Photography – With viewfinder – Selective or adjustable viewfinder field
Reexamination Certificate
2001-03-15
2004-03-23
Adams, Russell (Department: 2851)
Photography
With viewfinder
Selective or adjustable viewfinder field
C396S382000, C396S384000, C359S676000, C359S689000, C359S784000
Reexamination Certificate
active
06711356
ABSTRACT:
This application is based on Japanese Patent Application No. 2000-83247, filed on Mar. 21, 2000, the contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a real-image variable-magnification viewfinder, and more particularly to a real-image variable-magnification viewfinder that offers a high zoom ratio and is suitable for use in a digital camera.
2. Description of Prior Art
As image-sensing devices such as CCDs (charge-coupled devices) for use in digital cameras become smaller and smaller, there has been an increasingly greater demand for compact high-zoom-ratio real-image viewfinders. However, most conventional real-image variable-magnification viewfinders offer zoom ratios of 3× or lower, and those offering zoom ratios over 3× are large. For example, the real-image variable-magnification viewfinder proposed in Japanese Patent Application Laid-Open No. H6-102453 has an objective optical system composed of, from the object side, a positive lens unit, a negative lens unit, a positive lens unit, and a fourth lens unit including at least one negative lens element. However, this viewfinder offers a zoom ratio of only about 2× and lower. The real-image variable-magnification viewfinder proposed in U.S. Pat. No. 5,225,927 has an objective optical system composed of, from the object side, a positive lens unit, a negative lens unit, a positive lens unit, and a fourth lens unit that has a weak optical power. However, this viewfinder, too, offers a zoom ratio of up to about 2.5× to 3.5×, and is, in addition, large.
The real-image variable-magnification viewfinder proposed in U.S. Pat. No. 5,694,244 has an objective optical system composed of, from the object side, a positive lens unit, a negative lens unit, and a positive lens unit. This viewfinder offers a zoom ratio over 3×, but is large. The real-image variable-magnification viewfinder proposed in U.S. Pat. No. 6,035,145 has an objective optical system composed of four lens units arranged in the following order, from the object side, a positive lens unit, a negative lens unit, a positive lens unit, and a negative lens unit. This viewfinder achieves zooming from the wide-angle end to the telephoto end by moving the second lens unit toward the image-plane side and moving the third lens unit monotonically toward the object side. This viewfinder offers a zoom ratio of up to about 4×, but is rather large.
As proposed in Japanese Patent Application Laid-Open No. H6-102453 and U.S. Pat. No. 5,225,927 mentioned above, in conventional real-image variable-magnification viewfinders having an objective optical system starting with a positive lens unit, a negative lens unit, and a positive lens unit, zoom solutions are commonly designed so that the second lens unit is almost solely responsible for zooming. Such zoom solutions, however, inevitably require a long movement distance in the second lens unit and a high power in the third lens unit. As a result, large aberration is caused by the third lens unit and large variations occur in the aberration caused by the second lens unit as zooming is performed. This makes miniaturization of such viewfinders impossible. On the other hand, giving the second and third lens units approximately equal shares of the overall zoom ratio as proposed in U.S. Pat. Nos. 5,694,244 and 6,035,145, mentioned above, helps miniaturize such viewfinders, but only to an insufficient degree.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a real-image variable-magnification viewfinder that, despite having a high zoom ratio of over 3×, is compact and offers satisfactory optical performance.
To achieve the above object according to one aspect of the present invention, a real-image variable-magnification viewfinder is provided with: an objective optical system having a positive optical power for forming a real image in an optical path; an eyepiece optical system having a positive optical power for transmitting the real image formed by the objective optical system to a pupil of an observer; and an erecting optical system disposed in the optical path for inverting the real image formed by the objective optical system. The objective optical system is provided with three lens units arranged in the following order, from the object side; a first lens unit having a positive optical power; a second lens unit having a negative optical power; and a third lens unit having a positive optical power. In this viewfinder, as zooming is performed from the wide-angle end to the telephoto end, at least one of the second and third lens units are moved in such a way that the second and third lens units come closer to each other. In addition, the following conditional formulae are fulfilled:
−0.75<m
2W
<−0.3
−2<m
2T
<1.05
−0.75<m
3W
<−0.3
−
2
<m
3T
<−1.05
L
2
>L
3
where
m
2W
represents the lateral magnification of the second lens unit at the wide-angle end;
m
2T
represents the lateral magnification of the second lens unit at the telephoto end;
m
3W
represents the lateral magnification of the third lens unit at the wide-angle end;
m
3T
represents the lateral magnification of the third lens unit at the telephoto end;
L
2
represents the movement distance of the second lens unit over the entire zoom range; and
L
3
represents the movement distance of the third lens unit over the entire zoom range.
According to another aspect of the present invention, a real-image variable-magnification viewfinder is provided with an objective optical system having a positive optical power for forming a real image in an optical path; an eyepiece optical system having a positive optical power for transmitting the real image formed by the objective optical system to a pupil of an observer; and an erecting optical system disposed in the optical path for inverting the real image formed by the objective optical system. The objective optical system is provided with four lens units arranged in the following order, from the object side; a first lens unit having a positive optical power; a second lens unit having a negative optical power; a third lens unit having a positive optical power; and a fourth lens unit. In this viewfinder, as zooming is performed from the wide-angle end to the telephoto end, at least one of the second and third lens units are moved in such a way that the second and third lens units come closer to each other. In addition, the following conditional formulae are fulfilled:
−0.75<m
2W
<−0.3
−2<m
2T
<−1.05
−0.75<m
3W
<−0.3
−2<m
3T
<−1.05
L
2
>L
3
−0.1<PW
4
<0.04
where
m
2W
represents the lateral magnification of the second lens unit at the wide-angle end;
m
2T
represents the lateral magnification of the second lens unit at the telephoto end;
m
3W
represents the lateral magnification of the third lens unit at the wide-angle end;
m
3T
represents the lateral magnification of the third lens unit at the telephoto end;
L
2
represents the movement distance of the second lens unit over the entire zoom range;
L
3
represents the movement distance of the third lens unit over the entire zoom range; and
PW
4
represents the optical power (mm
−1
) of the fourth lens unit.
According to another aspect of the present invention, a real-image variable-magnification viewfinder is provided with: an objective optical system having a positive optical power for forming a real image in an optical path; an eyepiece optical system having a positive optical power for transmitting the real image formed by the objective optical system to a pupil of an observer; and an erecting optical system disposed in the optical path for inverting the real image formed by the objective optical system. The objective optical system is provided with four lens units arranged in the following order, from the object side: a first lens unit having a positive optical power; a seco
Minolta Co. , Ltd.
Nguyen Michelle
Sidley Austin Brown & Wood LLP
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