WO2013031564A1

WO2013031564A1 - Optical system and imaging device

Info

Publication number: WO2013031564A1
Application number: PCT/JP2012/070965
Authority: WO
Inventors: 慶延岸根
Original assignee: 富士フイルム株式会社
Priority date: 2011-08-26
Filing date: 2012-08-20
Publication date: 2013-03-07
Also published as: JP2014219426A

Abstract

Provided is a thin optical system that can eliminate reversal of resolution at a center part and peripheral parts and reversal of resolution in a tangent line direction (S direction) and radial direction (T direction) to obtain images which do not cause discomfort. Also provided is an imaging device using the same. The peak position (Vt) for defocus MTF in the T direction outside of the axis and the peak position (Vs) for defocus MTF in the S direction outside of the axis differ. The peak position for defocus MTF on the axis and the peak position (Vs) for defocus MTF in the S direction outside of the axis are in the best focus position. The peak position (Vt) for defocus MTF in the T direction is shifted to the plus side of the best focus position. Thus, the defocus MTF outside of the axis is smaller than the defocus MTF on the axis, and the defocus MTF outside the axis in the T direction is smaller than the defocus MTF outside the axis in the S direction; the range of defocus in which images which do not cause discomfort can be obtained is expanded.

Description

Optical system and imaging apparatus

The present invention relates to a thin optical system and an imaging apparatus having a short optical total length.

In recent years, a thin imaging unit is standardly mounted on a portable terminal such as a cellular phone. In addition, thin compact digital cameras are also popular in digital cameras that are dedicated machines. In addition, such a thin digital camera is also required to take a high-quality image on a large screen, and an improvement in the number of pixels of an optical system such as a lens or an image sensor is desired. In particular, an image pickup unit mounted on a mobile phone or the like is used for shooting various subjects such as a close-up view of a character string on a paper surface, a distant view of a landscape or a person, or a subject in which these are mixed. From this point, it is difficult to incorporate a complicated mechanism such as a zoom mechanism. For this reason, a digital camera mounted on a mobile phone or the like is required to be able to photograph various subjects with a large screen and high image quality without using a zoom mechanism or the like.

On the other hand, as the distance between the last lens of the optical system and the imaging surface of the image sensor becomes shorter due to the reduction in thickness, the chief ray angle (Chief Ray Angle (CRA)) increases in the peripheral part, so the resolution in the peripheral part is the center. The resolution may be lower than that of the portion, and the image quality may deteriorate. As an optical system that improves the image quality in such a peripheral portion, for example, an optical system that can obtain a substantially constant resolution regardless of the image height by intentionally curving the imaging surface. Known (Patent Document 1). The chief ray angle is an incident angle at which a light beam passing through the center of the stop is incident on the imaging surface of the image sensor. Although it depends on the lens type, the principal ray angle is maximized at a position where the image height is approximately 70% or more.

JP 2008-249909 A

As described above, the thin digital camera has a problem that the resolution tends to decrease in the peripheral portion, but it is not sufficient to simply improve the resolution in the peripheral portion. For human vision, the resolution of the central part is high and the resolution of the peripheral part is low. For example, when the resolution of the central part and the peripheral part is reversed and the resolution of the peripheral part is higher than the central part, the image was taken. The image feels strange. Therefore, when improving the resolution of the peripheral part, it is not preferable that the resolution of the peripheral part greatly exceeds the resolution of the central part.

For the same reason, as shown in FIG. 15, in the captured image 30, the radial direction (the intersecting direction of the tangential surface of the lens and the imaging surface; hereinafter referred to as the T direction) and the tangential direction (the sagittal surface of the lens) And the imaging plane (hereinafter referred to as the S direction), human vision is tolerant to a decrease in resolution in the T direction (so-called image blur). However, this is a case where the resolution in the S direction exceeds the resolution in the T direction. On the contrary, when the resolution in the T direction exceeds the resolution in the S direction, the image feels uncomfortable regardless of the resolution. For this reason, it is not preferable that the resolution in the T direction greatly exceeds the resolution in the S direction.

The optical system described in Patent Document 1 improves the in-plane uniformity of the depth of focus and resolution by intentionally curving the imaging surface into a spherical shape. This is because when the gap between the curved imaging surface of the optical system and the planar imaging surface of the image sensor is small, that is, when the maximum value of CRA is relatively small (for example, CRA as described in Patent Document 1). This is an effective method. However, when applied to a thin optical system having a larger CRA (for example, an optical system having a CRA of 35 degrees or more), the gap between the curved imaging surface and the planar imaging surface is increased, and the in-plane resolution is uniform. Even if the resolution is improved, the adverse effect that the resolution is lowered as a whole is greater.

In addition, recent thin digital cameras often have an autofocus function that automatically adjusts the focus (hereinafter referred to as the AF function). However, if the accuracy of the AF function is poor, the best focus is always taken. It is not always done. When shooting in such a defocused state, the in-plane non-uniformity of the resolution as described above and the deterioration in image quality due to the reversal of the resolution in the T direction and the S direction become more prominent, resulting in an uncomfortable image. The same applies to the case where the manual focus adjustment has failed or the photographing using the optical system having no AF function.

The present invention provides a thin optical system and an imaging apparatus capable of solving the reversal of the resolution in the central portion and the peripheral portion and the reversal of the resolution in the S direction and the T direction and obtaining an uncomfortable image. For the purpose.

In order to achieve the above object, the optical system of the present invention forms an image on an image sensor, has a maximum principal ray angle of 35 degrees or more, and has radial and tangential directions on the axis. The focus MTF peak position and the off-axis tangential defocus MTF peak position are within ± 5% of the focal length from the best focus position, and the off-axis defocus MTF peak position is off-axis. The peak position of the defocus MTF in the tangential direction is shifted in the plus or minus direction.

The maximum chief ray angle is the maximum angle among the angles at which a plurality of chief rays having different angles of incidence on the diaphragm are incident on the imaging surface of the image sensor. For example, the chief ray angle connecting the images at the position where the image height is 100% is the maximum chief ray angle. Depending on the aberration of the optical system and the like, the angle of the principal ray connecting an image with an image height of less than 100% may be the maximum principal ray angle.

”Axis” refers to the paraxial region. More specifically, “on the axis” has an image height less than 10% that can be regarded as substantially coincident with a point corresponding to the optical axis of the optical system on the imaging surface (or captured image) of the image sensor. An area is included.

”Off-axis” means outside the paraxial region. That is, the off-axis point is a point at a position away from the point corresponding to the optical axis of the optical system on the imaging surface (or the captured image) of the image sensor. More specifically, an area having an image height of 10% or more with respect to a point corresponding to the optical axis of the optical system (an imaging surface or the center of a captured image) is an off-axis area.

At a spatial frequency of 1/4 of the Nyquist frequency of the image sensor, the defocus range where the off-axis radial defocus MTF value is 0.2 or more is Wt, and the off-axis tangential defocus MTF peak. When the shift amount of the peak position of the defocus MTF in the radial direction off the axis with respect to the position is defined as Dv, it is preferable that Dv ≧ 0.05 Wt is satisfied.

It is preferable that the direction in which the peak position of the defocus MTF in the radial direction is shifted is the same at an arbitrary image height. The shift amount is preferably larger as the peak value of the defocus MTF in the radial direction is smaller.

The off-axis radial defocus MTF peak position shift amount is Dv, the off-axis radial defocus MTF value is 0.2 or more, the defocus range is Wt, and the off-axis radial defocus MTF is defocused. When the peak value of the focus MTF is P1 and the peak value of the defocus MTF in the radial direction on the axis is P0, it is preferable that the shift amount Dv satisfies Dv ≧ 0.05 Wt × (P0 / P1).

The ratio of the optical total length to the focal length is preferably 1.1 or less.

An imaging apparatus of the present invention includes an optical system capable of focus adjustment, and an image sensor that photoelectrically converts an image formed by the optical system, the maximum principal ray angle of the optical system being 35 degrees or more, and an axis The peak positions of the upper radial and tangential defocus MTFs and the peak position of the off-axis tangential defocus MTF are within ± 5% of the best focus position. The peak position of the focus MTF is shifted in the plus or minus direction with respect to the peak position of the defocus MTF in the tangential direction.

The focus adjustment of the optical system is performed within a range in which the defocus MTF on the axis is larger than the defocus MTF off the axis and the defocus MTF in the T direction off the axis is smaller than the defocus MTF in the S direction off the axis. It is preferable to provide automatic focusing means for performing.

In the present invention, since the peak position of the defocus MTF in the radial direction and the peak position of the defocus MTF in the tangential direction are different from each other in the off-axis direction, the resolution is reversed between the central portion and the peripheral portion, and the S direction. And the resolution inversion in the T direction are resolved, and an image with no sense of incongruity can be obtained.

It is a block diagram which shows schematic structure of an imaging device. It is explanatory drawing which shows the structure of an optical system. It is explanatory drawing which shows the structure of the thin optical system which does not use a correction lens. It is a graph which shows MTF of a thin optical system. It is a graph which shows the change of MTF with respect to the defocus of a thin optical system. It is a graph which shows defocus MTF of the thin optical system which shifted the peak position of off-axis defocus MTF of T direction. It is a graph which shows the defocus MTF of the thin optical system which shifted the peak position of the off-axis defocus MTF of the T direction to the minus side. It is a graph which shows defocus MTF of the thin optical system which shifted the peak position of off-axis defocus MTF of S direction. 10 is a graph showing a defocus MTF of a thin optical system in which a peak position of off-axis defocus MTF in the T direction and a peak position of focus MTF off the axis in the S direction are shifted in opposite directions. It is explanatory drawing which shows typically the shift amount of the peak position Vt of the off-axis defocus MTF of the T direction in intermediate image height. It is explanatory drawing which shows typically the example from which the shift amount Dv of the peak position Vt of the off-axis defocus MTF of a T direction differs according to a peak value. It is explanatory drawing which shows the structure of the conventional optical system which is not thin. It is a graph which shows MTF of the conventional optical system which is not thin. It is a graph which shows the change of MTF with respect to the defocus of the conventional optical system which is not thin. It is a schematic diagram showing T direction and S direction on the image | photographed image.

As shown in FIG. 1, the imaging device 10 includes an optical system 11, an image sensor 12, an AF control unit 13, an image processing unit 14, a display unit 16, a memory 17, and the like.

The optical system 11 forms an image of a subject on the imaging surface IP (see FIG. 2) of the image sensor 12. Since the imaging device 10 is a thin digital camera mounted on, for example, a mobile phone, the optical system 11 is also thin and is disposed in the vicinity of the image sensor 12. Hereinafter, the optical system 11 has an image circle of φ4.56 mm (100% image height 2.28 mm), which is larger than the image sensor 12.

The image sensor 12 is a CCD or CMOS device, and has an imaging surface IP in which a plurality of pixels are two-dimensionally arranged. The image sensor 12 photoelectrically converts an image formed on the imaging surface IP and outputs image data to the AF control unit 13 and the image processing unit 14. Here, the size of the image sensor 12 is, for example, ¼ inch and the pixel pitch is 1.4 μm (5M pixels, 2600 × 1960 pixels, 3.640 mm × 2.744 mm). In this case, the Nyquist frequency Ny determined from the pixel pitch of the image sensor 12 is about 360 lines / mm, and the image sensor 12 can resolve an image having a spatial frequency of about 360 lines / mm or less. Since the Nyquist frequency Ny, which is particularly well resolved, is about 90 lines / mm (hereinafter referred to as Ny / 4), the MTF for defocus described later is evaluated based on the spatial frequency of Ny / 4. To do.

The AF control unit 13 automatically determines an appropriate focal length based on input image data and the like, and inputs the result to the optical system 11. The optical system 11 performs focus adjustment by moving the lens L1 (see FIG. 2) and the like in the optical axis direction based on a signal input from the AF control unit 13. Also, the determination of an appropriate focal length performed by the AF control unit 13 is performed within a predetermined range according to the characteristics of the optical system 11 described later. The image processing unit 14 performs various image processing such as gamma correction processing on the image data obtained by the image sensor 12. Image data subjected to image processing by the image processing unit 14 is displayed on the display unit 16 or stored in the memory 17.

As shown in FIG. 2, the optical system 11 is an optical system substantially composed of one or more lenses, and includes, for example, a diaphragm (not shown), an objective lens group L0, and a correction lens L1. An optical system consisting essentially of one or more lenses means, in addition to one or more lenses or lens groups, lenses having substantially no power, optical elements other than lenses, such as a diaphragm and a cover glass, lens flanges, lenses It includes a barrel, a part or all of the image sensor (image sensor 12), a mechanism part such as a camera shake correction mechanism, and the like. The diaphragm of the optical system 11 is provided, for example, in the lens group L0, in front of the lens group L0, between the lens group L0 and the correction lens L1, and behind the correction lens L1 (on the imaging surface IP side).

The objective lens group L0 is a lens group that includes a plurality of approximately 2 to 5 lenses and has a predetermined focal length (eg, 3 mm) as a whole. The correction lens L1 is a lens for correcting the MTF of the optical system 11. Specifically, the correction lens L1 is a lens for correcting the MTF mode with respect to the focus shift of the lens unit L0. As will be described later, the correction lens L1 corrects the peak position of the MTF with respect to the focus shift in the T direction outside the optical axis.

For comparison with the optical system 11 of the present invention, a conventional optical system (non-thin normal optical system) 19 is shown in FIG. The optical system 19 includes a diaphragm (not shown), a lens group L2, and a lens L3. The lens group L2 has a longer focal length (for example, from 4 mm) than the lens group L0 of the thin optical system 11 of the present invention. The lens L3 is a lens corresponding to the correction lens L1 of the thin optical system 11 of the present invention, but the lens L2 does not have an action of correcting the MTF of the optical system 19 unlike the correction lens L1. The diaphragm of the conventional optical system 19 is provided, for example, in the lens group L2, in front of the lens group L2, between the lens group L2 and the lens L3, and behind the lens L3 (on the imaging surface IP side).

Comparing the optical system 11 of the present invention with the conventional optical system 19, the optical system 11 of the present invention is thin with a short back focus and has a short optical total length (focal length). An image having the same image height as that of the system 19 is formed. Therefore, the maximum principal ray angle CRA1 of the optical system 11 of the present invention is larger than the maximum principal ray angle CRA2 of the conventional optical system 19 (CRA1> CRA2).

In the present specification, an optical system having a large maximum chief ray angle means an optical system having a maximum chief ray angle of approximately 35 degrees or more (CRA ≧ 35 degrees). In this specification, a thin optical system refers to an optical system having a small ratio of optical total length to focal length (about 1). For example, the ratio of optical total length to focal length is 1.1 or less (optical total length / focus). The distance ≦ 1.1) is generally satisfied. However, these are performances required for a thin digital camera mounted on a portable terminal such as a mobile phone, but the limitation on the thin point imposed on the realistic optical system 11 is that the imaging device 10 It depends on the specific configuration of the mobile terminal etc. to be installed.

As shown in FIG. 13, the MTF of the conventional optical system (focal length 4 mm) 19 decreases as the spatial frequency increases. Also, the MTF on the optical axis corresponding to the central portion of the photographed image (hereinafter referred to as the on-axis MTF) is substantially the same in the S direction and the T direction. On the other hand, the decrease rate of the MTF outside the optical axis (hereinafter referred to as off-axis MTF) corresponding to the peripheral portion of the captured image differs between the S direction and the T direction, and the decrease rate of the MTF in the T direction is larger.

Further, as shown in FIG. 14, the conventional optical system 19 shows a change in MTF (hereinafter referred to as defocus MTF) with respect to focus shift (hereinafter referred to as defocus) at a spatial frequency of Ny / 4. As the amount of focus increases, both the on-axis defocus MTF and the off-axis defocus MTF decrease. In this case, the on-axis defocus MTF is larger than the off-axis defocus MTF in the most part including the best focus position, but the on-axis defocus MTF and the off-axis defocus MTF are larger or smaller in the part where the defocus amount is large. The relationship may be reversed and the off-axis defocus MTF may exceed the on-axis defocus MTF. The best focus position is the peak position of the on-axis defocus MTF at a spatial frequency of Ny / 4. In FIG. 14, the position of 0 μm defocus is the best focus position.

As described above, when the conventional optical system 19 is photographed at a focal position where the off-axis defocus MTF exceeds the on-axis defocus MTF, the resolution of the peripheral portion becomes larger than the resolution of the central portion of the image, and there is a sense of incongruity. Become an image. For this reason, it is preferable to perform the focus adjustment within a range in which the magnitude relationship between the off-axis defocus MTF and the on-axis defocus MTF is not reversed.

Furthermore, the off-axis defocus MTF differs in the way of decreasing the MTF with respect to the defocus in the T direction and the S direction. Specifically, in the region E1 including the best focus position, the off-axis defocus MTF in the S direction is larger than the off-axis defocus MTF in the T direction, but when the amount of defocus increases, the S The off-axis defocus MTF in the direction and the T direction may be reversed, and the off-axis defocus MTF value in the T direction may exceed the off-axis defocus MTF value in the S direction. In this way, when the off-axis defocus MTF in the T direction and the off-axis defocus MTF in the S direction are reversed and the off-axis defocus MTF in the T direction exceeds the off-axis defocus MTF in the S direction, The image looks strange in the surrounding area. For this reason, the focus adjustment is preferably performed within a range where the off-axis defocus MTF in the T direction is smaller than the off-axis defocus MTF in the S direction.

For this reason, the focus adjustment is performed within a range in which the off-axis defocus MTF is smaller than the on-axis defocus MTF and the off-axis defocus MTF in the T direction is smaller than the off-axis defocus MTF in the S direction. Is preferred. In the case of the conventional optical system 19 having a focal length of 4 mm shown in FIG. 14, the defocus range that satisfies this condition is about 45 μm. Further, in a conventional captured image, an image having an MTF smaller than 0.2 has insufficient resolution and is difficult to visually recognize. Therefore, if the MTF is smaller than 0.2, the image is blurred in the first place, so that the off-axis defocus MTF and the on-axis defocus MTF are reversed, the off-axis defocus MTF in the T direction and the axis in the S direction. Difficult to remember due to reversal of outside defocus MTF.

In the case of the conventional optical system 19 having a focal length of 4 mm, the off-axis defocus MTF exceeds the on-axis defocus MTF, or the off-axis defocus MTF in the T direction is larger than the off-axis defocus MTF in the S direction. In this range, the MTF is smaller than about 0.2, and even if the off-axis defocus MTF in the T direction and the off-axis defocus MTF in the S direction are reversed in the range where the MTF is 0.2 or more, the difference between them is small. Therefore, in the conventional optical system, even if reversal of the off-axis defocus MTF and the on-axis defocus MTF, or the off-axis defocus MTF in the T direction and the off-axis defocus MTF in the S direction occur, it is difficult to feel discomfort. .

On the other hand, in the case of the optical system (focal length 3 mm) 20 shown in FIG. As shown in FIG. 4, in the thin optical system 20, both the on-axis MTF and the off-axis MTF decrease as the spatial frequency increases, as in the conventional optical system 19. However, the on-axis MTF and the off-axis MTF in the S direction are substantially the same as those of the conventional optical system 19, but the off-axis MTF in the T direction of the thin optical system 20 decreases more steeply than the conventional optical system 19. . This is because the maximum chief ray angle increases due to the thinness, and light quantity deficiency (so-called vignetting) is likely to occur in the light imaged on the peripheral portion.

As shown in FIG. 5, when the defocus MTF of the thin optical system 20 is viewed, the on-axis defocus MTF and the off-axis defocus MTF in the S direction are almost the same as the defocus MTF of the conventional optical system 19. However, the characteristics of the off-axis defocus MTF in the T direction are significantly different. Specifically, in the case of the thin optical system 20, the off-axis defocus MTF in the T direction has a method of decreasing with respect to the defocus amount as compared with the on-axis defocus MTF and the off-axis defocus MTF in the S direction. It is moderate. As a result, the range in which the off-axis defocus MTF is smaller than the on-axis defocus MTF is smaller than that of the conventional optical system 19 (see FIG. 14).

Further, the width of the region E1 in which the off-axis defocus MTF in the T direction is smaller than the off-axis defocus MTF in the S direction is reduced, and the off-axis defocus MTF in the T direction is larger than the off-axis defocus MTF in the S direction. The width of the region E2 is expanded. In this case, the region E2 in which the off-axis defocus MTF in the T direction is larger than the off-axis defocus MTF in the S direction has a larger MTF range of 0.2 or more than the conventional optical system, and the T direction. The off-axis defocus MTF and the off-axis defocus MTF in the S direction are reversed, and the gap is larger than that of the conventional optical system (see FIG. 14). For this reason, in the thin optical system 20, the off-axis defocus MTF is smaller than the on-axis defocus MTF, and the off-axis defocus MTF in the T direction is smaller than the off-axis defocus MTF in the S direction. The range of the defocus amount that can capture an image with no image is about 40 μm, which is narrower than the conventional optical system 19. Here, an example in which the focal length is 3 mm is shown as the thin optical system 20, but the above-described tendency appears more strongly in a thinner optical system.

From the above, the thin optical system 11 of the present invention mounted on the imaging device 10 has the following characteristics by the correction lens L1, so that the on-axis defocus MTF is off-axis defocused while being thin. The range in which the off-axis defocus MTF in the T direction is smaller than the off-axis defocus in the S direction is larger than the MTF.

As shown in FIG. 6, the thin optical system 11 of the present invention is an optical system having the same focal length of 3 mm as the thin optical system 20 shown in FIG. 3, but is off-axis defocused MTF in the T direction by the correction lens L1. The optical position is shifted in the plus direction with respect to the peak position Vs of the off-axis defocus MTF in the S direction. In this way, when the peak position Vt of the off-axis defocus MTF in the T direction is shifted from the best focus position, the position of the intersection of the graph of the off-axis defocus MTF in the T direction and the off-axis defocus MTF in the S direction. The on-axis defocus MTF is larger than the off-axis defocus MTF and the off-axis defocus MTF in the T direction is smaller than the off-axis defocus in the S direction is about 47 μm. Compared to the case of the optical system 20 (see FIG. 5, 40 μm), the defocus range in which an image without a sense of incongruity is obtained is extended. Note that in the case of the conventional optical system 19 (see FIG. 14), or in the case of the optical system 20 simply reduced in thickness, the peak position of the on-axis defocus MTF and the peak position Vt of the off-axis defocus MTF in the T direction. The peak position Vs of the off-axis defocus MTF in the S direction is the best focus position.

As described above, the peak position Vt of the off-axis defocus MTF in the T direction can be shifted to expand the defocus range in which an uncomfortable image can be obtained. When the shift amount of Vt is Dv and the defocus range in which the off-axis defocus MTF in the S direction is 0.2 or more is Wt, the shift amount Dv is preferably 0.05 Wt or more (Dv ≧ 0.05 Wt). This is because when the shift amount Dv is smaller than 0.05 Wt, the defocus range in which an image without a sense of incongruity is obtained has a small extended range, and it is difficult to realize the effect. Further, the shift amount Dv is preferably 0.5 Wt or less. This is because if the shift amount Dv is too large, the off-axis defocus MTF value in the T direction is too small in the vicinity of the best focus position, and the image is always blurred in the T direction in the peripheral portion of the image. If the shift amount Dv is approximately 0.5 Wt or less, it is possible to extend the defocus range in which an image that is not too blurred in the T direction and has no sense of incongruity is placed in the peripheral portion of the image.

Note that the AF control unit 13 of the imaging apparatus 10 does not simply perform focus adjustment so that the main subject is in the best focus, but the above-described on-axis defocus MTF is larger than the off-axis defocus MTF, and T Focus adjustment is performed so that all the subjects are within the range in which the off-axis defocus MTF in the direction is smaller than the off-axis defocus MTF in the S direction (in the range of 47 μm in FIG. 8). As a result, even if there are subjects that are far and near, and there are subjects that are not photographed with the best focus, it is possible to obtain an image that does not feel uncomfortable over the entire range.

In the above-described embodiment, the example in which the peak position Vt of the off-axis defocus MTF in the T direction is shifted in the plus direction has been described. However, as illustrated in FIG. Vt may be shifted to the negative side. In this case as well, the range in which the on-axis defocus MTF is larger than the off-axis defocus MTF and the off-axis defocus MTF in the T direction is smaller than the off-axis defocus in the S direction is extended as described above. Can do.

In the above-described embodiment, an example has been described in which the defocusing range in which an image without a sense of incongruity is obtained by shifting the peak position Vt of the off-axis defocusing MTF in the T direction is illustrated in FIG. In this way, the peak position Vs of the off-axis defocus MTF in the S direction is shifted without shifting the peak position Vt of the off-axis defocus MTF in the T direction. Thus, the peak position Vs of the off-axis defocus MTF in the S direction may be shifted to the opposite side of the shift direction (for example, plus side) of the peak position Vt of the focus MTF. However, as shown in FIGS. 8 and 9, the off-axis defocus MTF in the S direction changes in substantially the same manner as the on-axis defocus MTF. Therefore, if the off-axis defocus MTF in the S direction is shifted, the on-axis defocus MTF changes. The range in which the off-axis defocus MTF in the S direction exceeds the focus MTF is increased, and the defocus range in which an image without a sense of incongruity is obtained may be narrowed. For this reason, it is preferable to shift the peak position Vt of the on-axis defocus MTF mainly in the T direction as in the above-described embodiment, and when shifting the peak position of the off-axis defocus MTF in the S direction, It is preferable that the range in which the off-axis defocus MTF in the direction exceeds the on-axis defocus MTF is not expanded. The peak position Vt of the off-axis defocus MTF in the T direction is shifted, and the range of the off-axis defocus MTF in the S direction exceeding the on-axis defocus MTF is not expanded. If the peak position Vs is shifted, it is possible to extend the defocus range in which an image with a sense of incongruity can be obtained more than when only the peak position Vt of the off-axis defocus MTF in the T direction is shifted.

In the above-described embodiment, the example in which the peak position Vt of the off-axis defocus MTF in the T direction at the position where the image height is 100% (maximum principal ray angle) has been described. However, as illustrated in FIG. The peak position Vt of the off-axis defocus MTF in the T direction at the image height is preferably shifted in the same direction (for example, all plus side as shown in FIG. 10). This is because when the shift direction of the peak position Vt of the off-axis defocus MTF in the T direction differs depending on the image height, the resolution decreases constantly with increasing image height, such as the resolution in the T direction is high only at a certain image height. This is because there are many cases where the image is not used, and the image tends to be uncomfortable. Furthermore, for the same reason, it is preferable that the shift amount Dv of the peak position Vt of the off-axis defocus MTF in the T direction at each image height increases monotonously as the image height increases. For example, in FIG. 10, the shift amount Dv of the peak position Vt at the image height Y = 10% (Y = 10%), the shift amount Dv of the peak position Vt at the image height Y = 50% (Y = 50%), the image Assuming that the shift amount Dv (Y = 90%) of the peak position Vt at high Y = 90%, Dv (Y = 90%) ≧ Dv (Y = 50%) ≧ Dv (Y = 10%). The same applies to other image heights.

Further, the peak position Vt of the off-axis defocus MTF in the T direction at each image height depends on the curve shape of the off-axis defocus MTF in the S direction and the off-axis defocus MTF in the T direction at each image height. Each may be different. However, at any image height, the shift amount Dv is preferably 0.05 Wt or more with reference to the defocus range Wt in which the off-axis defocus MTF in the S direction at each image height is 0.2 or more ( Dv ≧ 0.05 Wt).

In the above-described embodiment, the example in which the peak position Vt of the off-axis defocus MTF in the T direction is shifted regardless of the peak value of the off-axis defocus MTF in the T direction (the MTF value at the peak position Vt) has been described. However, the shift amount Dv of the peak position Vt of the off-axis defocus MTF in the T direction is preferably larger as the peak value is smaller. For example, as shown in FIG. 11, when the shift amount Dv1 of the peak position Vt1 when the peak value is large and the shift amount Dv2 of the peak position Vt2 when the peak value is small are compared, the shift amount Dv2 is greater than the shift amount Dv1. Is also large (Dv2> Dv1). More specifically, with reference to the peak value P0 of the on-axis defocus MTF, the defocus range where the off-axis defocus MTF in the S direction is 0.2 or more is Wt, and the peak value of the off-axis defocus MTF in the T direction. Is set to P1, the shift amount Dv is preferably 0.05 Wt × (P0 / P1) or more (Dv ≧ 0.05 Wt × (P0 / P1)). This is because, as the peak value is smaller, the off-axis defocus MTF in the T direction becomes broader, so that the effect of shifting the peak position of the off-axis defocus MTF in the T direction can be sufficiently recognized. This is because the peak position Vt needs to be shifted correspondingly. When the shift amount Dv is smaller than 0.05 Wt × (P0 / P1), even if the defocus range in which an image without a sense of incongruity is obtained is expanded, the increase in the range is minute, and a remarkable effect Is hard to appear.

In the above-described embodiment, the thin optical system 11 is illustrated with a focal length of 3 mm. However, the optical system 11 has an arbitrary focal length, and is based on the imaging device 10 on which the optical system 11 is mounted.

The thin optical system 11 includes, for example, the following Table 1 (focal length 3 mm, shifted in the plus direction), Table 2 (focal length 3 mm, shifted in the minus direction), and Table 3 (focal length 2 mm, shifted in the plus direction). As shown in (2), at least one surface of the correction lens L1 can be realized as an aspherical surface. In Tables 1 and 2 below, an example in which one surface is an aspherical surface is shown, but both surfaces of the lens L1 may be aspherical. When two or more lenses are used, at least one of these lenses is used. By making the surface an aspherical surface, the performance of the thin optical system 11 described in the above embodiment can be obtained.

In Table 1, the radius of curvature and the unit of the radius are mm. The lens group L0 with surface numbers 2 and 3 is an ideal lens for simulation (focal length 3 mm) and has no aberration and thickness (thickness 0 mm). The same applies to Table 2 below.

In Table 3, the radius of curvature and the unit of the radius are mm. The lens group L0 with surface numbers 2 and 3 is an ideal lens for simulation (focal length 2 mm) and has no aberration and thickness (thickness 0 mm).

In Tables 1 to 3 described above, an ideal lens having no aberration and no thickness but only a focal length is defined as the lens group L0. However, a lens group L0 using a realistic lens having an aberration and a thickness may be used. it can. When the lens group L0 is configured by a realistic lens, the lens group L0 by the ideal lens in Tables 1 and 2 may be replaced with the lens group L0 by a realistic lens. Although an example in which the lens group L0 is configured by one ideal lens has been described here, the lens group L0 does not have to be formed by one lens, and the number of lenses constituting the lens group L0 is arbitrary. The realistic lens unit L0 is composed of about 2 to 5 lenses as described above. Therefore, the actual optical system 11 includes about 3 to 6 lenses including the correction lens L1.

In the above Tables 1 to 3, an example in which one side of the correction lens L1 is aspherical has been described, but both sides of the correction lens L1 may be aspherical. In the above-described embodiment and Tables 1 to 3, an example in which the correction lens L1 is configured by one lens has been described. However, the correction lens L1 may be a lens group including a plurality of lenses. Also in this case, a plurality of surfaces of the correction lens L1 may be aspherical.

In the above-described embodiment, the defocus MTF is evaluated based on the spatial frequency Ny / 4 (lines / mm). However, when the defocus MTF is evaluated for other spatial frequencies, it is substantially the same as the above-described embodiment. Are the same. For example, in the above-described embodiment, it has been described that it is preferable that the shift amount Dv is 0.05 Wt or more at the spatial frequency Ny / 4 (lines / mm). If obtained, the conditional expression to be satisfied changes. However, if the shift amount Dv is 0.05 Wt or more at the spatial frequency Ny / 4 (lines / mm), the conditional expressions derived at other spatial frequencies are substantially the same. The same applies to the other conditional expressions described in the above embodiment.

The various MTFs described in the above embodiment are values evaluated with the d-line (587.5618 nm). The conventional optical system (FIG. 12), the thin optical system (FIG. 3), and the thin type of the present invention. The F number of the optical system 11 (FIG. 2) is about 2.4.

In the above-described embodiment, the example in which the optical system 11 includes the stop, the lens group L0, and the correction lens L1 has been described. However, if the above-described conditions can be satisfied for the defocus MTF, the specific configuration of the optical system 11 will be described. Is optional. For example, the presence or absence of a diaphragm, the lens group L0, the correction lens L1, the arrangement of the diaphragm, and the like are arbitrary. In the above-described embodiment, the optical system 11 including the correction lens L1 is described as an example in addition to the lens group L0. However, the lens group L0 and some lenses constituting the lens group L0 correspond to the correction lens L1. The effect (power) to perform may be incorporated. Furthermore, the number of lenses constituting the lens group L0 and the correction lens L1 is arbitrary, and each lens may include a lens having no power or a cover ballast. Further, it is preferable to use an image sensor having a small incident angle dependency such as an organic CMOS sensor.

In the above-described embodiment, the peak of the on-axis defocus MTF and the off-axis defocus MTF in the S direction are accurately in the best focus (defocus 0 μm). The peak position of the defocus MTF is, for example, the focal length. An error of about ± 5% is allowed. That is, if the peak position of the off-axis defocus MTF in the S direction is within ± 5% of the focal length from the peak position of the on-axis defocus MTF, it can be considered that there is a peak at the best focus position. When there is a difference larger than ± 5% of the focal length, the effect of shifting the peak appears.

In the above-described embodiment, the imaging device 10 mounted as a part of the function of a mobile phone or the like has been described as an example. However, if a device dedicated to shooting such as a so-called compact digital camera is thin, the present invention The invention can be suitably used.

In the above-described embodiment, an example in which focus adjustment is automatically performed by the AF control unit 13 has been described. However, the present invention is also suitable for an optical system that manually performs focus adjustment or an optical system that does not have a focus adjustment function. Can be used.

DESCRIPTION OF SYMBOLS 10

Imaging device

11, 19, 20 Optical system L0 Objective lens group L1 Correction lens IP Imaging surface

Claims

In an optical system for forming an image on an image sensor,
The maximum chief ray angle is 35 degrees or more,
The peak position of the defocus MTF in the radial direction and the tangential direction on the axis, and the peak position of the defocus MTF in the tangential direction off the axis are within ± 5% of the best focus position,
An optical system in which the off-axis radial defocus MTF peak position is shifted in the plus or minus direction with respect to the off-axis tangential defocus MTF peak position.
At a spatial frequency that is ¼ of the Nyquist frequency of the image sensor, the defocus range where the off-axis radial defocus MTF value is 0.2 or more is Wt, and the off-axis tangential defocus is 2. The optical system according to claim 1, wherein Dv ≧ 0.05 Wt is satisfied, where Dv is a shift amount of a defocus MTF peak position in the off-axis radial direction with reference to the MTF peak position.
3. The optical system according to claim 2, wherein the direction in which the peak position of the defocus MTF in the radial direction is shifted is the same at an arbitrary image height.
3. The optical system according to claim 2, wherein the shift amount increases as the peak value of the defocus MTF in the radiation direction decreases.
The off-axis radiation defocus MTF peak position shift amount is Dv, the off-axis radiation defocus MTF value is 0.2 or more, and the defocus range is Wt, and the off-axis radiation is off-axis radiation. When the peak value of the defocus MTF in the direction is P1, and the peak value of the defocus MTF in the radial direction on the axis is P0, the shift amount Dv satisfies Dv ≧ 0.05Wt × (P0 / P1). 5. The optical system according to the fourth item.
The optical system according to claim 1, wherein the ratio of the total optical length to the focal length is 1.1 or less.
In an imaging apparatus including an optical system capable of focus adjustment and an image sensor that photoelectrically converts an image formed by the optical system,
The optical system has a maximum principal ray angle of 35 degrees or more, and the peak position of the defocus MTF in the radial and tangential directions on the axis and the peak position of the defocus MTF in the tangential direction off the axis are from the best focus position. An imaging apparatus which is located within ± 5% and whose off-axis radial defocus MTF peak position is shifted in the plus or minus direction with respect to the off-axis tangential defocus MTF peak position.
Focus adjustment of the optical system within a range where the on-axis defocus MTF is larger than the off-axis defocus MTF and the off-axis T-direction defocus MTF is smaller than the off-axis S-focus defocus MTF. The image pickup apparatus according to claim 7, further comprising an automatic focus adjustment unit that performs the operation.