Optical: systems and elements – Lens – Selective magnification by exchanging or adding a lens...
Reexamination Certificate
1999-03-04
2001-09-11
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Lens
Selective magnification by exchanging or adding a lens...
C359S763000, C359S771000
Reexamination Certificate
active
06288847
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an attachment lens detachably for extending the focal length of a main camera lens and camera such as a television camera and a video camera with which the attachment lens is used together, and, more particularly, to an attachment lens detachably incorporated between a main camera lens and a camera such as a television camera and a video camera equipped with a color separation optical system or assembly and/or filters which imposes restraints on optical dimensions such as a back-focal distance.
1. Description of Related Art
With an increased demand of high resolution for the entire area of a charge coupled device (CCD) installed as an image forming device in cameras such as a TV camera and a video camera, the camera lens is required to have optical performance to form the whole area of image with a high and uniform resolution. On the other hand, there have been put to practical use various types of attachment lenses which extend the focal length of a camera lens with which the attachment lens is used together without shifting the focal point of the camera lens. Such an attachment lens is essential to provide the overall lens system with a high resolution power. One of such attachment lenses known from, for example, Japanese Unexamined Patent Publication No. 7-174968 has a magnification between approximately 1.1 and 1.2 which is suitably used together with a still camera or a video camera having a relatively short back-focal distance.
TV camera lenses and video camera lenses which are required to have a relatively high resolution power are essential to have a long back-focal distance because TV cameras and video cameras incorporate a color separation optical system and/or a filter assembly in front of an imaging device therein. Such a TV camera lens or a video camera lens is generally required to have a relatively long distance to an exit pupil. If the exit pupil is at a short distance, there occurs color shading due to a difference in incident angles of principal light rays incident upon upper and lower margins of a dichroic mirror of the color separation optical assembly. Therefore, the attachment lens of this type is essential to have a long distance to the exit pupil as well as a long back-focal length.
TV camera lenses and video camera lenses are designed so as to appropriately balance the axial chromatic aberrations of the lens according to glass kinds used for color separation optical assemblies. If the attachment lens is able to change the balance of axial chromatic aberrations of the TV camera lens or the video camera lens, the attachment lens is exceedingly conveniently used together with TV cameras and video cameras which are equipped color separation optical assemblies made of different types of glasses.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide an attachment lens for use with a camera lens of a TV camera or a video camera equipped with a color separation optical system which keeps high resolution power of the camera lens without causing a change in aberrations.
It is another object of the invention to provide an attachment lens for use with a camera lens of a TV camera or a video camera equipped with a color separation optical system which provides a long back-focal distance and a long distance to an exit pupil.
It is a further object of the invention to provide an attachment lens for use with a camera lens of a TV camera or a video camera equipped with a color separation optical system which can convert balance of axial chromatic aberrations of the camera lens according to glass types of the color separation optical system.
The foregoing objects of the invention are accomplished by an attachment lens optical system which is disposed between a camera lens and an imaging device to extend the overall focal length of the camera lens, the attachment lens optical system comprising, in order from the object side, a first lens group comprising of one of a biconcave lens element and a negative power meniscus lens element having a convex object side surface, a second lens group including a cemented doublet comprising a biconcave lens element and a biconvex lens element, a third lens group including at least a biconvex lens element, and a fourth lens group desirably with a negative power which includes at least a biconcave lens element.
The attachment lens optical system has a magnification &bgr; between 1.1 and 1.3. where &bgr; is the magnification of the attachment lens optical system.
The attachment lens optical system desirably satisfies the following condition:
0.1≦|
f
1
/f
4
|≦0.7
where f
1
and f
4
are the overall focal lengths of the first and fourth lens groups, respectively.
Further, the attachment lens optical system desirably satisfies the following conditions:
1.8<
N
1
1.8<
N
21
&ngr;
22
<&ngr;
21
&ngr;
44
<30
where N
1
and N
21
are the indices of refraction of one of a biconcave lens element and a negative power meniscus lens element having a convex object side surface within the first lens group and the biconcave lens element of the second lens group, respectively, and &ngr;
21
, &ngr;
22
and &ngr;
44
are the Abbe numbers of the biconcave lens element of the second lens group, the biconvex lens element of the lens group and the biconcave lens element of the fourth lens group, respectively.
REFERENCES:
patent: 4154508 (1979-05-01), Nakamura
patent: 4239340 (1980-12-01), Ogino
patent: 5253112 (1993-10-01), Suzuki et al.
patent: 7-174968 (1995-07-01), None
Fuji Photo Optical. Co., Ltd.
Schwartz Jordan M.
Young & Thompson
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