Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
2002-02-13
2003-10-28
Robinson, Mark A. (Department: 2872)
Optical: systems and elements
Diffraction
From grating
C359S676000, C359S747000, C359S686000, C359S781000, C396S072000
Reexamination Certificate
active
06639721
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to zoom lens systems and optical apparatuses using the same, and more particularly a compact, high-performance and ultra wide-angle zoom lens system especially capable of satisfactorily correcting transverse chromatic aberration, and an optical apparatus using the same. The inventive zoom lens system and optical apparatus are suitable for still cameras, such as a single-lens reflex camera, electronic still cameras, and video cameras.
An interchangeable lens for use with a single-lens reflex camera requires a back focal distance long enough to secure a space to arrange a quick return mirror, etc. in a lens system at a side of an image surface. A digital camera using a CCD also requires a back focal distance long enough to secure a space to arrange a low-pass filter, an infrared light cutting filter, etc.
A negative lead type zoom lens system of negative refractive power has conventionally been known as a so-called ultra wide-angle lens which has a focal length shorter than 20 mm at an wide-angle end when converted for a 35 mm single-lens reflex camera.
For example, Japanese Laid-Open Patent Applications Nos. 60-87312, 2-201310, 5-241073, 4-235515, etc. propose zoom lens systems that have four lens units including, in order from an object side, a first unit of negative refractive power, a second unit of positive refractive power, a third unit of negative refractive power, and a fourth unit of positive refractive power, and that move at least two lens units among these lens units for zooming. The lens system has a retro-focus type as a zoom type, which may secure the above back focal distance with a simple lens unit structure. Such a simple lens structure type advantageously reduces the cost, but disadvantageously makes the miniaturization difficult since it has an increased stop diameter and large moving amount of each lens unit necessary for zooming in realizing high range zooming and large aperture arrangement.
A zoom lens system has been proposed in Japanese Lad-Open Patent Applications Nos. 11-174328 and 11-174329, which ameliorates these disadvantages and attempts to miniaturize the entire lens system as well as realizing high range zooming.
Each of these references arranges a zoom lens system including totally four lens units of, in order from an object side, negative refractive power, positive refractive power, negative refractive power, and positive refractive power, and achieves zooming by properly moving predetermined lens units.
The above conventional wide-angle zoom lens systems have had specific disadvantages in the way of satisfactorily correcting the distortion, curvature of field and transverse chromatic aberration to improve the good optical performance. They are generated because the lens system forms such an asymmetrical refractive power arrangement with respect to a stop as arranges the negative refractive power at the object side and the positive refractive power at the image side. In particular, the transverse chromatic aberration remains much between an intermediate image height and a maximum image height, causing color blurs when a bright, high intensity subject is shot. Use of anomalous dispersion glass is known as a method for correcting the transverse chromatic aberration, but the anomalous dispersion glass is expensive. In addition, it does not provide any remarkable effect on the miniaturization of the lens system.
In order to correct the chromatic aberration, in addition to a method using a combination of different dispersion glass materials, Japanese Laid-Open Patent Application Nos. 4-213421 and 6-324262 and U.S. Pat. No. 5,268,790 have propose an optical system that provides a lens surface or part of the optical system with a diffraction optical element having a diffraction effect to correct the chromatic aberration.
In general, enhanced refractive power of a lens unit or an increased moving amount of each zooming lens unit to realize a compact zoom lens system with high range zooming would increase aberration, in particular, chromatic aberration that fluctuates with zooming, whereby it becomes difficult to obtain good optical performance throughout the entire zoom range.
BRIEF SUMMARY OF THE INVENTION
Accordingly, it is an exemplary object of the present invention to provide a zoom lens system and optical apparatus using the same, which properly arranges a lens in each lens unit and a diffraction optical surface in the lens unit, thereby facilitating a high range zooming ratio, satisfactorily correcting the transverse chromatic aberration that fluctuates with zooming, and exhibiting good optical performance throughout the entire zoom range.
Another exemplary object of the present invention is to provide a compact zoom lens system having a focal length of about 28 mm when converted into a focal length for a 35 mm single-lens reflex camera, and an optical apparatus using the same, which have good optical performance, in particular in corrected transverse chromatic aberration.
A zoom lens system of one aspect of the present invention includes, in order from an object side, a first lens unit of negative refractive power, a second lens unit of positive refractive power, a third lens unit of negative refractive power, and a fourth lens unit of positive refractive power, the zoom lens system zooming from a wide-angle end to a telephoto end while varying a separation between respective lens units so that conditional expressions D
1
W>D
1
T, D
2
W<D
2
T, and D
3
W>D
3
T are satisfied where D
1
W is a separation between i-th and (i+1)-th lens units at a wide-angle end, DiT is a separation between the i-th and (i+1)-th lens units at a telephoto end, wherein a diffraction optical element is included in at least one of the lens units.
The diffraction optical element may be included in the fourth lens unit, and a conditional expression 15<f
4
DOE/ft<1500 may be satisfied where f
4
DOE is a focal length of only a diffraction component of the diffraction optical element, and ft is a focal length of an entire lens system at the telephoto end.
The diffraction optical element may be included in the first lens unit, and a conditional expression −1500<f
1
DOE/ft<−15 may be satisfied where f
1
DOE is a focal length of only a diffraction component of the diffraction optical element and ft is a focal length of an entire lens system at the telephoto end.
The diffraction optical element may be included in the first and fourth lens units, wherein conditional expressions 15<f
4
DOE/ft<1500 and −1500<f
1
DOE/ft<−15 may be satisfied where f
1
DOE is a focal length of only a diffraction component of the diffraction optical element included in the first lens unit, f
4
DOE is a focal length of only a diffraction component of the diffraction optical element included in the fourth lens unit, and ft is a focal length of an entire lens system at the telephoto end.
Conditional expressions 0.7<|f
1
|/{square root over ((fw·ft))}<1.1, 0.6<f
2
/{square root over ((fw·ft))}<1.4, 1.05<|f
3
|/f
2
<1.5, and 1.05<f
4
/f
2
<2.5 may be satisfied where fw and ft are focal lengths of an entire lens system at the wide-angle and telephoto ends, respectively, and fi is a focal length of the i-th lens unit.
Preferably, a conditional expression 1.6<SKw/fw<2.8 is satisfied where fw is a focal length of an entire lens system at the wide-angle end, and SKw is a back focal distance at the wide-angle end.
The first lens unit may have such an aspheric surface that negative refractive power of the aspheric surface decreases from a center of the lens to a peripheral of the lens.
The second and fourth lens units may move together during zooming.
The first lens unit may move while drawing a convex locus at a side of an image surface during zooming.
The above zoom lens system may further comprise a stop adjacent to the third lens unit at the object side or an image side.
The fourth lens unit may have such an aspheric surface that
Boutsikaris Leo
Morgan & Finnegan , LLP
Robinson Mark A.
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