Optical system and imaging device

An optical system includes: a main mirror (11) having a shape of a portion of a convex paraboloid which includes an opening in a center and is rotationally symmetric; a second-reflection mirror (12) which further reflects light reflected by the main mirror (11), and has a shape of a portion of a concave paraboloid which is rotationally symmetric; at least one lens which forms an image of the light reflected by the second-reflection mirror (12); and a lens barrel (14) holding the at least one lens, and a position of a front principal point of the at least one lens coincides with a focal position of the second-reflection mirror (12), and an optical axis of the at least one lens is tilted with respect to a rotational axis of each of the convex paraboloid and the concave paraboloid.

This application is a U.S. National Stage entry of PCT/JP2010/002470 filed Apr. 5, 2010, the content of which is hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to optical systems which allow wide-angle image capturing, and relates particularly to an optical system having a single viewpoint.

BACKGROUND ART

Recently, a concept of “wearable computing” using a constant-wearable device has been proposed. Particularly, a camera intended for constant wearing and constant capturing (hereinafter, described as a “wearable camera”) is capable of recording an experience of a wearer lively as it is, and various applications thereof can be considered.

One of features necessary for such a wearable camera is an angle of view comparable to a human visual field. Conventionally, for an optical system which allows such a wide angle of view, an optical system using a fisheye lens or a convex mirror has been used. Above all, an optical system using a parabolic mirror or a hyperboloidal mirror for the convex mirror has a feature of single viewpoint, that is, having properties that reflected light converges on a single point. For example, Patent Literature 1 discloses a configuration of an omnidirectional visual sensor having a single viewpoint as described above. With a configuration disclosed in Patent Reference 1, as shown inFIG. 1, light205proceeding to a focal point202of a hyperboloidal portion201of one of two sheets is reflected toward a focal point204of a hyperboloidal portion203of the other of the two sheets. Thus, it is possible to obtain an image having a single viewpoint by providing a mirror having a shape of the hyperboloidal portion201of the one of the two sheets and positioning a lens at the focal point204of the hyperboloidal portion203of the other one of the two sheets. Note that in the case of a set lens including a plurality of lenses, the same effect can be produced by positioning the front principal point of the lens group at the focal point204of the hyperboloidal portion203of the other one of the two sheets.

An advantage of having a single viewpoint is to allow a captured image to have the same projection characteristics as those of a general camera. This produces advantageous effects such as allowing applying, to the captured image, general image processing based on image geometry, or to convert the captured image into a general perspective projection image without distortions.

SUMMARY OF INVENTION

On the other hand, compactness is another important feature for an optical system in a wearable camera. In the optical system disclosed in Patent Literature 1, which uses a hyperboloidal mirror for a convex mirror, the geometric characteristics thereof render a distance between the mirror and the lens dependent on the curvature of the hyperboloid. To reduce the distance between the mirror and the lens, it is necessary to increase the curvature of the hyperboloid; however, a larger curvature results in an out-of-focus image unless a sufficient depth of field is secured. It is possible to reduce defocusing level by stopping down an aperture, but this results in a darker image in compensation.

On the other hand, disclosed as another technique for downsizing is, for example, an optical system which reduces a substantial distance between the mirror and the lens by deflecting a light path through plural reflections, and Patent Literature 2 discloses such an omnidirectional optical system.

However, the configuration disclosed in Patent Literature 2 deflects the light path in a height direction, but this does not change the light path in a mirror-radius direction. Accordingly, this allows downsizing only in the height direction. Although it is possible to improve size in the mirror-radius direction by introducing another mirror for deflecting the light path into the radius direction as well, not only does this cause difficulty in designing of the optical system that allows introducing such another mirror, but it also causes another practical problem of making it more difficult to position the mirrors without errors as the number of mirrors increases.

In addition, generally, a horizontal human visual field is approximately 200 degrees. Thus, in the case of using a convex mirror to obtain such a horizontal visual field, as shown inFIG. 2A, it is only necessary to use a portion311of a convex mirror301, as shown inFIG. 2B, instead of using a whole circumference of the convex mirror301. However, as shown inFIG. 2B, when an optical axis16of a lens group13coincides with an axis305of the convex mirror301, an invalid imaging region312in which the convex mirror is not reflected is generated within the angle of view of the lens group13. This problem also arises in a system which performs a plurality of reflections as described in the configuration in Patent Literature 2.

The present invention is conceived for the reasons described above, and it is an object of the present invention to provide a compact optical system which allows wide-angle image capturing, has a single viewpoint, and suppresses generation of the invalid region in the capture image.

An optical system according to an aspect of the present invention includes: a main mirror having a shape of a portion of a convex paraboloid which includes an opening in a center and is rotationally symmetric; a second-reflection mirror which further reflects light reflected by the main mirror, the second-reflection mirror having a shape of a portion of a concave paraboloid which is rotationally symmetric; at least one lens which forms an image of the light reflected by the second-reflection mirror; and a lens barrel holding the at least one lens, and a position of a front principal point of the at least one lens coincides with a focal position of the second-reflection mirror, and an optical axis of the at least one lens is tilted with respect to a rotational axis of each of the convex paraboloid and the concave paraboloid.

With this configuration, as shown inFIG. 3B, it is possible to capture (perform imaging of) a second-reflection mirror12(an image of a main mirror11reflected therein) to a full extent within the angle of view of a lens group13by positioning the optical axis16of the lens group13at a tilt, in contrast to the case where, as shown inFIG. 3A, an invalid imaging area312exists within the angle of view of the lens group13when the optical axis16of the lens group13(at least one lens) coincides with the rotation axes of a convex paraboloid and a concave paraboloid.FIGS. 3C and 3Dschematically show images obtained by a system represented byFIGS. 3A and 3B, respectively, withFIG. 3Dclearly showing a smaller invalid region in which the image in the second-reflection mirror12is not reflected than an invalid region inFIG. 3C. Note thatFIG. 3Dis a schematic diagram which looks enlarged lengthwise from a real image, but in practice an almost semicircular image can be obtained. In addition, a hatched portion in the semicircular center is an image of an opening of the main mirror.

Thus, each of the main mirror11and the second-reflection mirror12has a shape of a portion of a corresponding one of the convex paraboloid and concave paraboloid that are rotationally symmetric. This allows reducing size in the mirror-radius direction compared to the conventional optical system having entire shapes of the convex paraboloid and concave paraboloid. Accordingly, it is possible to provide a compact optical system which allows wide-angle image capturing, has a single viewpoint, and suppresses generation of an invalid region in the capture image.

In addition, the at least one lens may be a lens group including at least two lenses, and the lens group may be included in a zoom lens.

Preferably, the optical system described above further includes a movement portion having a structure with which the lens barrel is moved forward and backward along an optical axis of the lens group, and the movement portion has a structure with which the lens barrel is moved to a point at which a variation in the position of the front principal point is offset, and at which the position of the front principal point of the lens group coincides with the focal position of the second-reflection mirror, the variation being caused by change in a zoom factor of the lens group.

With this configuration, it is also possible to fix the position of the front principal point when obtaining an enlarged image by zooming, thus allowing obtaining an advantage of having a single viewpoint equally in zooming.

In addition, the lens group may have a configuration in which the front principal point is not moved by change in a zoom factor.

With this configuration, it is also possible to fix the position of the front principal point when obtaining an enlarged image by lens zooming, thus allowing obtaining an advantage of having a single viewpoint equally in zooming.

In addition, the optical system described above may further include a gimbal mechanism holding the lens barrel and allowing the optical axis of the lens group to rotate in a biaxial direction, and a rotational axis in each direction of the gimbal mechanism may pass through the focal position of the second-reflection mirror.

With this configuration, it is possible to match a rotational center of the lens group in pan-tilt motion with the position of the front principal point of the lens group, thus making it possible to hold the single viewpoint equally in pan-tilt motion.

[Advantageous Effects of Invention]

Accordingly, it is possible to provide a compact optical system which allows wide-angle image capturing, has a single viewpoint, and suppresses generation of an invalid region in a capture image.

DESCRIPTION OF EMBODIMENTS

First, an optical system according to a first embodiment of the present invention will be described.

FIG. 4is a diagram schematically showing a configuration of an optical system10according to the first embodiment.FIGS. 4(a),4(b), and4(c) show a front view, a right side view, and a top view of the optical system10, respectively.

The optical system10includes: a main mirror11, a second-reflection mirror12, a lens group13, a lens barrel14, and a base15. The lens barrel14is fixed to the base15so that an optical axis16of the lens group13is tilted with respect to a rotational axis17of the second-reflection mirror12. Note that the rotational axis17of the second-reflection mirror12and the rotational axis of the main mirror11should preferably be parallel. Illustrated here is the case where the two rotational axes coincide with each other.

The main mirror11reflects a light ray proceeding from space to the second-reflection mirror12. The main mirror11has an opening in the center, and includes a portion of a convex surface which is rotationally symmetric. Such a rotationally-symmetric surface, for example, is a quadric surface defined by Expression (1).

c represents a curvature of a curved surface, k represents a conic constant, and r represents a distance from a central axis of the quadric surface. For example, a paraboloid can be obtained where the conic constant k=−1, and a hyperboloid can be obtained where k<−1. The rotational axis of the main mirror11described above is a rotational axis of the rotationally-symmetric convex surface described above.

The second-reflection mirror12reflects the reflected light from the main mirror11, toward the lens group13. As with the main mirror11, the second-reflection mirror12includes a portion of a concave surface that is rotationally symmetric. This rotationally-symmetric concave surface is also defined by Expression (1) above. The rotational axis of the second-reflection mirror12described above is the rotational axis of the rotationally-symmetric concave surface described above.

The lens group13is made of plastic or glass, and collects light rays reflected from the second-reflection mirror12. Note thatFIG. 4illustrates, for ease of reference, the lens group13as a single lens, but according to an embodiment of the present invention, the number of lenses is not limited to one, and two or more lenses may be used.

The lens barrel14holds a positional relationship between the respective lenses included in the lens group13under a specific condition.

The base15holds the main mirror11, the second-reflection mirror12, and the lens barrel14under a specific condition.

Here described is a condition with which the light reflected by the second-reflection mirror12converges on a single point at the focal position of the second-reflection mirror12.

Generally, to realize the single viewpoint using a reflecting mirror having a quadric surface, there are techniques of using a hyperboloidal mirror and using a parabolic mirror. Of these, the technique of using a hyperboloidal mirror is disclosed in Patent Literature 1.

The other technique of using a parabolic mirror, as shown inFIG. 5, takes advantage of the properties that a light ray503A, which is reflected by the parabolic convex mirror501and then proceeds to a focal point502of the parabolic convex mirror501, becomes parallel light503B that is parallel to the rotational axis504of the paraboloid (hereinafter described as “parallel light”). It is possible to collect such parallel light, using a telecentric lens505(particularly an object-side telecentric lens) as a lens for forming the parallel light503B into an image.

However, the telecentric lens505is often a “dark” lens generally having a small aperture, and this is likely to increase the size of the entire lens, thus having a disadvantage of not being suited for downsizing of the entire optical system.

In contrast, another imaging technique that does not use the telecentric lens505is, as shown inFIG. 6, a technique to combine a parabolic convex mirror601and a parabolic concave mirror602. The parabolic concave mirror602has properties that render parallel light603B convergent on a focal point605of the parabolic concave mirror602. Since reflected light603C, reflected by the parabolic concave mirror602, is no longer parallel light, it is possible to collect the reflected light603C using an ordinary lens that is not the telecentric lens505. That is, a light ray603A proceeding to a focal point604of the parabolic convex mirror601is reflected by the parabolic convex mirror601and turns into the parallel light603B, and is subsequently reflected toward a focal point605of the parabolic concave mirror602. Positioning an ordinary lens at a focal point605of the parabolic concave mirror602makes it possible to collect the reflected light603C using the parabolic concave mirror602. In the case of using the lens group including plural lenses, it is possible to deal with the lens group in the same manner as in the case of using a single lens, by positioning the lens group so that the front principal point of the lens group coincides with the focal point605of the parabolic concave mirror602. Thus, since the light, which is reflected by the parabolic convex mirror601and proceeds to the parabolic concave mirror602, turns into the parallel light603B, it is possible to change the distance between the parabolic convex mirror601and the parabolic concave mirror602.

FIG. 7shows how a light ray is reflected by the optical system according to the first embodiment, based on the properties described above. However, it is assumed here that the curved surface included in the main mirror11is a portion of a convex paraboloid, and that the curved surface included in the second-reflection mirror12is a portion of a concave paraboloid. A light ray72A proceeding to a focal point71of the main mirror11is reflected by the main mirror11and turns into parallel light72B, which is then incident on the second-reflection mirror12. Furthermore, the parallel light72B is reflected by the second-reflection mirror12toward the focal point thereof, to be collected by the lens group13.

Note that the lens group13is placed at a position at which the focal point71of the main mirror11and the front principal point of the lens group13coincide with each other.

Note that according to this optical system, it is possible to capture an image of the main mirror11as shown inFIG. 3D.

As described above, according to the first embodiment of the present invention, it is possible to capture the second-reflection mirror12(an image of the main mirror11reflected therein) to a full extent within the angle of view of the lens group13as shown inFIG. 3Bby positioning the optical axis16of the lens group13at a tilt, in contrast to the case where, as shown inFIG. 3A, an invalid imaging area312exists within the angle of view of the lens group13when the optical axis16of the lens group13coincides with the rotation axes of the main mirror11and the second-reflection mirror12.FIGS. 3C and 3Dschematically show an image obtained by the system represented byFIGS. 3A and 3B, respectively, withFIG. 3Dclearly showing a smaller invalid region in which the image in the second-reflection mirror12is not reflected than an invalid region inFIG. 3C. Note thatFIG. 3Dis a schematic diagram which looks enlarged lengthwise from a real image, but in practice an almost semicircular image can be obtained. In addition, a hatched portion in the semicircular center is an image of an opening of the main mirror11.

In addition, each of the main mirror11and the second-reflection mirror12has a shape of a portion of a corresponding one of the convex paraboloid and concave paraboloid that are rotationally symmetric. This allows reducing size in the mirror-radius direction compared to the conventional optical system having the entire shapes of the convex paraboloid and concave paraboloid. Accordingly, it is possible to provide an optical system which allows wide-angle image capturing, has a single viewpoint, and is compact.

In addition, as is clear from a comparison between the conventional optical system shown inFIG. 3Aand the optical system according to the present embodiment as shown inFIG. 3B, tilting the optical axis16of the lens group13results in a smaller angle of view of the lens group13required for capturing the entire second-reflection mirror12. Generally, the larger the angle of view of the lens is, that is, the shorter the focal distance is, the larger the influence of a falloff in light amount at edges; thus, it is possible to reduce such influence to a greater extent for a smaller angle of view.

An optical system according to a second embodiment of the present invention has a mechanism which allows holding a single viewpoint even in zoom and in pan-tilt motion of a lens.

The following will describe an optical system according to the second embodiment of the present invention.

FIG. 8is a diagram schematically showing a configuration of an optical system100according to the second embodiment.FIGS. 8(a),8(b), and8(c) show a front view, a right side view, and a top view of the optical system100, respectively.

The optical system100has a configuration which further includes, in addition to the optical system10according to the first embodiment, a holding portion20and a control unit30. Since the function of the configuration except for the holding portion20and the control unit30is the same as the configuration of the optical system10according to the first embodiment, the following will describe only a portion different from the optical system10.

The holding portion20has a function to fix the lens barrel14to the base15, with a position and posture of the lens barrel14held in a changeable state. The holding portion20includes a movement portion21and a two-axis gimbal22.

FIGS. 9A and 9Bare diagrams schematically showing a configuration of the holding portion20:FIG. 9Ais a cross-sectional view of the holding portion20along an optical axis of the lens group13, andFIG. 9Bis a diagram of the holding portion20as seen from a direction of the optical axis.

The movement portion21has a function to move the lens barrel14forward and backward along the optical axis of the lens group13. For example, a specific configuration includes an external barrel and a cam around the lens barrel14, and rotating the lens barrel14around the optical axis of the lens group13allows the lens barrel14to move forward and backward. In addition, the lens barrel14may be moved forward and backward along the optical axis, with a groove provided in one of the lens barrel14and the external barrel, and with a protruding portion provided in the other to fit the groove.

The two-axis gimbal22has a function to hold the lens barrel14in a rotatable state centering on a specific point. The two-axis gimbal22is attached such that the rotational center thereof coincides with the focal position of the second-reflection mirror12. That is, the rotation axis extended in each direction of the two-axis gimbal22passes through the focal position of the second-reflection mirror12, with the rotational axis extended in each direction of the two-axis gimbal22coincident with the focal position of the second-reflection mirror12, and with the lens barrel14moving forward and backward along the optical axis of the lens13.

The control unit30includes: a CPU, a random access memory (RAM), a read-only memory (ROM) in which a control program is stored, and an input unit such as a button. The control unit30performs zooming by changing the positional relationship between the respective lenses included in the lens group13, in accordance with the operation performed by a user of the optical system100. Along with this, the control unit30determines an amount of movement of the lens barrel14, and causes the movement portion21to move the lens barrel14. That is, the control unit30moves the lens barrel14so that the front principal point of the lens group13constantly coincides with the focal point of the second-reflection mirror12.

Note that the amount of movement of the lens barrel14is previously determined by calculation as below. Specifically, the process includes: previously calculating the amount of displacement in position of the front principal point of the lens group13that is caused by change in zoom factor; and calculating, as the amount of movement of the lens barrel14, the amount of movement of the lens group13which offsets the measured amount of movement of the position of the front principal point and is used for matching the position of the front principal point of the lens group13with the focal position of the second-reflection mirror12. Note that adjusting a cutting state of the grove in the cam of the movement portion21allows the zoom factor and the movement amount of the lens barrel14to work in relation to each other.

Note that the position of the control unit30inFIG. 8is a mere example, and in practice it is possible to attach the control unit30at an arbitrary position.

As described above, according to the second embodiment of the present invention, in addition to the advantageous effect described in the first embodiment, it is possible to fix the position of the front principal point even when capturing an enlarged image by lens zooming, thus allowing obtaining an advantage of having a single viewpoint equally in zooming.

In addition, it is possible to match the rotational center of the lens group13in pan-tilt motion with the position of the front principal point of the lens group13, thus making it possible to continuously hold the single viewpoint equally in pan-tilt motion.

An optical system according to a third embodiment of the present invention includes, in addition to a main mirror, a plurality of sub mirrors and a mechanism intended to obtain distance information from a reflection image from each of the main and the sub mirrors.

The following will describe the optical system according to the third embodiment of the present invention.

FIG. 10is a diagram schematically showing a configuration of an optical system200according to the third embodiment.FIGS. 10(a),10(b), and10(c) show a front view, a right side view, and a top view of the optical system200, respectively.

The optical system200includes sub mirrors50in addition to the optical system10according to the first embodiment. Since the function of the configuration except for the sub mirror50is the same as the configuration of the optical system10according to the first embodiment, the following will describe only a portion different from the optical system10.

The sub mirror50reflects a light ray from space toward a second-reflection mirror12. The sub mirror50includes a rotationally-symmetric convex surface, such as a quadric surface defined by Expression (1). At least one sub mirror50is included, and, inFIG. 10, two sub mirrors50are provided; however, the number of such mirrors is not limited by the present embodiment. Note that the rotational axis of the sub mirrors50should preferably coincide with the rotational axis of the main mirror11.

FIG. 11shows, as an example, an image of a parking lot captured using the optical system200. As shown inFIG. 11, cars, buildings in the neighborhood, and the sky are reflected in projection images of the main mirror11and the sub mirrors50.

Note that when providing the sub mirrors50, it is preferable to position the sub mirrors50such that no invalid region is generated in a final projection image. For example,FIG. 12Ashows an image which is captured when the two sub mirrors are positioned so as to be reflected on an image having an aspect ratio of 4:3. This image includes an image11A of the main mirror11and images50A of the sub mirrors50. Assuming that a is a radius of the sub mirrors50in the image, it is proved that4ais a maximum size that does not cause the sub mirrors50to overlap with each other. In addition, considering the case of obtaining a 180-degree horizontal angle of view, the invalid region is the smallest where the radius of the main mirror11in the image is2awith the center thereof located to a full extent in a lower part of the image. In this case, assuming that θ is an angle defined by centers A and B of the sub mirrors50and a center O of the main mirror11, θ is calculated according to Expression (2) below.

In addition, this angle θ is larger in a horizontally-long image as shown inFIG. 12B, and is smaller, on the contrary, in a vertically-long image. This image includes the image11A of the main mirror11and the images50A of the sub mirrors50. For example, as shown inFIG. 12B, θ=90° when positioning the sub mirrors50such that no invalid region is generated in an image having an aspect ratio 2:1. In the case of using, for an imaging system, a solid-state imaging device such as a charge coupled device (CCD) and a complementary metal oxide semiconductor (CMOS), aspect ratios 4:3, 3:2, 16:9, and the like are generally used for the image, and the invalid region in the image is smaller where 30°≦θ≦90° in the case of using such aspect ratios.

Here, a technique of obtaining the distance information from reflected images from the plural mirrors is described. Each mirror described here is a parabolic mirror.

As shown inFIG. 13, the case where two parabolic mirrors MA and MB are located in space is considered. It is assumed that CAand CBare curvatures of the parabolic mirrors MA and MB, respectively. In addition, it is assumed that OAand OBare position vectors of the projection points when vertexes of the parabolic mirrors MA and MB are projected respectively onto an image plane.

Here considered is a case where a light ray1201from infinity is incident on the parabolic mirrors MA and MB. It is possible to consider the light ray1201from infinity as parallel light; thus, in the case of capturing images of these parabolic mirrors MA and MB using a telecentric lens, the light ray proceeding to the focal points of the parabolic mirrors MA and MB is projected respectively onto points PAand PBon the image plane by reflection. In this context, pixel values at the points PAand PBare equal, and the relationship between the two points is defined by Expression (3) below.

Next, as shown inFIG. 14, it is assumed that there is an object1301at a nearby point. In addition, it is assumed that the object1301is located at a position such that the light ray1302from infinity is projected onto the point PA. In this context, the image of the object1301formed by the parabolic mirror MB is an image formed when a light ray1303, proceeding from the object1301toward the focal point of the parabolic mirror MB, is reflected by the parabolic mirror MB and is then projected onto the point PB′. Accordingly, the pixel values at the points PAand PBare not equal, which proves that the images projected onto the points PAand PBare not formed of the light from infinity, that is, proves that the object1301is located close to the optical system. This technique allows obtaining the distance information indicating whether or not the object1301is close to a boundary assumed to be located in a predetermined distance. The distance determined to be the boundary is defined such that the distance between PBand PB′on the image is equivalent to exactly one pixel.

Note that the case of using the parabolic mirror is described here; however, even in the case of using a mirror having another shape, it is also possible to detect likewise whether or not the object1301is located nearby, provided that it is possible to previously obtain the positional relationship between positions onto which the light ray from infinity is projected.

For the parabolic mirrors MA and MB described above, it is possible to use the main mirror11, and one or two sub mirrors50.

In addition, in the case of using three or more mirrors, it is possible to use a technique of judging the above relationship for each set of mirrors sharing the same field of view among combinations of the mirrors, and performing majority decision or the like on the results, so as to obtain distance information more reliably. For example, it is possible to reliably calculate distance information, using the three mirrors, that is, the main mirror11and the two sub mirrors50of the optical system200.

As described above, according to the third embodiment of the present invention, it is possible, in addition to the advantageous effect described in the first embodiment, to calculate the distance information from the reflection images of the main mirror11and the sub mirrors50.

Thus far, embodiments of the present invention have been described, but the present invention is not limited to the description above but can also be performed in a variety of forms for achieving the object of the present invention and purposes associated with the object, and may also be performed as below, for example.

For example, in the first to third embodiments, the quadric surface represented by Expression (1) has been described as an equation representing a mirror shape, but what is known as an aspheric shape defined by Expression (4) below may also be used.

In addition, in the second embodiment, another configuration of the lens group13may be such that the position of the front principal point does not change by zooming, instead of using the movement portion21. By using the lens group thus configured, it is also possible to produce an advantageous effect of fixing the position of the front principal point of the lens group13. Patent Literature 3 discloses a technique of varying the focus, with the position of the front principal point held at a point in a zoom lens. Use of this technique allows realizing a configuration which does not change the position of the front principal point by zooming.[PTL 3] Japanese Examined Patent Application Publication No. 61-10047

In addition, in the second embodiment, the configuration may also be such that: instead of using a cam for the movement portion21, an external barrel is provided around the lens barrel14, and the lens barrel14is held slidable forward and backward; a ROM in the control unit30previously holds an amount of sliding corresponding to the zoom factor; and the amount of movement is determined with reference to the recorded amount of sliding.FIG. 15is a diagram showing an example of a data table recorded in the ROM in the control unit30. This data table records the zoom factor and the amount of movement of the lens barrel14in association with each other, and indicates, for example, that the amount of movement of the lens barrel14is 2 mm at zoom factor 1.2.

In addition, in the third embodiment, the main mirror11and the sub mirrors50are separately formed, but these mirrors may also be formed into a single configuration in advance. Forming these mirrors into a single configuration causes no misalignment when attaching the sub mirrors50, thus reducing errors in distance information to be obtained.

In addition, all the first to third embodiments have a configuration that includes an optical system only, but these embodiments are also applicable as an imaging device which incorporates the optical system described in such embodiments.

It should be considered that the embodiments described above are not limitative but illustrative in every aspect. The scope of the present invention is presented not by the description above but by the claims, and is intended to include all the changes and modifications without departing from the meaning and scope equivalent to the claims.

The present invention is applicable as an optical system which allows a wider field with suppressed size, and also allows capturing an image which retains a feature of single viewpoint equally in zooming and in pan-tilt operation. The present invention is particularly applicable when it is intended to keep an entire device compact.