Zoom lens

Details Zoom lens Zoom lens

2003-05-20

2004-11-30

Spector, David N. (Department: 2873)

Optical: systems and elements

Lens

With variable magnification

C359S685000

Reexamination Certificate

active

06825989

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to a zoom lens, and more particularly to a high-aperture-ratio, high zoom-ratio zoom lens system including a wide-angle zone which has a phototaking field angle of at least 70° suitable for cameras in general, and video cameras or digital cameras in particular.
In recent years, attention has been paid on digital cameras (electronic cameras) which are potential next-generation cameras superseding silver-salt 135 mm film (usually called Leica size) cameras. For digital cameras for general users, single-focus lenses having a diagonal field angle of about 60° or zoom lenses of about 3 magnifications using the same at wide-angle ends go mainstream. For high-class users, on the other hand, zoom lenses must be further extended to the wide-angle or telephoto end, and be compatible with TTL optical finders as well. As a matter of course, such zoom lenses are required to have ever higher performance. For zoom lenses having a diagonal field angle of about 75° at the wide-angle end and about 7 to 10 magnifications and compatible with TTL optical finders, some are now commercially available for the aforesaid silver-salt 135 mm film cameras. However, wide-angle, high-zoom-ratio zoom lenses, which are well suitable for image-pickup formats considerably smaller in size than the film camera formats and are fast as expressed by an F-number of about 2.0 to 2.8 at the wide-angle end, are little known except those for TV cameras and other commercial purposes.
SUMAMRY OF THE INVENTION
The state of the art being like this, an object of the present invention is to provide a wide-angle, high-zoom-ratio zoom lens, and especially a zoom lens system which is compatible with a TTL optical finder having a diagonal field angle of at least 70° at the wide-angle end and about 7 to 10 magnifications, and is fast as well, as expressed by an F-number of about 2.0 to 2.8 at the wide-angle end.
To achieve this object, the present invention basically provides
a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of the zoom lens system during zooming and has positive refracting power, a second lens group which moves toward an image side of the zoom lens system along the optical axis during zooming from a wide-angle end to a telephoto end of the zoom lens system and has negative refracting power, and a rear lens group having at least two movable subgroups or, alternatively,
a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of the zoom lens system during zooming and has positive refracting power, a second lens group which moves toward an image side of the zoom lens system along the optical axis during zooming from a wide-angle end to a telephoto end of the zoom lens system and has negative refracting power, and a rear group which is located subsequent to the second lens group and has at least two spacings variable during zooming.
Such constructions are favorable for achieving high zoom ratios while various aberrations are minimized. The present invention having such basic constructions has the following characteristic features.
According to the first embodiment of the present invention, there is provided a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of the zoom lens system during zooming and having positive refracting power, a second lens group which moves toward an image side of the zoom lens system along the optical axis during zooming from a wide-angle end to a telephoto end of the zoom lens system and a rear lens group having at least two spacings variable during zooming, wherein a focal length f
1
of the first lens group satisfies the following condition (1):
6
<f
1
/L<
20  (1)
where L is a diagonal length of an effective image pickup surface located in the vicinity of an image-formation plane.
When the lower limit of 6 to condition (1) is not reached, spherical aberrations remain under-corrected at the telephoto end. When the upper limit to 20 is exceeded, the amount of zooming movement of the movable groups increases, and so the overall size of the zoom lens system tends to increase.
More preferably, condition (1) should be reduced to
6.5
<f
1
/L<
16  (1′)
Most preferably, condition (1) should be reduced to
7
<f
1
/L<
12  (1″)
According to the second embodiment of the present invention, there is provided a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of the zoom lens system during zooming and having positive refracting power, a second lens group which moves toward an image side of the zoom lens system along the optical axis during zooming from a wide-angle end to a telephoto end of the zoom lens system and a rear lens group having at least two movable subgroups or a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of the zoom lens system during zooming and having positive refracting power, a second lens group which moves toward an image side of the zoom lens system along the optical axis during zooming from a wide-angle end to a telephoto end of the zoom lens system and a rear lens group having at least two spacings variable during zooming, wherein a focal length f
1
of the first lens group and anomalous dispersion &Dgr;&thgr;
gF
of a medium of at least one positive lens in the first lens group satisfy the following conditions:
6
<f
1
/L<
20  (1)
0.015<&Dgr;&thgr;
gF
<0.1  (2)
where L is a diagonal length of an effective image pickup surface located in the vicinity of an image-formation plane.
It is here noted that the anomalous dispersion &Dgr;&thgr;
gF
of each medium (vitreous material) is defined by
&thgr;
gF
=A
gF
+B
gF
·&ngr;
d
+&Dgr;&thgr;
gF
with the proviso that &thgr;
gF
=(n
g
−n
F
)/(n
F
−n
C
) and &ngr;
d
=(n
d
−1)/(n
F
−n
C
) wherein n
d
, n
F
, n
C
and n
g
are refractive indices with respect to d-line, F-line, C-line and g-line, respectively, and A
gF
and B
gF
are each a linear coefficient determined by two vitreous material types represented by glass code 511605 (available under the trade name of NSL7, Ohara Co., Ltd. with &thgr;
gF
=0.5436 and &ngr;
d
=60.49) and glass code 620363 (available under the trade name of PBM2, Ohara Co., Ltd. with &thgr;
gF
=0.5828 and &ngr;
d
=36.26);that is, A
gF
is 0.641462485 and B
gF
is −0.001617829.
When the lower limit of 0.015 to condition (2) is not reached, short wavelength longitudinal chromatic aberrations remain under-corrected at the telephoto end, and so colors are likely to bleed out at the edges of a subject having a large luminance difference. Any inexpensive medium exceeding the upper limit of 0.1 is little available, and opposite chromatic aberrations occur above 0.1.
More preferably, conditions (2) and (3) should be reduced to
6.5
<f
1
/L<
16  (1′)
0.020<&Dgr;&thgr;
gF
<0.08  (2′)
Most preferably, conditions (2) and (3) should be reduced to
7
<f
1
/L<
12  (1″)
0.025<&Dgr;&thgr;
gF
<0.06  (2″)
According to the third embodiment of the present invention, there is provided a zoom lens system comprising, in order from an object side thereof, a first lens group which is movable along an optical axis of the zoom lens system during zooming and has positive refracting power, a second lens group which moves toward an image side of the zoom lens system along the optical axis during zooming from a wide-angle end to a telephoto end of the zoom lens system, has negative refracting power and comprises at least three negative lens elements while a positive lens element is located nearest to an image side of the second lens group, or

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