Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2001-11-15
2002-12-17
Lester, Evelyn A (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S684000, C359S654000, C359S677000, C359S740000, C359S569000, C359S566000, C359S742000
Reexamination Certificate
active
06496310
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical system and an optical apparatus using the same, which are suitable for an optical apparatus such as a silver halide photographic camera, a video camera, an electronic still camera, a digital camera and a camera for reading an image (optical system for a copying machine), which satisfactorily maintain an imaging performance by combining a refractive optical element and a diffractive optical element.
In particular, the present invention is suited as a retrofocusing type image pick-up optical system with a long back focus, which satisfactorily maintains an imaging performance by combining a reflective optical element and a diffractive optical element.
2. Related Background Art
A solid image pick-up element used in an image pick-up apparatus in recent years has made progress in miniaturization with high definition by the development of manufacturing technology year by year. In the current state, a size of one pixel has been reduced to the order of several &mgr;m. Thus, an image pick-up lens of a higher performance is required for an optical system used in the image pick-up apparatus and, in particular, it is desired to correct chromatic aberration of magnification to be extremely small in a wave band of a visible range. There is a method of using anomalous dispersion glass as a method of satisfactorily correcting chromatic aberration of magnification including the g line. Such an image pick-up lens is proposed, for example, in Japanese Patent Application Laid-open Nos. 5-134174, 7-181376 and 7-248447. In the embodiments disclosed in these laid-open patent applications, chromatic aberration of magnification is corrected in a wide wave length range including the g line using two to three pieces of anomalous dispersion glass. However, it is very difficult to process such anomalous dispersion glass.
In addition, recently, a diffractive optical element (DOE) utilizing a diffraction phenomenon attracts attention as a chromatic aberration correction element. An optical property of the DOE is different from that of a conventional refraction lens and has reverse dispersibility and anomalous dispersibility characteristics of an Abbe number &ngr;=−3.45 and a partial dispersion ratio &thgr;gf=0.296. This is described in, for example, the thesis “Hybrid diffractive-refractive lenses and achromats”, Thomas Stone and Nicholas George, Applied Optics, 27, 14, 2960-2971 (1988. 7. 15). It is well known that chromatic aberration correction is possible with only one member by providing a DOE of a positive weak optical power in a refraction lens of a positive optical power utilizing these characteristics. In recent years, embodiments of various image pick-up lenses have been suggested utilizing the characteristics of the DOE. For example, the embodiment of U.S. Pat. No. 5,148,314 to Chungte W. Chen describes an eyepiece, a Petzval type lens, a large aperture lens, a telephoto lens, and the like, and discloses that chromatic aberration and off-axis aberration can be corrected by using a DOE in an optical system.
In addition, Japanese Patent Application Laid-open Nos. 6-331898 (corresponding to U.S. Pat. No. 5,631,779), 6-331887 (corresponding to U.S. Pat. No. 5,629,799), 6-347700 corresponding to U.S. Pat. No. 5,631,779) and 6-324262 (corresponding to U.S. Pat. No. 5,790,321), Japanese Patent Translation Publication No. 8-508116 (corresponding to U.S. Pat. No. 5,543,966), and the like disclose the embodiments in which a diffractive optical element is applied to an objective lens, a projection lens, a standard lens for photography and the like. Moreover, as examples in which a diffractive optical element is applied to a wide-angle lens, there are enumerated Japanese Patent Application Laid-open Nos. 10-11577 (corresponding to U.S. Pat. No. 5,982,942), 11-64728 (corresponding to U.S. Pat. No. 6,010,537) and 2000-66092.
Since image pick-up lenses described in Japanese Patent Application Laid-open Nos. 10-11577 and 11-64728 correct on-axis chromatic aberration and chromatic aberration of magnification, optical power of a diffractive optical element tends to be strong on an axis. When optical power of a diffractive optical element becomes strong on an axis, there arise the following problems.
a. Since a grating pitch of a concentric grating forming the diffractive optical element narrows and the number of orbicular zones increases, it becomes difficult to make the diffractive optical element.
b. Since an aspect ratio (height/pitch of a grating) of an element becomes larger, a percentage of diffraction light (additional diffraction light, also known as unnecessary diffraction light) of orders other than a design order increases to make larger flare.
c. Since a diffraction angle becomes larger, a spot by additional diffraction light on an image forming surface becomes large.
d. Flare caused by additional diffraction light is conspicuous around a light source when a high luminance light source enters a screen.
These problems will be hereinafter described more specifically.
FIGS. 12A and 12B
show a conventional optical system using a diffractive optical element that utilizes diffraction light of a positive first order.
FIG. 12A
shows light beam of a positive first order (first order diffraction light) and
FIG. 12B
shows light beam of a positive second order. As shown in the figures, the light beam of the positive second order (second order diffraction light) defocuses greatly.
FIG. 13
shows a spot on a surface of an image obtained by the conventional optical system using the diffractive optical element that utilizes diffraction light of a positive first order. A unit of a numerical value of the optical system which is used here is mm. As shown in the figure, since the light beam defocuses greatly, it forms a spot of approximately 6 mm on the surface of the image. Although not shown in the figure, light beam of a zero-th order defocuses in the opposite direction of the positive second order and forms a spot of the same size.
Originally, although a diffraction efficiency is desirably 100% in an objective order, the 100% diffraction efficiency cannot be realized due to limitations of design and tolerance. Even if a laminated DOE is used, a maximum value of the diffraction efficiency is in the order of 98% and the remaining 1 to 2% of light becomes additional diffraction light.
For example, if a person
2
is photographed against light as a subject with the sun
1
in the background as shown in
FIG. 14
using such an optical system, flare
3
due to additional diffraction light appears around the silhouette of the person
2
. In addition, flare due to the additional diffraction light is also conspicuous around a high luminance light source.
SUMMARY OF THE INVENTION
The present invention has been devised in view of the above drawbacks, and it is an object of the present invention to provide an optical system in which chromatic aberration of magnification of a high order is satisfactorily corrected and flare caused by additional diffraction light (unnecessary diffraction light) is suppressed to as minimum as possible while maintaining a high imaging performance up to peripheral part of a field angle.
In order to attain the above-mentioned object, an optical system of an embodiment to be described later is characterized in that it is provided with a diffractive optical element which has a configuration such that a phase given to incident light from an optical axis toward the peripheral part in the radial direction is not reversed in an effective area and an absolute value of optical power on the optical axis is made minimum.
If this diffractive optical element is disposed ahead of an aperture stop of the optical system (at the object side in an image-taking system), the diffractive optical element is preferably configured such that optical power decreases from the optical axis toward the peripheral part in the radial direction in the effective area.
Conversely, if the diffractive optical element
Canon Kabushiki Kaisha
Lester Evelyn A
Morgan & Finnegan , LLP
LandOfFree
Optical system and optical apparatus provided with the same does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical system and optical apparatus provided with the same, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical system and optical apparatus provided with the same will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2994410