JP2005176150A - Imaging device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an imaging device which enables an MTF characteristic on a periphery at a distance from an optical axis to be corrected so as to be nearly equal to the MTF characteristic in the center of the optical axis. <P>SOLUTION: The imaging device is provided with a lens optical system 10, a CCD sensor 11, and an image processing portion 14 performing sharpening processing to an image generated by the CCD sensor 11. The image processing portion 14 performs an orthogonal conversion to the image (inputted image) generated by the CCD sensor 11, sets a parameter of a BPF to an intermediate frequency range in which enhancement of sharpening is desired, and a sharpening degree (gain) is set so that the sharpening degree (gain) of the intermediate frequency range in which the enhancement of sharpening is desired is enhanced and is proportional to square of a distance from an optical axis of the lens optical system 10. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an imaging apparatus such as a digital camera, and more particularly to an imaging apparatus that performs digital processing based on an image captured by an imaging element through a lens optical system having a predetermined modulation transfer function characteristic.

  In general, in an imaging apparatus such as a digital camera, moire due to a folding signal occurs when a high-frequency component that is 1/2 or more of the sampling frequency of the imaging element is included in the incident light. Since this moire is difficult to remove by image processing by an electric circuit, an optical low-pass filter is disposed in the optical path between the optical lens that causes light to enter the image sensor and the image sensor to remove high-frequency components. .

By the way, in general, a modulation transfer function characteristic (hereinafter referred to as MTF characteristic: Modulation Transfer Function) representing the sharpness of an image of a lens optical system including an optical lens is not uniform due to the influence of aberration, and the resolution at the center of the optical lens is high. The resolution decreases as you go to the periphery.
FIG. 6 exemplifies MTF characteristics when a normal optical lens is used. (A) shows the characteristics on the optical axis, (b) shows the characteristics at an image height of 50%, and (c) shows the image height at 70%. (D) shows the characteristic at the position farthest from the optical axis. As shown in the figure, the MTF characteristics deteriorate as the distance from the optical axis increases.
On the other hand, the optical low-pass filter disposed in the optical path between the optical lens and the image sensor is made of quartz, and its thickness is constant from the peripheral part to the central part. Accordingly, since the frequency characteristics of the optical low-pass filter are uniform throughout, the MTF characteristics in the peripheral portion are not improved even if the optical lens having the MTF characteristic is combined with the optical low-pass filter.

  Although it is possible to improve the MTF characteristics of the peripheral part to some extent by using a special optical lens, the use of such a special optical lens is not preferable because it causes an increase in cost. Therefore, there is a demand for a technique for improving the MTF characteristics of the peripheral portion as well as the MTF characteristics of the central portion by image processing while using the conventional optical lens and the optical low-pass filter without using the optical filter.

  The present invention has been made in view of such circumstances, and an object thereof is to make it possible to correct the MTF characteristics of the peripheral part away from the optical axis so as to be substantially the same as the MTF characteristic of the central part of the optical axis. It is to provide an image pickup apparatus.

  In order to achieve the above object, an aspect of the present invention is an image pickup apparatus that picks up an image of a subject by forming an optical image of the subject on an image pickup surface of an image pickup device using a lens optical system, and is obtained by imaging. First sharpness setting means for setting a first sharpness degree for the image so that a sharpness degree of a specific spatial frequency component of the image is increased, and the image obtained by imaging A second sharpness setting means for setting a second sharpness for the image based on a relationship between a distance from the center and a modulation transfer function characteristic of the lens optical system; and the first sharpening Sharpening processing means for performing a sharpening process on the image based on the degree and the second sharpening degree.

  Preferably, the second sharpness setting unit sets the second sharpness for the image so as to be proportional to the square of the distance from the center of the image.

  According to the imaging apparatus of the present invention, it is possible to correct the MTF characteristics in the peripheral part away from the optical axis so as to be substantially the same as the MTF characteristic in the central part of the optical axis, so that the resolution is high in a wide range. A clear image can be obtained.

Embodiments Hereinafter, embodiments of an imaging apparatus according to the present invention will be described with reference to the accompanying drawings.
FIG. 1 is a diagram illustrating an overall configuration of an imaging apparatus 1 in the present embodiment. The imaging apparatus 1 is roughly composed of an optical system, a signal processing system, a recording system, a display system, and a control system.

The optical system includes a lens optical system 10 and a CCD sensor 11.
The lens optical system 10 includes an optical lens facing the subject, an optical low-pass filter (not shown), and the like. In the lens optical system 10, the optical image of the subject is condensed by the optical lens, and an image of the subject is formed on the CCD sensor 11.

The lens optical system 10 has a predetermined MTF characteristic determined by the curvature, aberration, and the like of the optical lens.
In general, the MTF characteristics deteriorate as the distance from the optical axis center increases due to aberrations or the like, and at the same spatial frequency, the square of the distance from the optical axis center and the MTF value are in an inversely proportional relationship. That is, as the distance from the center of the optical axis increases, the MTF characteristics deteriorate at an accelerated rate.

  The CCD sensor 11 is composed of, for example, a CCD image sensor provided with a color filter, and photoelectrically converts an object image formed by the lens optical system 10.

The signal processing system is a correlated double sampling circuit (CDS: Correlated Double) for reducing noise by sampling the electrical signal output from the CCD sensor 11.
Sampling) CDS 12, A / D converter 13 that converts an analog signal output from CDS 12 into a digital signal, and predetermined image processing to be described later are performed on the digital signal output from A / D converter 13. And an image processing unit 14.

  The recording system includes a memory 15 that stores a control program executed by the control unit 16 and compressed data of the image data generated by the image processing unit 14.

  The display system includes a D / A converter 18 that converts image data processed by the image processing unit 14 and stored in a built-in image memory, and an LCD (Liquid Crystal) that functions as a finder by displaying an input image. The display unit 19 including a display) is included.

  The control system includes a timing generator 17 for controlling the operation timing of the CCD sensor 11 to the A / D converter 13, an operation input unit 20 for inputting a shutter operation and other commands by the user, and a sharpening process for the input image. Thus, the image processing unit 14 that corrects the MTF characteristics and the control program stored in the memory 15 are read out, based on the read control program, a user command input from the operation input unit 20, and the like. The control unit 16 includes a CPU (Central Processing Unit) that controls the entire imaging apparatus 1.

In the imaging apparatus 1, an optical image of a subject is incident on a CCD sensor 11 via a lens optical system 10, and is photoelectrically converted by the CCD sensor 11 into an electrical signal. The obtained electrical signal is removed of noise components by the CDS 12, digitized by the A / D converter 13, and then temporarily stored in an image memory built in the image processing unit 14.
In a normal state, an image signal is constantly overwritten at a constant frame rate in the image memory built in the image processing unit 14 by the control of the signal processing system by the timing generator 17. The image signal of the image memory built in the image processing unit 14 is converted into an analog signal by the D / A converter 18, and a corresponding image is displayed on the display unit 19.

The display unit 19 also serves as a finder for the imaging apparatus 1. After the user presses (operates) the shutter button included in the operation input unit 20, the control unit 16 holds the image signal immediately after the shutter button is pressed to the timing generator 17, that is, image processing. The signal processing system is controlled so that the image signal is not overwritten in the image memory of the unit 14. Then, the image data held in the image memory of the image processing unit 14 is compressed by a predetermined method and recorded in the memory 15.
With the operation of the imaging apparatus 1 as described above, the acquisition of one piece of image data is completed.

Next, processing executed in the image processing unit 14 will be described with reference to FIG.
2A shows the MTF characteristic of the lens optical system 10, FIG. 2A shows the MTF characteristic at the center of the optical axis, FIG. 2B shows the MTF characteristic at a position slightly away from the optical axis, and FIG. The MTF characteristics at the position farthest from the optical axis (most peripheral part) are shown. In FIG. 2, (B) shows the relationship between the sharpness (a gain described later) executed in the image processing unit 14 and the spatial frequency.

As shown in FIG. 2A, the MTF characteristics deteriorate as the distance from the optical axis increases. Therefore, the image processing unit 14 adjusts the sharpening degree according to the distance from the optical axis.
On the other hand, in order to improve the MTF characteristics, it is not preferable to increase the sharpness uniformly in the entire spatial frequency region, and as shown in FIG. 2B, the sharpness is particularly increased in the intermediate spatial frequency region Bf. Is desirable. That is, if the sharpening degree is excessively increased in the low frequency region and the high frequency region of the spatial frequency, an image with an unnatural impression may be generated, and this needs to be avoided.
From this point of view, as will be described later, the image processing unit 14 is configured to set the sharpening degree (gain) according to the distance from the optical axis and the spatial frequency range.

FIG. 3 shows an example of the configuration of the image processing unit 14 in the present embodiment. As shown in FIG. 3, the image processing unit 14 includes an orthogonal transform unit 141, a parameter setting unit 142 serving as a first sharpening degree setting unit, a second sharpening degree setting unit, and a sharpening unit serving as a sharpening processing unit. And the conversion processing unit 143.
The orthogonal transform unit 141 performs orthogonal transform on the image (input image Iin) input from the A / D converter 13 and outputs the result to the parameter setting unit 142. The orthogonal transformation performed by the orthogonal transformation unit 141 refers to two-dimensional Fourier transformation, and is performed by FFT (fast Fourier transformation), whereby the spatial frequency characteristics of the input image Iin can be obtained.
The parameter setting unit 142 sets a spatial frequency for a bandpass filter (BPF) executed in the subsequent sharpening processing unit 143. Specifically, the spatial frequency region Bf to be corrected among the MTF characteristics of the lens optical system 10 is set in advance. For example, the set value (spatial frequency region) may be variable according to the distance from the optical axis. .
The sharpening processing unit 143 performs a sharpening process on the input image using the BPF set by the parameter setting unit 142 and a gain set according to the distance from the optical axis center, and generates an output image Iout To do.

Further, the image sharpening process executed by the sharpening processing unit 143 will be described with reference to FIG.
In FIG. 4, an image input unit 51 receives the input image Iin input from the A / D converter 13 and separates it into a horizontal direction component, a vertical direction component, and an oblique direction component.
The vertical LPFs 52 and 54 are vertical low-pass filters (LPF), and the horizontal LPFs 53 and 55 are horizontal low-pass filters (hereinafter LPF).
The horizontal direction BPF 56 and the vertical direction BPF 57 are band-pass filters (hereinafter referred to as BPF) in the horizontal direction and the vertical direction, respectively. The gains 58 to 61 are gains for adjusting the degree of image sharpening (sharpness).
With the configuration described above, the input image Iin is sharpened in the horizontal, vertical, and diagonal directions, then they are combined, the overall sharpening degree is adjusted, and the sharpening process ends.

Here, the sharpening process performed in each direction will be described by taking the horizontal direction sharpening process as an example.
First, LPF is applied in a direction orthogonal to the direction in which sharpening is performed, and components unnecessary for subsequent processing are removed. That is, as shown in FIG. 3, in the case of sharpening in the horizontal direction, vertical LPF is applied (vertical LPF 52).
Next, the spatial frequency to be sharpened is extracted by a BPF in the horizontal direction (horizontal direction BPF 56), and a gain indicating the degree of sharpening is applied to the extracted portion (gain 58), thereby achieving horizontal sharpening. finish.

  In the sharpening processing unit 143, various known techniques such as a method using a Laplacian filter can be applied to the sharpening processing executed according to the gain. For example, by changing the filter coefficient of the Laplacian filter according to the gain value, sharpening processing according to the gain can be performed.

Next, the gain (sharpness) set in each of the gains 58 to 61 shown in FIG. 4 will be described.
5A and 5B are diagrams illustrating the relationship between the distance R from the optical axis and the gain. FIG. 5A is a definition of the distance R from the optical axis on the imaging surface, and FIG. 5B is the distance R from the optical axis. An example of the gain relationship is shown respectively.
FIG. 5A shows a state in which the light beam from the subject forms an image on the imaging surface of the CCD sensor 11 through the optical lens of the lens optical system 10, and in the image captured by the CCD sensor 11, R is the optical axis. Defined by distance from center.
In FIG. 5B, the gain (sharpness) set in each of the gains 58 to 61 shown in FIG. 4 is the distance from the optical axis, as exemplified by the relationship between the distance R from the optical axis and the gain. It is set to be proportional to the square of R.
As a result, it is possible to correct the MTF characteristic of the lens optical system 10 that decreases at an acceleration according to the distance R from the optical axis. That is, as shown in FIG. 2A, the apparent MTF characteristic can be raised with respect to the imaging surface at a position away from the optical axis.

  As described above, according to the imaging device 1 of the present embodiment, the lens optical system 10 having a predetermined MTF characteristic and the CCD that generates an image by photoelectrically converting the optical image formed by the lens optical system 10. The image processing unit 14 includes a sensor 11 and an image processing unit 14 that performs a sharpening process on an image generated by the CCD sensor 11, and the image processing unit 14 performs orthogonal transformation to orthogonally transform an image (input image) generated by the CCD sensor 11. Based on the parameters set by the transform unit, the parameter setting unit for setting the BPF parameters for the intermediate frequency region for which sharpening is to be emphasized with respect to the spatial frequency characteristics of the input image obtained by the orthogonal transform, and the parameter set unit The sharpening degree (gain) in the intermediate frequency region where sharpening is to be emphasized is set to be high. In addition, the sharpening degree of the image to be sharpened is proportional to the square of the distance from the center of the image with respect to the sharpening degree with respect to the center of the image (the optical axis of the lens optical system 10). (Gain) is set. The following effects can be obtained as a result of performing the sharpening process of the input image with the sharpening degree set as described above.

  That is, the image processing unit 14 sharpens the input image based on the sharpness (gain) according to the distance from the optical axis on the imaging surface, and as a result, the MTF characteristics in the peripheral portion are equal to the MTF characteristics in the central portion of the optical axis. Thus, a wide range and high resolution image can be obtained.

  Since the sharpening degree (gain) is set according to the square of the distance from the optical axis so as to interpolate the MTF characteristic of the lens optical system 10 that decreases at an acceleration according to the distance R from the optical axis, The sharpening process does not result in an unnatural impression.

  Since the sharpening degree (gain) is set so as to match the MTF characteristic of a conventional general optical lens, the optical lens used for the lens optical system of the image pickup apparatus is a general optical used conventionally. A lens can be used, and it is not necessary to use a special optical lens.

The contents of the present embodiment are not limited to the contents described above, and various modifications can be made without departing from the scope of the present invention.
For example, in the image processing unit 14, BPF is applied to the spatial frequency characteristic orthogonally transformed by the orthogonal transformation unit 141. However, even when a gain (sharpness) having a magnitude corresponding to the spatial frequency is set. Good. That is, a strong gain is applied in the intermediate spatial frequency range where the MTF value needs to be greatly improved, and a relatively weak gain is applied in the high frequency and low frequency spatial frequency regions where the MTF value does not need to be greatly improved. May be.

In the above description, the gain of the sharpening process executed in the sharpening processing unit 143 is changed in accordance with the square of the distance from the optical axis center so as to be adapted to a general optical lens. However, if the relationship between the distance from the center of the optical axis and the MTF characteristic is clear, the gain can be set according to the relationship.
For example, the MTF characteristic of the lens optical system 10 actually used in the imaging device 1 is measured using the distance from the optical axis center as a variable, and mapping data (the distance between the optical axis center and the gain between the optical axis center and the gain) is measured. The sharpening process may be performed based on the relationship.

It is an example of the block diagram of an imaging device. It is the figure which illustrated the process performed in an image process part, (A) shows an MTF characteristic, (B) shows the sharpening degree with respect to the spatial frequency required in order to correct | amend an MTF characteristic, respectively. It is a block diagram of an image processing part. It is a block diagram of a sharpening process part. It is a figure which shows the relationship between the distance R from an optical axis, and a gain, (A) is the definition of the distance R from the optical axis in an imaging surface, (B) is the relationship between the distance R from an optical axis, and a gain. An example is shown respectively. Examples of MTF characteristics of a lens optical system when a normal optical lens is used, (a) shows characteristics on the optical axis, (b) shows characteristics at an image height of 50%, and (c) shows image height at 70%. (D) shows the characteristic at the position farthest from the optical axis.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Imaging device 10 ... Lens optical system 11 ... CCD sensor 12 ... CDS
DESCRIPTION OF SYMBOLS 13 ... A / D converter 14 ... Image processing part 141 ... Orthogonal transformation part 142 ... Parameter setting part 143 ... Sharpening process part 15 ... Memory 16 ... Control part 17 ... Timing generator 18 ... D / A converter 19 ... Display part 20 ... operation input part

Claims (2)

An imaging device that forms an optical image of a subject on an imaging surface of an imaging element by a lens optical system and images the subject,
A first sharpness setting means for setting a first sharpness for the image so that the sharpness of a specific spatial frequency component of the image obtained by imaging is increased;
Second sharpness setting means for setting a second sharpness for the image based on the relationship between the distance from the center of the image obtained by imaging and the modulation transfer function characteristic of the lens optical system When,
An imaging apparatus comprising: a sharpening processing unit that performs a sharpening process on the image based on the first sharpening degree and the second sharpening degree. The second sharpening degree setting means includes:
The imaging device according to claim 1, wherein the second sharpening degree for the image is set to be proportional to the square of the distance from the center of the image.

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