Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2002-07-01
2004-09-14
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S684000
Reexamination Certificate
active
06791762
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a high zoom ratio lens, and more particularly, it relates to a compact and lightweight high zoom ratio lens that is approximately 75 degrees in shooting angle of view at a wide-angle end, approximately f/3 to f/4 in F-number at the wide-angle end, approximately f/6 to f/7 at a telephoto end, and approximately 10 in zoom ratio, and that is suitable for a single-lens reflex camera, a video camera, an electronic still camera, and the like.
BACKGROUND ART
In the prior art, there have been proposed a variety of zoom lenses such as high zoom ratio having 4, 5 and 6 lens groups. Such highly multi-lens groups are advantageous in view of correcting aberrations, but they tend to lead to adverse effects of cost increase due to an increased number of components including a cam barrel and of resultantly making the whole lens larger. Instead, reduced lens groups such as a 2 lens groups encounter difficulty in attaining both enhancing a zoom ratio and downsizing because of a significantly reduced number of parameters.
As to an improvement of a high zoom ratio lens of emphatically down-sized and weight-reduced design, a 4 lens group zoom lens is suitably implemented, with four of the lens groups having their respective refractive power of positive, negative, positive, and positive levels in order from the closest to an object, which is equivalent to those disclosed in Japanese Patent Laid-Open No. 2000-330016 and Japanese Patent Laid-Open No. 2001-117005, and is also equivalent to Zoom Lens 185D (28 to 300 in focal length and f/3.5 to f/6.3) available from Tamron Incorporated.
Although a trend of down-sizing the high zoom ratio lenses has been drastic and rapid in recent years, improvements in the prior art are still yet bulky and heavy for practical use, compared with ordinary standard zoom lenses of approximately &phgr;62 mm in diameter of a filter in front of the high zoom ratio lens and roughly 3.5 in zoom ratio. In addition to that, insufficiently reduced dimensions of the prior art high zoom ratio lenses cause not only imbalance with more greatly down-sized and weight-reduced bodies of single-lens reflex cameras but also aesthetically unsatisfied appearance and poor portability.
One of factors of the high zoom ratio lenses staying larger than the standard zoom lenses is that displacement of each lens group is increased to attain higher zoom ratio. Additionally, associated variation in aberration is also increased, which, in turn, causes a difficulty in correcting the aberration throughout the focal range. To overcome such a disadvantage, several approaches have been attempted, including ways of reducing a refractive power of each lens group to correct aberration, increasing the number of lenses in each lens group to correct aberration without reducing the refractive power of the lens group, configuring aspherical surface to correct aberration, and so forth.
However, the ways other than that of using an aspheric geometry unavoidably make the whole lens dimensions larger. As with the way of using the aspheric geometry, however, simply increasing the number of aspherical surfaces causes further problems of a performance degradation due to a poor surface precision and of an increase in cost for a metal mold. For instance, the previously mentioned Zoom Lens 185D is comprised of 15 pieces of lenses, creating 3 of the aspheric surfaces, and this model is 93.6 mm in full lens length at a wide-angle end and &phgr;72 mm in filter diameter. Thus, in contrast with the standard zoom lenses (focal length 28 to 105, zoom ratio 3.75, f/3.5 to f/4.5), this type of zoom lens is 18 mm or more longer in the entire length of optics and 10 mm or more larger in filter effective diameter.
The zoom lenses as disclosed in Japanese Patent Laid-Open Nos. 2000-330016 and 2001-117005, when compared with the model 185D, are almost the same in effective diameter of the filter despite having some variation in the entire lens length of the optics at a wide-angle end. Thus, similar to the model 185, these lenses disclosed in Japanese Patent Laid-Open Nos. 2000-330016 and 2001-117005 are longer in the entire length of their respective optics and 10 mm or more larger in effective diameter of their filters, in comparison with the standard zoom lenses.
The present invention is made, allowing for the above mentioned disadvantages in the prior art high zoom ratio lenses, and accordingly, it is an object of the present invention to provide a compact and lightweight high zoom ratio lens which is configured in 4 lens group zoom format with a deployment of positive, negative, positive, and positive refractive power in order, being approximately 75 degree in shooting angle of view at the wide-angle end, approximately f/3 to f/4 at the wide-angle end, and approximately f/6 to f/7 at the telephoto end so as to implement an enhanced variable zoom power up to approximately 10, which is yet as large as the above-mentioned ordinary standard zoom lenses (&phgr;62 mm in diameters of their filters proximal to an object).
SUMMARY OF THE INVENTION
A zoom lens according to the present invention includes first to fourth lens groups each comprised of a plurality of lenses, the lens groups having respective refractive powers of positive, negative, positive, and positive levels in order from the closest to an object. In zooming from the wide-angle end to a telephoto end, the first and second lens groups have a an air-filled space widened therebetween, the second and third lens groups have an air-filled space narrowed therebetween, and the third and fourth lense groups have an air-filled space narrowed therebetween while the first, third, and fourth lens groups move toward the object, and in focusing, the second lens group alone moves, under the conditions described as follows:
0.05<|f2|/(fT−fW)<0.07 (1)
0.3<f1/(fT−fW)<0.5 (2)
0.2<f4/(fT−fW)<0.3 (3)
0.37<Z2/Z<0.47 (4)
where fW is a focal length of the entire optics at a wide-angle end, fT is a focal length of the entire optics at a telephoto end, f2 is a focal length of the second lens group, f1 is a focal length of the first lens group, f4 is a focal length of the fourth lens group, Z2 is equal to B2T/B2W, where B2W is an imaging magnification of the second lens group at the wide-angle end with B2W<0), B2T is an imaging magnification of the second lens group at the telephoto end with B2T<0, and Z is equal to fT/fW.
Another aspect of the present invention, as defined in claim
1
, is characterized by additional conditions as in the following (A) to (D):
(A) In zooming or focusing, a condition is satisfied as follows:
2.7<(
TLW−
0.5×
FLT
/tan &agr;
W
)/
fW<
3.3 (5)
where TLW is the full length of the entire optics (from a vertex of a foremost lens to an image plane) at the wide-angle end, FLT is an aperture of a filter located proximal to an object, and a W is a half angle of view.
(B) In zooming or focusing, a condition is satisfied as follows:
−0.1<1/&bgr;3
w<
0<1/&bgr;3
T<
0.3 (6)
where &bgr;3w is an imaging magnification of the third lens at the wide-angle end, and &bgr;3T is an imaging magnification of the third lens group at the telephoto end.
(C) In zooming or focusing, a condition is satisfied as follows:
DWENP<
28 (7)
where DWENP is a distance from a vertex of a surface of the first lens group closest to an object to the center of an entrance pupil.
(D) In zooming or focusing, conditions are satisfied as follows:
0.15<|&bgr;2
W|<
0.3 (8)
e
0<5 (9)
h
1+
e
0×tan &agr;
W+fW
/(2×
FW
)<25 (10)
where e0 is a distance from a vertex of a foremost surface of the first lens group closest to an object to a frontal principal point of the first lens group, and h1 is a height at which extensions of chief rays incident at a half angle of view upon the wide-angle end transversely pass
Jacobson & Holman PLLC
Schwartz Jordan M.
Tamron Co., Ltd.
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