Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
1999-07-07
2001-05-22
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S684000, C359S689000
Reexamination Certificate
active
06236516
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a zoom lens and an optical apparatus having the same and, more particularly, to a zoom lens of a high magnification range suited to be used in single-lens reflex cameras or video cameras.
2. Description of Related Art
For zoom lenses, there have been known a focusing method of moving the first lens unit, or the so-called “front focus” method, and another focusing method of moving the second or later lens unit, or the so-called “inner focus” or “rear focus” method.
In general, the zoom lenses of the inner or rear focus type have their first lens units made smaller in diameter to admit of the effective light beam, than when the front focus type is in use. So, there is an advantage of assuring improvements of the compact form of the entire lens system. Moreover, the focusing lens unit is relatively small in size and light in weight. So, particularly for the auto-focus cameras that have recently become the main stream of development, fast focusing becomes possible to perform. Even this constitutes a characteristic feature of the inner or rear focus type zoom lens.
As one of the zoom lenses of such inner or rear focus type, mention may be made of the zoom lens that comprises, in order from an object side, a positive first lens unit, a negative second lens unit and succeeding lens units whose overall power is positive. All the separations between the adjacent lens units are made to vary to effect zooming. In this so-called “positive lead” type of zoom lens, the second lens unit of negative refractive power is used for focusing purposes, as disclosed in, for example, Japanese Laid-Open Patent Applications No. Hei 3-228008 (corresponding to U.S. Pat. No. 5,144,488), No. Hei 5-119260 (corresponding to U.S. Pat. No. 5,528,427), and No. Hei 6-230285.
This method, particularly when applied to the high range zoom lens including the standard region, not only brings the above-described features into full play, but also maintains good stability of optical performance throughout the entire focusing range from an infinitely distant object to a closest object.
Here, for the positive lead type of zoom lens, an explanation is made about the variation of magnification of the negative second lens unit during zooming of the entire zoom lens.
In general, the negative second lens unit has a negative fractional magnification at the wide-angle end and, as zooming advances to the telephoto end, the absolute value of the magnification increases. Moreover, since the second lens unit is the main variator of the positive lead type zoom lens, the magnification of the second lens unit increases by a large amount (or changes in such a direction as to approach “−1” from the negative fractional magnification) during zooming of the entire zoom lens from the wide-angle end to the telephoto end. Particularly in the case of the high range zoom lens, such an increase becomes conspicuous.
Now, the relationship between the magnification of the focusing lens unit and the focusing sensitivity (the ratio of the amount of shift of the focal plane to the amount of movement of the focusing lens unit) can be expressed by the following formula:
ES=(1−&bgr;f
2
)×&bgr;r
2
where
ES: the focusing sensitivity,
&bgr;f: the magnification of the focusing lens unit, and
&bgr;r: the combined magnification of all the lens units disposed on the image side of the focusing lens unit.
It is understandable from the formula described above that, when the absolute value of magnification of the focusing lens unit is “1”, the focusing sensitivity takes a value of “0”, and as the absolute value of magnification of the focusing lens unit departs from “1”, the focusing sensitivity becomes greater.
In the positive lead type of zoom lens, however, as described before, the magnification of the negative second lens unit varies in a direction from the negative fractional magnification to “−1” during zooming of the entire zoom lens from the wide-angle end to the telephoto end. For this reason, when shooting a close object with a zoom lens of high range, the focusing sensitivity of the negative second lens unit becomes small at or near the telephoto end, so that the total focusing movement is remarkably increased. Further, in a case where the magnification of the negative second lens unit takes a value of “−1” in a middle way of zooming, the focusing sensitivity becomes “0”, so that it happens that the focusing becomes impossible.
BRIEF SUMMARY OF THE INVENTION
The present invention has been made to solve the problems described above. It is, therefore, an object of the invention to provide a zoom lens of compact form arranged such that even if the zoom lens has a high range, the required amount for a certain focusing range of movement of the focusing lens unit is prevented from extremely increasing.
To attain the above object, in accordance with an aspect of the invention, there is provided a zoom lens comprising, in order from a longer conjugate side to a shorter conjugate side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, and a succeeding lens group that is composed of a plurality of lens units or one lens unit and whose overall refractive power is positive, wherein, during zooming from a wide-angle end to a telephoto end, the separation between the first lens unit and the second lens unit increases and the separation between the second lens unit and the succeeding lens group decreases, and wherein the second lens unit includes a front lens subunit of negative refractive power and a rear lens subunit of negative refractive power disposed on the shorter conjugate side of the front lens subunit, focusing being performed by moving the rear lens subunit, and the following condition being satisfied:
0.3<|f2a|/{square root over ((fw×ft))}<0.9
where fw and ft are focal lengths at the wide-angle end and the telephoto end of the zoom lens, respectively, and f2a is a focal length of the front lens subunit.
Another object of the invention is to provide an optical apparatus having the zoom lens of the invention described above.
The above and further objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.
REFERENCES:
patent: 5144488 (1992-09-01), Endo et al.
patent: 5528427 (1996-06-01), Tanaka et al.
patent: 5581404 (1996-12-01), Misaka et al.
patent: 5627682 (1997-05-01), Nagaoka
patent: 5691851 (1997-11-01), Nishio et al.
patent: 5815320 (1998-09-01), Hoshi et al.
patent: 5831772 (1998-11-01), Nishio et al.
patent: 5859729 (1999-01-01), Misaka
patent: 3-228008 (1991-10-01), None
patent: 5-119260 (1993-05-01), None
patent: 6-230285 (1994-08-01), None
Canon Kabushiki Kaisha
Epps Georgia
Fitzpatrick ,Cella, Harper & Scinto
Thompson Timothy
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