JPH10148754A - Digital still camera - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a still image of high image quality by making an image- pickup lens single body satisfy a specified condition when the best value MTF of defocusing at the center of a screen on (e) line of the image pickup lens not passing through a low-pass filter is taken as the function of a specified spatial frequency. SOLUTION: In an optical system 31, a luminous flux from a subject is transmitted through the image pickup lens, and transmitted through an infrared ray cut filter RF, etc., then, an image is formed on a solid image pickup element CCD. And, the image-pickup lens single body satisfys the flowing condition in the case an MTF(u) is the best value MTF(modulation-transfer-function) in defocusing at the center of the screen at the (e) line of the image pickup lens no passing through the low-pass filter with (u) as a spatial frequency; 0.98< MTF(1/4P)}/}1-(λF/πP)}<1, provided that P denotes the smallest distance between pixels of solid image pickup elements, F denotes an open F number of the image-pickup lens, λ denotes the wavelength of (e) line and πdenotes a ratio of the circumference of a circle to its diameter.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a digital still camera, and more particularly, to a digital still camera for receiving a still image through a solid-state image sensor through an image pickup lens passed through a low-pass filter.

[0002]

2. Description of the Related Art Conventionally, there are two types of cameras using a solid-state image sensor, one for a moving image and the other for a still image. A camera for a still image requires a higher image quality than a camera for a moving image such as a video camera. Is done. Further, in a camera using a solid-state imaging device, in order to prevent the occurrence of moire due to the periodicity of pixel intervals of the solid-state imaging device and interference of a pattern structure of a subject, for example, a stitch pattern, a quartz plate called a low-pass filter is used. By using the birefringence of (2), the image is doubled to prevent the equivalent aperture ratio from increasing.

[0003]

However, it has been found that it is difficult to obtain a sufficiently satisfactory image quality as the research on the image quality of the still image by the optical system of the imaging lens through the low-pass filter is repeated.

Therefore, as a result of trial and error with respect to improvement of the image quality of a still image of a camera using a solid-state imaging device, an MTF (Modur) of an imaging lens alone which does not pass through a low-pass filter has been obtained.
It has been found that satisfactory image quality can be obtained by satisfying the conditional expressions described below regarding the operation-Transfer-Function.

An object of the present invention has been made in view of the above problems, and an object of the present invention is to provide a digital still camera using a solid-state image pickup device capable of obtaining a still image quality better than before.

[0006]

The object of the present invention is achieved by the following means. That is, in the digital still camera according to the first aspect of the present invention, in the digital still camera which obtains a still image by receiving an image obtained by an imaging lens having passed through a low-pass filter with a solid-state imaging device, the imaging lens does not pass through the low-pass filter. When the MTF of the best value of defocus at the center of the screen at the e-line is MTF (u) where u is a spatial frequency, 0.98 <{MTF (1 / 4P)} / ｛1- (λF / πP )｝ <1 .. (1) where P: minimum distance (mm) between pixels of the solid-state imaging device F: open F number of imaging lens λ: wavelength of e-line (0.546 × 10 ⁻³ mm) π Pi is characterized by satisfying the following condition.

A digital still camera according to a second aspect of the present invention is the digital still camera according to the first aspect, further comprising: 0.93 <{MTF (1 / 2P)} / {1- (2λF / πP)}. <1... (2) is satisfied.

According to a third aspect of the present invention, in the digital still camera, the minimum interval P (m) between the pixels of the solid-state imaging device is provided.
m) satisfies the following conditional expression: 0.002 <P <0.008 (3)

Here, the above conditional expressions (1), (2),
Prior to the description of (3), FIG. 1 shows a relationship diagram between MTF and spatial frequency used in the description. In the figure, the vertical axis indicates MTF, and the horizontal axis indicates spatial frequency. Also, U _N is the Nyquist frequency, with MTF of the shown solid line (a) an imaging lens impervious to low-pass filter (optical system), a dotted line (b) is an optical system when passed through a low-pass filter MT
F is shown. Also, P point the MTF value of a single imaging lens when the spatial frequency is U _N / 2, Q point spatial frequencies respectively show MTF value of a single imaging lens in U _N.

[0010] Here, to describe the conditional expressions, conditional expressions (1) Nyquist frequency of the actual imaging lens for MTF value of the approximate expression will be described later, of an ideal lens which is a physical limit due to the influence of diffraction U _N The ratio to the MTF value at P / 2 (point P in FIG. 1) is shown. Note that the Nyquist frequency U
_N is a limit value of a spatial frequency at which moire of a pitch (interval) P of solid-state imaging pixels does not occur. Nyquist frequency U
_N is generally represented by U _N = １ ／ P.

In order to prevent the occurrence of moire, a low-pass filter, but the spatial frequency It is known to be a sufficiently small value MTF at least U _N, U
The MTF value at the point of _N / 2 affects the quality of image quality (contrast, resolution), and also affects the MTF value at a spatial frequency equal to or lower than _UN / 2. When the value goes below the lower limit of the conditional expression (1), in the still image,
Sufficient image quality cannot be obtained.

The conditional expression (2) is a physical limit value due to the influence of diffraction.
Nyquist frequency U _N of the actual imaging lens for F value
Represents the ratio to the MTF value (point Q in FIG. 1).

When the value goes below the lower limit of the conditional expression (2), sufficient image quality cannot be obtained in a still image.

Further, conditional expression (3) represents the range of the minimum interval P (mm) between pixels of the optimal solid-state image sensor. When the upper limit of conditional expression (3) is exceeded, it is necessary to obtain a sufficient number of pixels. Is not preferable because the screen size becomes large and the imaging lens becomes large. When the value goes below the lower limit of the conditional expression (3), the influence of the diffraction of the lens becomes large, and the MTF is greatly reduced only by increasing the F-number of the aperture, so that a sufficient image quality can be obtained for a bright subject. I can't do anything.

Here, the approximate expression of the MTF will be described. The MTF on the optical axis of the ideal lens is expressed as follows: λ is the wavelength of the e-line,
F is the F number when the imaging lens is open, u is the spatial frequency,
When θ = cos ⁻¹ λFu, MTF (u) = 1 / π (2θ−sin2θ). When the spatial frequency u is not so large, θ = cos ⁻¹ λFu = π / 2−λFu − ／ (λF
u) ³ and neglecting the third or higher order of λFu, θ = π / 2−λ
Fu, and when this is substituted, sin2θ = sin (π−2λFu) = sin2λFu
^{= 2λFu- (λFu) 3/3} ! +... Ignoring the third order or more of λFu, sin2θ =
sin2λFu, these are substituted, and the MTF
(U) = 1− (4λFu / π).

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A schematic configuration of a digital still camera according to the present invention will be described with reference to the drawings. FIG. 2 is a schematic configuration diagram of a digital still camera. In FIG. 2, a digital still camera 30 includes an optical system 31 and a solid-state imaging device CC.
D, an imaging circuit 32, a frame memory 33, and the like.

The digital still camera 30 is a high-quality digital still camera, and the optical system 31 transmits a light beam from a subject through an imaging lens, further passes through an infrared cut filter RF, a low-pass filter LF, and a cover glass CG, and captures a solid image. An image is formed on the element CCD. Further, the image data is compressed by the image pickup circuit 32, temporarily stored in the frame memory 33, and taken out to obtain a high-quality still image.

The image pickup lens alone has an MTF of the best value of defocus at the center of the screen at the e-line of the image pickup lens which does not pass through the low-pass filter.
Assuming that F (u), 0.98 <{MTF (1 / 4P)} / ｛1- (λF / π
P)｝ <1 0.93 <{MTF (１ ／ P)} / ｛1- (2λF /
πP)｝ <1. Here, P is the minimum distance (mm) between pixels of the solid-state imaging device, F is the open F number of the imaging lens, λ is the wavelength of the e-line (0.546 × 10 ⁻³ mm), and π is the pi. .

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below along with comparative examples with reference to the drawings. The reference numerals used in this embodiment are as follows. f is the focal length (mm), F is the aperture value, R
Is the radius (mm), D is the interval (mm), Nd is the refractive index, ν
d is the Abbe number of the d-line, and P is the minimum interval P (mm) between pixels of the solid-state imaging device.

An MTF value of an imaging lens described later is e.
This is a value obtained by a wave optics simulation considering diffraction at a line (0.546 × 10 ⁻³ mm). However,
The MTF value of the lens is obtained by replacing the low-pass filter with a cover glass having the same optical path length in all embodiments of the present invention, and does not include the effect of the low-pass filter.

Next, the solid-state image pickup device (CC
Describing D), the solid-state imaging device is described in the first and third embodiments.
Then, Sony IC × 084K, format is 1 /
3 inch, unit pixel size is 7.4μ × 7.4μ, effective pixel number is 659 (H) × 494 (V) and about 330,000 pixels C
CD is used. In the second embodiment, the format is 1 / 2.72 inch with Matsushita Electronics MN3773.
The unit pixel size is 4.6 μ × 4.6 μ, and the number of effective pixels is 11
A CCD of 52 (H) × 872 (V) and about 1 million pixels is used.

Next, the low-pass filter used in the embodiment will be described with reference to an exploded view of the low-pass filter shown in FIG. 3. The low-pass filter LF uses a quartz plate, and uses three quartz plates having different crystal axis directions. ing. The configuration of the low-pass filter LF is the optical axis OA, O
B, OC make 45 ° with respect to the imaging lens optical axis LO,
The projection of the optical axes OA, OB, and OC onto a plane P perpendicular to the optical axis LO of the imaging lens is 0 with respect to the longitudinal direction of the screen.
Three crystal plates LFA, L forming angles of °, 45 °, and 90 °
A combination of FB and LFC is used. Also,
The thickness of each quartz plate is as shown in Table 1 below.

[0023]

[Table 1]

Embodiment 1 FIG. 4 is a sectional view of an optical system of an imaging lens according to Embodiment 1 of the present invention. Table 2 shows optical data of Example 1.

[0025]

[Table 2]

The MTF value of this imaging lens at e-line is as follows.

MTF (１ ／ P) = MTF (67.6) = 0.852 MTF (１ ／ P) = MTF (33) = 0.93 (Example 2) The imaging lens according to Example 2 of the present invention is described. FIG. 5 is a sectional view of the optical system. Table 3 shows the numerical data of Example 2.
Shown in

[0028]

[Table 3]

The MTF value of this lens at the e-line is as follows.

MTF (１ ／ P) = MTF (109) = 0.756 MTF (１ ／ P) = MTF (54.3) = 0.882 (Third Embodiment) An imaging lens according to a third embodiment of the present invention is described. FIG. 6 shows a sectional view of the optical system. Table 4 shows numerical data of the third embodiment.

[0031]

[Table 4]

The MTF value of this lens at the e-line is as follows.

At the wide-angle end, MTF (1 / 2P) = MTF (67.5) = 0.820 MTF (1 / 4P) = MTF (33) = 0.922 MTF (1 / 2P) = at an intermediate focal length MTF (67.5) = 0.828 MTF (1 / 4P) = MTF (33) = 0.922 MTF (1 / 2P) = MTF (675) = 0.850 MTF (1 / 4P) = MTF (33) = 0.926 (Comparative Example) A comparative example is an imaging lens of a four-unit zoom lens, and FIG. 7 is a sectional view of an optical system of the imaging lens of the comparative example.
The figure shows the wide-angle end. Focal length is f = 4.5
9-16.02-44.12, and each F number is F = 1.67-2.05-2.44. 1/3
The CCD of inch format 570,000 pixels, the effective pixel number is 330,000 pixels, and the unit pixel size of this CCD is P =
0.006 mm.

Table 5 shows values corresponding to the conditional expressions (1) and (2) of the embodiment and the comparative example.

[0035]

[Table 5]

As shown in Table 5, all the embodiments satisfy the above conditional expressions.

(Evaluation) As an evaluation method, an image was output to a video monitor, and the inspector visually evaluated the appearance of the image. The image of the resolution test chart placed at an object distance of 2 m is loaded into the frame memory of the test unit, and the black and white checkerboard bleed placed at 2 m is evaluated. × ”.

“O” indicates a satisfactory image quality level (sharpness, contrast) “Δ” indicates a somewhat problematic image quality level “×” indicates a problematic image quality level The evaluation results are shown in Table 6 below.

[0039]

[Table 6]

As shown in Table 6 above, the examples of the present invention showed good, satisfactory and high image quality.

[0041]

According to the configuration described above, the following effects can be obtained.

In the first aspect, the MTF of the best value of the defocus at the center of the screen at the e-line of the imaging lens that does not pass through the low-pass filter is defined as MTF (u), where u is a spatial frequency.
0.98 <{MTF (1 / 4P)} / ｛1- (λF / π
P) Since the condition of｝ <1 was satisfied, a digital still camera using a solid-state imaging device capable of obtaining good still image and high image quality was obtained.

According to a second aspect, in the digital still camera according to the first aspect, further, 0.93 <{MTF (1 / 2P)} / ｛1- (2λF /
πP)｝ <1 was satisfied, so that a digital still camera using a solid-state imaging device capable of obtaining better still image quality and higher image quality was obtained.

According to the third aspect, the minimum distance P (mm) between the pixels of the solid-state imaging device satisfies the following conditional expression: 0.002 <P <0.008. The size of the imaging element is the best, the camera does not increase, and the MTF does not decrease much even if the F value is increased.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the relationship between MTF and spatial frequency used in the description.

FIG. 2 is a schematic configuration diagram of a digital still camera according to the present invention.

FIG. 3 is an exploded perspective view of a low-pass filter.

FIG. 4 is an optical system cross-sectional view of the imaging lens according to the first embodiment of the present invention.

FIG. 5 is an optical system sectional view of an imaging lens according to a second embodiment of the present invention.

FIG. 6 is a sectional view of an optical system of an imaging lens according to a third embodiment of the present invention.

FIG. 7 is an optical system sectional view of an imaging lens of a comparative example.

[Explanation of symbols]

 Reference Signs List 30 digital still camera 31 optical system 32 imaging circuit 33 frame memory CCD solid-state imaging device CG cover glass LO imaging lens optical axis LFA, LFB, LFC quartz plate LF low-pass filter OA, OB, OC optical axis RF infrared cut filter

Claims (3)

[Claims] 1. A digital still camera for obtaining a still image by receiving an image from an imaging lens passed through a low-pass filter with a solid-state imaging device, and defocusing the imaging lens at the center of the screen at e-line without passing through the low-pass filter. M is the best value of
Assuming that F (u), 0.98 <{MTF (1 / 4P)} / ｛1- (λF / π
P)｝ <1 where P; minimum distance (mm) between pixels of the solid-state imaging device F; open F number of imaging lens λ; wavelength of e-line (0.546 × 10 ⁻³ mm) π; A digital still camera characterized by satisfying certain conditions. 2. The digital still camera according to claim 1, further comprising: 0.93 <{MTF (1 / 2P)} / ｛1- (2λF /
πP)｝ <1. A digital still camera characterized by satisfying the following condition: 3. A minimum interval P between pixels of the solid-state imaging device.
The digital still camera according to claim 1, wherein (mm) satisfies the following conditional expression: 0.002 <P <0.008.