JPH03267913A - Image pickup optical system - Google Patents

Abstract

PURPOSE:To make an incident light beam incident vertically on the image pickup surface of a small-sized image pickup element at its peripheral part by providing positive refracting power to at least one of optical elements in an optical element group. CONSTITUTION:This image pickup optical system is an image pickup optical system such as a video camera, an electronic still camera, and a fiber scope and at least one optical element in the optical element group has the positive refracting power. For example, the projection surface of an optical low-pass filter 2 is formed as a lens part 2a which has positive refracting power. Therefore, the light beam which is projected by a photographing lens 1 and made incident on the peripheral part of the image pickup surface of the image pickup element 3 is refracted positively by the lens part 2a and made incident on the image pickup surface almost vertically. Consequently, the constitution of the image pickup optical system which uses image pickup elements having a regular picture element array and removes moire fringes by the optical low-pass filter is small in size and simple and the system becomes telecentric. Namely, the light beam which is made incident on the peripheral part of the image pickup surface of the image pickup element is made incident on the image pickup surface almost vertically.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a video camera and an electronic still camera.

It relates to imaging optical systems such as fiberscopes.

[Conventional technology]

In a camera using an imaging device such as a CCD with a regular pixel array, if a subject image contains a frequency component higher than the Nyquist frequency of the imaging device, false signals called moiré fringes are generated. Moiré fringes also occur in fiberscopes because, like in CCDs, the fibers on the east end face of the fibers have a regular array.

Therefore, conventionally, in order to prevent the occurrence of moire fringes, an optical low-pass filter that limits high frequency components has been placed in the imaging optical system or the objective optical system.

Many optical low-pass filters have been proposed, including birefringence-type low-pass filters that utilize the birefringence of quartz crystals and phase-type low-pass filters that utilize the diffraction effect of light.

The imaging optical system using an optical low-pass filter has a first
The one shown in Figure 3 is common. In the figure, reference numeral 1 denotes a photographic lens, behind which an optical low-pass filter 2 is arranged, and behind it an image sensor 3 having a regular arrangement such as a CCD.

[Problem to be solved by the invention]

By the way, it is desirable that the exit pupil of the photographing lens 1 used here is far from the imaging surface of the image sensor 3. That is, it is desirable that the light rays incident on the periphery of the imaging surface be incident as close to perpendicular to the imaging surface as possible. That is, recent image pickup devices 3 such as CCDs have a structure in which one microlens 3b is attached to each pixel light receiving section 3a, as shown in FIG. 14(A), in order to increase its sensitivity. The light is collected by the microlens 3b and sent to the pixel light receiving section 3.
It guides light to a. At the center of the imaging surface, the light rays enter the imaging surface almost perpendicularly, so a sufficient amount of light can be obtained at the pixel light receiving section 3a, but as shown in FIG. When the light rays are incident on the imaging surface, the light rays are not sufficiently incident on the pixel light-receiving portions 3a, and as a result, the periphery of the reproduced image becomes dark.

However, in this type of conventional imaging optical system, due to design constraints such as miniaturization, the exit pupil of the imaging lens 1 is not always sufficiently far from the imaging surface of the imaging element 3, so the image shown in FIG. As shown, the light rays that enter the periphery of the imaging surface from the photographic lens 1 are obliquely intersecting the optical axis so as to move away from the optical axis.Therefore, the above-mentioned problems occur. , an imaging optical system using a so-called telecentric converter that makes the exit pupil of the photographic lens far away has been proposed in Japanese Patent Application Laid-Open No. 63-235910. There was a problem that the imaging optical system when doing so became large and complicated.

Furthermore, examples in which a lens in a photographic lens also functions as an optical low-pass filter are shown in Japanese Patent Laid-Open No. 64916 and Japanese Patent Laid-Open No. 61-917, but these do not change the exit pupil position. Since it is not intended for change, the above problem could not be resolved.

In view of the above-mentioned problems, the present invention provides an imaging optical system that uses an imaging element with a regular pixel arrangement and removes moiré fringes with an optical low-pass filter. In other words, it is an object of the present invention to provide an imaging optical system in which the incident light beam to the periphery of the imaging surface of an imaging element is made substantially perpendicular to the imaging surface.

[Means and effects for solving the problem] An imaging optical system according to the present invention includes a photographing lens, an imaging element having a regular pixel arrangement, and an optical element group including at least an optical low-pass filter disposed between them. In this imaging optical system, at least one optical element of the optical element group has positive refractive power, so that it is possible to extend the image to the periphery of the imaging surface of the imaging element without increasing the size of the imaging optical system. The incident light beam is made to enter the imaging surface almost perpendicularly.

That is, in the imaging optical system according to the present invention, as shown in FIG. 1, for example, the exit surface of the optical low-pass filter 2 is formed as a lens section 2a having positive refractive power, so that the imaging lens l can be ejected to take an image. The light rays incident on the periphery of the imaging surface of the element 3 are positively refracted by the lens portion 2a and are incident on the imaging surface almost perpendicularly.

Note that the refractive power of the lens surface preferably satisfies the following conditional expression (1).

(It O<-< 0.6 However, h is the arbitrary length of the diagonal length of the imaging surface of the image sensor 3, and f
is the focal length of the lens portion 2a having positive refractive power.

That is, equation (1) defines the refractive power of the lens section 2a disposed between the photographic lens 1 and the image sensor 3.

If the lower limit of conditional expression (1) is exceeded, the refractive power becomes negative and the effect is lost. If the upper limit is exceeded, distortion or chromatic aberration of magnification will occur, which is not preferable.

Alternatively, the position where the lens portion 2a is provided is preferably a position determined by the following conditional expression (2).

(2) 0 < - < 5 However, h is the length outside the diagonal length of the imaging surface of the image sensor 3, and D
is the air-equivalent optical path length from the imaging surface of the image sensor 3 to the lens portion 2a.

If the lower limit of conditional expression (2) is exceeded, interference between the lens section 2a and the imaging surface of the sensor element 3 will occur. Moreover, if the upper limit is exceeded, the amount of blur caused by the optical low-pass filter 2 becomes too large, and the number of pixels of the image pickup device 3 decreases, resulting in deterioration of the imaging performance of the photographic lens l.

〔Example〕

Hereinafter, the present invention will be described in detail based on the illustrated embodiments.

FIG. 2 is a diagram showing a first embodiment of the imaging optical system according to the present invention, and FIG. 3 is an enlarged view of the optical low-pass filter. In this case, the optical low-pass filter 2 includes a crystal birefringent plate. It is constructed by bonding a lens part 2a molded from glass or resin and having a positive refractive power to the rear surface of a birefringent low-pass filter part 2b formed by one or a plurality of filters stacked one on top of the other. Specifically, the lens portion 2a
Since the focal length f is 20 mm, the air-equivalent optical path length from the exit pupil of the photographic lens 1 to the lens section 2a is 20+n.
If it is about +n, the image side becomes telecentric, that is, the incident light on the imaging surface of the image sensor 3 becomes almost parallel to the optical axis and almost perpendicular to the imaging surface. In addition, the image sensor 3 is of each inch size (the diagonal length of the imaging surface is 8 mm).
) is used, and the diagonal length h of the imaging surface of the imaging element 3 is 4 mm. Therefore, h/f=0.2,
The above conditional expression (1) is satisfied. Also, there is a certain distance from the lens section 2a to the imaging surface, and the distance is set to 20 m.
If it is less than m, the above conditional expression (2) is also satisfied.

FIG. 4 shows an optical low-pass filter 2 according to a second embodiment, which is constructed by joining a lens portion 2a to the front surface of a birefringent low-pass filter portion 2b. In this case, the light rays are perpendicularly incident on the birefringent low-pass filter portion 2b up to the peripheral portion, which is preferable for the function of the low-pass filter.

FIG. 5 shows an optical low-pass filter 2 according to a third embodiment, which is constructed by joining a lens portion 2a to the front and rear surfaces of a birefringent low-pass filter portion 2b.

FIG. 6 shows an optical low-pass filter 2 according to a fourth embodiment, in which the surface opposite to the lattice surface of a phase-type low-pass filter portion 2C molded from glass or resin is formed as a lens surface 2d. It consists of

FIG. 7 shows an optical low-pass filter 2 according to a fifth embodiment, in which the grating surface of a phase-type low-pass filter portion 2c is formed on a lens surface 2d.

FIG. 8 shows an optical low-pass filter 2 according to a sixth embodiment, which is made by joining a gradient index lens portion 2e having a positive refractive power to the rear surface of a birefringent low-pass filter portion 2b. It is something.

Note that the refractive index distribution type lens portion 2e has a refractive index distribution such that the refractive index decreases as it moves away from the optical axis, so that positive refractive power can be obtained.

FIG. 9 shows the main part of the seventh embodiment, which is formed by forming the entrance surface or the exit surface of a gradient index lens 4 having positive refractive power as a phase plane 4a.

FIG. 1O shows the main part of the eighth embodiment, which is made by combining media with different refractive indexes nl and n2 to form a positive lens 5, and forming a cemented surface of the positive lens 5 with a phase plane 5a. It is. According to this positive lens 5, the difference between the refractive indexes n1 and 02 is much smaller than the difference between the refractive index of air, so it is effective in cases where it is not desired to remove high frequency components to some extent.

FIG. 11 shows the main part of the ninth embodiment, which includes a positive lens 6 near the image sensor side of the optical low-pass filter 2.
It consists of arranging.

FIG. 12 shows the main part of the tenth embodiment, which includes a lens section 7a provided on the front surface of a protective cup X-7 that covers the imaging surface of the imaging element 3. In this embodiment, the lens portion 7a may be molded from resin, the front surface of the protective cover 7 may be polished to form a lens surface, or the protective cover 7 may be constructed from a plastic lens sheet. It may also be used as a protective cover that has a lens effect.

It goes without saying that in each of the above embodiments, the lens surface is not limited to a spherical surface, but may also be a paraboloid, a hyperboloid, or other aspheric surface.

〔Effect of the invention〕

As described above, the imaging optical system according to the present invention uses an imaging element with a regular pixel arrangement and uses an optical low-pass filter to remove moiré fringes, and has a compact and simple configuration. , the incident light rays to the periphery of the imaging surface of the image sensor become incident on the imaging surface almost perpendicularly, and the periphery of the screen also becomes bright, which is an important practical advantage.

[Brief explanation of drawings]

FIG. 1 is a conceptual diagram of the imaging optical system according to the present invention, FIG. 2 is a diagram showing the first embodiment, FIG. 3 is an enlarged view of the optical low-pass filter of the first embodiment, and FIGS. 4 to 8 are diagrams showing the optical low-pass filters of the second to sixth embodiments, respectively, FIGS. 9 to 12 are diagrams showing the main parts of the seventh to tenth embodiments, respectively, and FIG. 13 is the conventional one. FIG. 14 is an enlarged view of a main part showing a problem in the conventional example, and FIG. 15 is a diagram showing the state of light rays incident on the image sensor from the photographing lens in the conventional example. 1...Photographing lens, 2...Optical low-pass filter 2a, 7a...Lens portion, 2b...Part of birefringent low-pass filter, 2c...Phase-type low-pass filter 1 part, 2d...lens surface, 2e...
・Refractive index gradient lens portion, 3...imaging element, 4 refractive index gradient lens, 4a, 5a...phase plane, 5, 6...
9.・Positive lens, 7... Protective cover (-11--
No. 3 Procedural amendment (voluntary) Indication Patent Application No. 68644 Imaging optical system Address: 5-19 Shinbashi, Minato-ku, Tokyo 105 113 Contents of the amendment to Figure 15 Correct it to r or J. (2) Delete "Optical low-pass filter... because the number of pixels decreases" in lines 2 to 4 of page 7 of the same. (3) No. 10 of the same. Page 4-5 lines [to some extent...
"is in effect and in force" is corrected as follows. "The height of the unevenness of the phase plane 5a can be made higher than that of the phase plane 4a of the seventh embodiment, which is advantageous in terms of manufacturing accuracy. The difference between the unevenness of the phase plane 4a is about the wavelength, and Accuracy is required (4) Figure 10 in the drawing is corrected as attached. Figure 10 JP-A-267913 (6)

Claims (1)

[Scope of Claims] An imaging optical system comprising a photographing lens, an imaging element having a regular pixel arrangement, and an optical element group including at least an optical low-pass filter disposed between them, at least of the optical element group. An imaging optical system characterized in that one optical element has positive refractive power.