Focus detection device and distance measurement device

[0001] 1. Field of the Invention

[0002] The present invention relates to a focus detection device and a distance measurement device which are mounted on optical equipment such as a camera. The present invention can allow obtaining high-contrast object images and performing excellent focus detection and distance measurement at all times.

[0003] 2. Related Background Art

[0004] The technology for optically measuring distances to objects which exist in a plurality of directions is described in, for example, the publications of U.S. Pat. Nos. 4,749,848 and 4,916,302. This technology can allow a photographer to obtain distance distribution information and defocus quantity distribution information for objects which exist in an objective field and to recognize the placement of the objects in the objective field based on these types of distribution information.

[0005] According to this approach, a camera having CCD imaging elements or the like is used to take an image of the objects and a pair of resulting images having a parallax with respect to each other are used to perform well-known correlation calculations on the parallax images to determine a defocus quantity. In addition, distances to the objects with respect to each calculation area can be determined based on the principle of triangulation. Thus, these calculations can be performed equally on each area of the resulting images to obtain such distance and defocus quantity distribution information.

[0006] Alternatively, there has been conventionally known a focus detection device for a TTL camera. This is an automatic focus detection device which uses a so-called pupil splitting method to detect focusing conditions of an image taking optical system from a relative deviation quantity of a plurality of object images generated by luminous fluxes from regions of each having a different pupil position in the image taking optical system. For example, this kind of autofocus device has been disclosed, which consists of an array pair, that is, a lens array positioned in the proximity of a primary image plane and a optical receiving element array positioned directly behind the lens array. Another example of this kind of autofocus device has been also disclosed, which consists of a field lens positioned on a primary image plane, two re-imaging lenses for re-imaging an image formed on the primary image plane onto a secondary image plane, and two image sensor arrays positioned on the secondary image plane.

[0007] Now, a prior art will be described below with reference to FIG. 6.

[0008] Reference numeral 601 denotes an image taking lens, 602 denotes a quick return mirror, 603 denotes a sub-mirror, 604 denotes a field lens, 605 denotes a diaphragm, 606 denotes a secondary imaging lens, 607 denotes an area sensor, 608 denotes a focus detection portion, 609 denotes a focus output portion, 610 denotes a focus lens, 611 denotes a pentaprism, 612 denotes an eyepiece, 613 denotes a photographer's eye, 614 denotes a shutter curtain, and 615 denotes a film plane.

[0009] A part of the luminous flux passing through the image taking lens 601 is imaged on the area sensor 607 by the quick return mirror 602 and the sub-mirror 603 through the field lens 604, the diaphragm 605, and the secondary imaging lens 606. Here, the detailed configuration of the focus detection optical system will be described below in detail with reference to FIG. 7.

[0010] The luminous flux is guided respectively from different pupil positions of the image taking lens 601 onto two imaging screens 607a and 607b of the area sensor 607 to be re-imaged under an imaging magnification determined by the field lens 604 and the secondary imaging lens 606. The area sensor 607 is positioned at a location optically equivalent to an image taking film plane with respect to the image taking lens 601 and imaging screens 607a and 607b have a field of view equal to a portion of an image taking screen or the image taking screen itself, respectively. The diaphragm 605 has a function of a diaphragm as well as that of an infrared cut filter, which removes undesired light.

[0011] The above-mentioned configuration can allow the imaging screens 607a and 607b to have a parallax of different pupil positions of a predetermined image taking lens. The imaging screens with this parallax are used to perform well-known correlation calculations on signals in opposed blocks in the focus detection portion 608 of FIG. 6 to determine a distance to an object in a previous block as well as a defocus quantity. For this purpose, it is preferable that a high-contrast image is used because the correlation calculations are performed to detect the quantity of displacement of the image. The correlation calculations cannot be performed correctly on a low-contrast image, for which the focus detection cannot be performed. This determination is performed on a predetermined block to obtain distance information or defocus quantity information and the result is provided by the focus output portion 609. Based on the provided result, the image taking lens 601 is driven to achieve a correct focal position for implementing autofocus.

[0012] The optical axis of the remainder of the luminous flux passing through the image taking lens 601 is bent by the quick return mirror 602 and the remainder of the luminous flux is guided through the focus lens 610, the pentaprism 611, and the eyepiece 612 to the photographer's eye 613 to be finally recognized as a field-of-view of image for image taking.

[0013] Thereafter, by pressing a shutter release button, the quick return mirror 602, the sub-mirror 603, and the shutter 614 are withdrawn to expose the film 615 (for taking the image).

[0014] The area sensor for focus detection usually has a sensitivity characteristic that it shows sensitivity over the entire visible spectrum. Since the area sensor performs sensitivity identification rather than color identification, it can produce outputs of the same level from inputs of different colors if they have the same sensitivity. This operation will be described below with reference to FIGS. 8A to 8C.

[0015] FIG. 8A shows an example of an object for image taking and in the drawing, a first color is represented by oblique-line hatching and a second color is horizontal-stripe hatching. In addition, it is assumed that the second color is located in the background for the first color and that the second color forms a higher-contrast image as compared with a stripe pattern of the first color.

[0016] FIG. 8B shows wavelength as an axis of abscissas and sensitivity as an axis of ordinates for modeling the sensitivity characteristic of the area sensor for focus detection, which shows that the area sensor has a sensitivity characteristic that it shows sensitivity over the entire visible spectrum.

[0017] FIG. 8C shows an image obtained by the area sensor for focus detection when the first and second colors shown in FIG. 8A have the wavelength and sensitivity as shown in FIG. 8B, respectively. A graph in the lower portion of FIG. 8C shows the output level of the output image described above in one-dimensional representation. This indicates that the first and second colors have different wavelengths and that the same output level is produced due to the sensor's sensitivity characteristic.

[0018] It may still have plenty of room for improvement in order to provide a focus detection device and a distance measurement device which can allow obtaining a high-contrast object image and performing excellent focus detection or distance measurement operations at all times.

[0019] One aspect of this invention has a plurality of light receiving sensors having different sensitivity characteristics, a focus detection circuit for performing focus detection based on the output of any one of the light receiving sensors, and a determination circuit for determining whether the focus detection result is appropriate, and if the determination circuit determines that the result is not appropriate, the focus detection circuit performs focus detection based on the output of another light receiving sensor. This can allow performing focus detection from a high-contrast object image at all times, irrespective of color of an object.

[0020] Another aspect of this invention has a plurality of light receiving sensors having different sensitivity characteristics, a distance measurement circuit for performing distance measurement based on the correlation among a plurality of pupil-split images obtained from the output of any one of the light receiving sensors or among a plurality of images with a parallax, and a determination circuit for determining whether the distance measurement result is appropriate, and if the determination circuit determines that the result is not appropriate, the distance measurement circuit performs distance measurement based on the output of another light receiving sensor. This can allow performing distance measurement through correlation calculations of a high-contrast object image at all times, irrespective of color of an object.

[0021] FIG. 1 is a block diagram for schematically showing a single-lens reflex camera according to each embodiment;

[0022] FIGS. 2A, 2B and 2C show several diagrams for explaining an effect of a first embodiment;

[0023] FIG. 3 is a flow chart for showing an operation of a major portion of the first embodiment;

[0024] FIG. 4 is a flow chart for showing an operation of a major portion of a second embodiment;

[0025] FIG. 5 is a flow chart for showing an operation of a major portion of a third embodiment;

[0026] FIG. 6 is a block diagram for schematically showing a prior single-lens reflex camera;

[0027] FIG. 7 is a perspective view for showing an optical system which performs focus detection through pupil splitting; and

[0028] FIGS. 8A, 8B and 8C are several diagrams for explaining problems of the prior system.

[0029] Now, the present invention will be described below in detail with reference to preferred embodiments.

[0030] (First Embodiment)

[0031] FIG. 1 is a block diagram for schematically showing a single-lens reflex camera according to a first embodiment of the present invention. In FIG. 1, the same components as those in FIG. 6 have the same reference numerals and they will not be further described below.

[0032] In FIG. 1, reference numeral 101 denotes a half mirror for splitting a luminous flux into two parts, one having a bent optical axis and the other passing through straight ahead. Reference numerals 102 and 108 denote field lenses, 103 and 109 denote diaphragms, 104 and 110 denote secondary imaging lenses, 105 and 111 denote area sensors, 106 denotes a first focus detection portion, 112 denotes a second focus detection portion, and 107 denotes a processed focus output portion.

[0033] An optical axis of a part of the luminous flux passing through the image taking lens 601 is bent by the quick return mirror 602 and the sub-mirror 603 and the part of the luminous flux is further splitted by the half mirror 101 into two parts, one of which is imaged on the area sensor 105 through the field lens 102, the diaphragm 103, and the secondary imaging lens 104. The image imaged on the area sensor 105 with different pupil positions is subject to well-known correlation calculations performed by the first focus detection portion 106 to obtain defocus quantity information of an object. The result is supplied to the processed focus output portion 107.

[0034] The other part of the luminous flux splitted by the half mirror 101 is similarly imaged on the area sensor 111 through the field lens 108, the diaphragm 109, and the secondary imaging lens 110. Then, defocus quantity information of the object is obtained by the second focus detection portion 112 and the result is supplied to the processed focus output portion 107 in a similar manner as described above. The processed focus output portion 107 makes a determination on the information from the first focus detection portion 106 and the information from the focus detection portion 112 and any information which is determined to be appropriate will be provided as a focus detection result. Thereafter, based on the focus detection result, the image taking lens 601 is driven for focusing in a similar manner as that for the prior system.

[0035] As described later, first focus detection means consisting of a series of components from the field lens 102 to the first focus detection portion 106 and second focus detection means consisting of another series of components from the field lens 108 to the second focus detection portion 112 have their respective optical filters of different pass wavelength regions positioned in their respective optical paths, and thus, these focus detection means will have different sensitivity characteristics.

[0036] Alternatively, the element 106 may be replaced by a first distance measurement portion, the element 112 by a second distance measurement portion, and the element 107 by a processed distance output portion to perform similar operations.

[0037] The plurality of sensors having different sensitivity characteristics, that is, these focus detection means or these distance measurement means, can be used as described above to allow performing excellent focus detection of an object for which the focus detection cannot be performed by the prior system.

[0038] With reference to FIGS. 2A to 2C, the operation and the effect of these focus detection means having different sensitivity characteristics will be described below.

[0039] FIG. 2A shows the same object image example that shown in FIG. 8 for describing the prior system. FIG. 2B shows the modeling of a first sensitivity characteristic and a second sensitivity characteristic which correspond to the sensitivity characteristics of the first and second focus detection means in this embodiment, respectively. FIG. 2C shows output images when the image of FIG. 2A is detected, to be obtained based on these two sensitivity characteristics.

[0040] A first output image shown in FIG. 2C has the first sensitivity characteristic that it shows sensitivity to the first color. As shown in the drawing, an output signal is provided for the first color but no sensitivity is shown to the second color with no output. However, the output signal exhibits so distinct contrast that two images of different pupil positions can be correlated for excellent focus detection. Similarly, a second output image shown in FIG. 2C shows sensitivity to the second color rather than to the first color, resulting in the output as shown in the drawing. Also in this case, the output signal exhibits so distinct contrast that the output image is suitable for focus detection.

[0041] Therefore, it should be understood that, according to this embodiment, either of the first and second output images can be used for focus detection on an object image for which the focus detection cannot be performed by the prior system.

[0042] Next, an approach to use the first focus detection result and the second focus detection result to obtain an optimal focus detection result will be described below with reference to a flow chart of FIG. 3.

[0043] At step S301, focus detection is performed with the first sensitivity characteristic. At the next step S302, a determination is made on whether or not the focus detection result is appropriate. At this time, the determination may be made based on the contrast, output level, or correlation value of two images having different pupil positions. Furthermore, they may be compared with preset values. If the focus detection is appropriate, the process proceeds with step S304. If it is not appropriate, the process proceeds with step S303 to perform focus detection with the second sensitivity characteristic, and then proceeds with step S304. At this step S304, the focus detection result is provided as an output.

[0044] As described above, the two focus detection means having different sensitivity characteristics can be switched appropriatly so that excellent focus detection results can be obtained at all times.

[0045] (Second Embodiment)

[0046] FIG. 4 is a flow chart for showing an operation of a major portion of a second embodiment according to the present invention. It is assumed that this camera has a similar configuration to that of the first embodiment described above.

[0047] The second embodiment is configured to switch the first and second focus detection means having different sensitivity characteristics appropriatly, depending on their respective contrast values.

[0048] At step S401 of FIG. 4, a contrast value in a predetermined field of view is calculated for each of or either one of two images from different pupil positions with the first sensitivity characteristic. The contrast calculation may be accomplished through any well-known technique or by obtaining a difference between the maximum and minimum values. Next, the process proceeds with step S402 to calculate a contrast value with the second sensitivity characteristic in a similar manner to that of step S401 described above. Then, the process proceeds to step S403 to compare the contrast values obtained at steps S401 and S402 described above to select focus detection means producing a higher contrast. At the following step S404, a determination is made on whether the image with the first sensitivity characteristic has a higher contrast, based on the comparison result of step S403 described above. The process proceeds with step S405 if it is higher or with step S406 if it is lower (that is, the image with the second sensitivity characteristic has a higher contrast).

[0049] When the process proceeds with step S405, a focus detection calculation is performed with the first sensitivity characteristic and then the process proceeds with step S407. When the process proceeds with step S406, a focus detection calculation is performed with the second sensitivity characteristic and then the process also proceeds with step S407. At step S407, a focus detection result of higher contrast selected from those obtained with the first and second sensitivity characteristics is provided as an output.

[0050] This operation can allow for selection of higher contrast focus detection so that the focus detection can be performed accurately to provide excellent stable focus detection results at all times.

[0051] (Third Embodiment)

[0052] FIG. 5 is a flow chart for showing an operation of a major portion of a third embodiment according to the present invention. It is assumed that this camera has a similar configuration to that of the first embodiment described above.

[0053] This third embodiment is configured to switch the first and second focus detection means having different sensitivity characteristics appropriatly, depending on a correlation value of two images from different pupil positions.

[0054] At step S501 of FIG. 5, a correlation value is calculated for two images from different pupil positions with the first sensitivity characteristic. The correlation value calculation may be accomplished through any well-known technique or by using the degree of matching between two images. Next, the process proceeds with step S502 to calculate a correlation value with the second sensitivity characteristic in a similar manner to that of step S501 described above. Then, the process proceeds to step S503 to compare the correlation values obtained at steps S501 and S502 described above to select focus detection means producing a higher correlation value. At the following step S504, a determination is made based on the comparison result of step S503 described above. If the image with the first sensitivity characteristic has a higher value of correlation between the two images from different pupil positions, the process proceeds with step S505. Otherwise (that is, if the image with the second sensitivity characteristic has a higher correlation value), the process proceeds with step S506.

[0055] When the process proceeds with step S505, a focus detection calculation is performed with the first sensitivity characteristic and then the process proceeds with step S507. When the process proceeds with step S506, a focus detection calculation is performed with the second sensitivity characteristic and then the process also proceeds with step S507. At step S507, a focus detection result of higher correlation selected from those obtained with the first and second sensitivity characteristics is provided as an output.

[0056] This operation can allow for selection of higher correlation focus detection so that the focus detection can be performed accurately to provide excellent stable focus detection results at all times.

[0057] According to the embodiments described above, at least two focus detection means having different sensitivity characteristics are switched appropriatly to perform focus detection, so that the present invention can produce a high-contrast object image at all times, irrespective of color of an object, thereby allowing for excellent focus detection. In order to achieve this, in the first embodiment, if the first focus detection means cannot perform focus detection, the system is switched to use another focus detection means, and in the second embodiment, image contrasts from these focus detection means are compared with each other to switch them based on the comparison result, and in the third embodiment, correlation values of two images with these focus detection means are compared with each other to switch them based on the comparison result.

[0058] In the second and third embodiments, focus detection may be performed at respective light receiving sensors to make a comparison depending on the resulting contrast information or correlation calculation values.

[0059] The above-mentioned embodiments have been described for the case of two pupil-split images but the present invention is not limited to this case and the number of images used may be two or more. Alternatively, at least two or more images with a parallax may be used for focus detection.

[0060] The above-mentioned embodiments have been described in the context of a camera which comprises two focus detection means having different sensitivity characteristics but more than two focus detection means may be used and these means may also apply to other optical equipment than such a camera. In a digital camera wherein a CCD or C-MOS is used as an image taking element, the image taking element for recording an object image may double as the focus detection device/distance measurement device in the above-mentioned embodiments.

[0061] In the above-mentioned embodiments, the sensitivity wavelength regions for the sensors are substantially identical as shown in FIG. 2 but these wavelength regions may have different widths. In the latter case, it is desirable that focus detection means with a narrower sensitivity wavelength region is first used for focus detection and if that means cannot perform focus detection, another focus detection means with a wider sensitivity wavelength region is used.

[0062] Moreover, it is needless to say that, when at least two or more images with a parallax are used, the above-mentioned focus detection means may be replaced by at least two distance measurement devices (devices for measuring the distance to an object) having different sensitivity characteristics to provide similar effects.

[0063] As described above, the above-mentioned embodiments can provide a focus detection device and a distance measurement device which can allow obtaining a high-contrast object image and performing excellent focus detection or distance measurement operations at all times.