Optical: systems and elements – Lens – Telephoto
Reexamination Certificate
2000-03-31
2001-11-13
Epps, Gerogia (Department: 2873)
Optical: systems and elements
Lens
Telephoto
C359S745000, C359S785000
Reexamination Certificate
active
06317275
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an internal focusing telephoto lens that is suitable for a photographing camera, a video camera or the like. More particularly, the present invention relates to the telephoto lens whose view angle is about 25 degrees and F-number is about 2 through 2.8.
In general, a fast telephoto lens tends to be long in the overall length and heavy in weight. Therefore, such a fast telephoto lens often employs an internal focusing mechanism. The internal focusing mechanism reduces barycentric movement during the focusing, allows high speed focusing due to low-load to the focusing mechanism and reduces variation of aberration though the use of a floating effect, as compared with a telephoto lens that employs a whole-elements focusing mechanism.
U.S. Pat. No. 4,852,984, Japanese patent provisional publications Nos. Hei 5-157964 and Hei 9-159911 disclose the internal focusing telephoto lenses.
The publication Hei 1-154112 discloses the telephoto lenses whose F-numbers are 1.2 through 1.4 and view angles are about 30 degrees. Each of the telephoto lenses of the publication includes a great number of lenses, while chromatic aberration, particularly chromatic aberration in spherical aberration and lateral chromatic aberration, and astigmatism are not sufficiently corrected.
The publication Hei 5-157964 discloses the telephoto lenses whose F-numbers are 1.8 and view angles are about 29 degrees. Since each of the telephoto lenses of the publication varies spherical aberration, astigmatism and distortion during the focusing, it cannot insure good optical performance over the full focusing range.
The publication Hei 9-159911 discloses the telephoto lenses whose F-numbers are 2.0 and view angles are about 19 degrees. The telephoto lenses of the first through third embodiments of the publication include a great number of lenses. The lenses of the fourth and fifth embodiments are narrow in the view angle and insufficient in the correction of astigmatism.
Further, when the telephoto lenses disclosed in the publications are scaled up for applying to a large format camera using Brownie film, chromatic aberration also scales up, which makes correction of chromatic aberration be deficient.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an internal focusing telephoto lens, which is capable of reducing the variations of chromatic aberration, spherical aberration, astigmatism or the like over the full focusing range where f-number is 2.0 through 2.8 and view angle is about 25 degrees while maintaining a relatively small lens number.
For the above object, according to the present invention, there is provided an improved internal focusing telephoto lens, which includes a first lens group I of positive refractive power, a second lens group II of negative refractive power and a third lens group III of positive refractive power, arranged in order from an object side. The first lens group I includes a first positive lens I-1, a second positive meniscus lens I-2 whose object side surface is convex, a third positive meniscus lens I-3 whose object side surface is convex and a fourth negative meniscus lens I-4 whose object side surface is convex, in order from the object side. The second lens group II is movable along an optical axis for focusing. The third lens group III includes a first meniscus lens III-1 whose object side surface is concave and a second positive lens III-2. Further, the telephoto lens of the present invention features that at least two positive lenses in the first lens group I satisfy the following conditions (1) and (2);
1.55<
n
I
<1.75 (1)
61<&ngr;
I
(2)
where n
I
is refractive index at d-line (588 nm) and &ngr;
I
is Abbe number of the lenses in the first lens group I.
The first lens III-1 in the third lens group III may consist of a biconcave lens and a biconvex lens that are cemented to each other.
Further, the following conditions (3) and/or (4) may be satisfied;
−10
<f/r
III-1-1
<−1.5 (3)
0.003
<d
III-1-III-2
/f
<0.25 (4)
where f is focal length of the entire lens system in an infinite focusing condition, r
III-1-1
is a radius of curvature of the object side surface of the first lens III-1 in the third lens group III and d
III-1-III-2
is a distance between the first lens III-1 and the second lens III-2 in the third lens group III.
REFERENCES:
patent: 4293197 (1981-10-01), Kobayashi et al.
patent: 4545654 (1985-10-01), Sato
patent: 4576443 (1986-03-01), Itoh
patent: 4610514 (1986-09-01), Nakamura
patent: 4786152 (1988-11-01), Arimoto
patent: 4852984 (1989-08-01), Takahashi et al.
patent: 5325234 (1994-06-01), Yoneyama
patent: 5-157964 (1993-06-01), None
patent: 9-159911 (1997-06-01), None
Asahi Kogaku Kogyo Kabushiki Kaisha
Epps Gerogia
Greenblum & Bernstein P.L.C.
Spector David N.
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