two image intensity distributions will become different in conformity with the respective amounts of defocus. For example, when the image-forming lens 6 is in front focus, the two image intensity distributions come near each other while blurring, and when the image-forming lens 6 is in rear focus, the two image intensity distributions conversely come off each other while blurring. Of these image intensity distributions, the light beam formed on the lenslets 11a.sub.1, 11a.sub.2, 11a.sub.3, . . . may be incident on the sensors 12a.sub.1, 12a.sub.2, 12a.sub.3, . . . of one sensor array on a sensor substrate 12 and the light beam formed on the lenslets 11b.sub.1, 11b.sub.2, 11b.sub.3 of the other lenslet array may be incident on the sensors 12b.sub.1, 12b.sub.2, 12b.sub.3, . . . of the second sensor array. In FIG. 5A, the distance in the direction of travel of light is depicted as being greater than the actual distance in order to make the field lenses 14a, 14b, the lenslets 11a.sub.1, 11a.sub.2, . . . , the sensor substrate 12, etc. readily understood. Now, considering a case where a respective sensor, for example, 12a.sub.2, of the sensor array is reversely projected, the light beam passes through the lenslet 11a.sub.2 and the field lens 14a and is reflected by one series of mirror surfaces 10a.sub.1, 10a.sub.2, . . . of the multi-surfaced mirror 10 and forms an image Ia on or near the exit pupil of the image-forming lens 6, and this image Ia is nearer to one side with respect to the optical axis O of the image-forming lens 6. When the light ray leaving the sensor 12a.sub.2 is imaged by way of the mirror surfaces 10b.sub.1, 10b.sub.2, . . . of the multi-surfaced mirror 10 which have another inclination, the image is formed on that side opposite to the image Ia with respect to the optical axis O, that is, on that side of the image Ib on which the image of the sensor 12b.sub.2 is formed by way of the mirror surfaces 10b.sub.1, 10b.sub.2, . . . of the multi-surfaced mirror 10, and it may deviate from the exit pupil of the image-forming lens 6.

Conversely, considering a thin light beam La passing through the center of the image Ia formed on the exit pupil of the image-forming lens 6, it is separated into two beams directed in two directions by the multi-surfaced mirror 10, as shown in FIG. 5B, to provide a light beam La.sub.1 reflected by the series of mirror surfaces 10a.sub.1, 10a.sub.2, . . . and a light beam La.sub.2 reflected by the other series of mirror surfaces 10b.sub.1, 10b.sub.2, . . . and the beam La.sub.1 is incident as a spot La.sub.1 on the sensor 12a.sub.2 and the beam La.sub.2 is incident as a spot La.sub.2 on the insensitive zone intermediate the sensors 12b.sub.2 and 12b.sub.3. This also holds true of a light beam Lb. In this manner, the sensors 12a.sub.2 and 12b.sub.2 are disposed in opposite directions with respect to the optical axes S.sub.1 and S.sub.2 of the respective lenslets 11a.sub.2 and 11 b.sub.2.

The polyhedral mirror 10 shown in FIG. 5A will now be described further. FIG. 7 shows the cross-sectional structure of the multi-surfaced mirror 10. The cross-section of the mirror surface thereof is in the form of a triangular wave, and incident light beam L may be wavefront-divided into two light beams La and Lb by inclined surfaces 10a.sub.n and 10b.sub.n. FIG. 8 shows the cross-sectional structure of another multi-surfaced mirror 10. A minute microprism structure 17 formed of a transparent material of triangular wave configuration is provided on the upper layer of a substrate 16, and the upper surface of the substrate 16 provides a reflecting surface 18. In this case, the light beam L is not directly reflected by the prism structure 17, but is reflected by the reflecting surface 18 and divided into light beams La and Lb by the prism structure 17. The cross-section of the multi-surfaced mirror 10 is not limited to the triangular wave form, but may be, for example, a trapezoidal wave form.

FIG. 9 shows another embodiment of the present invention. In the optical system of FIG. 5A, the image intensity distribution has been shifted to left and right by the multi-surfaced mirror 10, whereas in the present embodiment, the numerous mirror surfaces of the multi-surfaced mirror 10 are disposed horizontally so that two image intensity distributions are shifted forwardly and rearwardly. Also, in FIG. 5A, the series of lenslets 11a.sub.1, 11a.sub.2, . . . , 11b.sub.1, 11b.sub.2, . . . and the series of sensors 12a.sub.1, 12a.sub.2, . . . , 12b.sub.1, 12b.sub.2, . . . corresponding to the two image intensity distributions are arranged laterally in a row, whereas in the present embodiment, these are arranged in two forward and rearward series. Sensors 12a and 12b are projected as Ia and Ib upon the exit pupil of the image-forming lens 6 on the opposite sides of the optical axis of the image-forming lens 6.

In the above-described embodiments, the optical system is constructed with the multi-surfaced mirror 10 as the reflection type, but an optical system having a similar function can be constructed even if the multi-surfaced mirror 10 is of the transmission type.

The difference between the optical system of the present invention having such construction and the invention disclosed in U.S. Pat. No. 4,185,191 shown in FIG. 1 is that in the case of the present embodiment, the same image intensity distributions are spatially completely separately formed on the two series of surfaces of the lenslets and the respective image intensity distributions are received by two series of light-receiving systems each comprising multiple sets of a lenslet and a sensor and corresponding to the number of images.

As described above in detail, the optical system for detecting the state of focus in accordance with the present invention is a novel optical system in which at least two image intensity distributions are formed on the predetermined image-forming plane and which may be effectively used in a device for detecting in-focus and out-of-focus by utilizing the fact that detected images deviate relative to each other in accordance with the imaged condition thereof when the respective image intensity distributions are detected as by a CCD sensor array, and this optical system has the advantage that shading is hardly caused.