Optical: systems and elements – Lens – Telephoto
Reexamination Certificate
1998-09-29
2001-11-06
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Lens
Telephoto
C359S746000, C359S558000
Reexamination Certificate
active
06313958
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to photographic optical systems suited to silver-halide photographic cameras, video cameras, electronic still cameras or the like and, more particularly, to a diffractive-refractive photographic optical system of large relative aperture having a combination of a refractive optical system and a diffractive optical system and corrected well for the imaging performance of the entire optical system.
2. Description of Related Art
Telephoto lenses have generally a tendency to increase their longitudinal and lateral chromatic aberration as the focal length increases. To correct these chromatic aberrations, it has been the common practice to use a low-dispersion positive lens of fluorite or like material having an extraordinary partial dispersion in combination with a high-dispersion negative lens of another glass material, thereby obtaining achromatism.
However, the extraordinary partial-dispersion glass such as fluorite, though being advantageous for correcting chromatic aberrations, has a drawback that it is very expensive. The specific gravity of the extraordinary partial-dispersion glass is greater than that of the other low-dispersion glasses having no extraordinary partial dispersion. Therefore, the extraordinary partial dispersion glass has another drawback that the whole lens system becomes heavier. For example, fluorite has a specific gravity of 3.18, and FKO
1
has a specific gravity of 3.63. On the other hand, FK
5
, which has small extraordinary partial dispersion, has a specific gravity of 2.46, and BK
7
, which also has small extraordinary partial dispersion, has a specific gravity of 2.52.
Furthermore, the surface of the extraordinary partial-dispersion glass is relatively susceptible to scratches. Some large-relative-aperture lenses to which the extraordinary partial-dispersion glass is applied are liable to crack when the temperature changes rapidly. Also, in a case where the extraordinary partial-dispersion glass is used in a lens (positive lens) disposed closest to the object side, in order to prevent this lens from being damaged by scratches or cracks, there is a need usually to use a protection glass in the form of a parallel flat plate. So, yet another drawback is produced that the entirety of the lens system increases in weight and cost as much as this protection glass.
With the glasses having no extraordinary partial dispersion left in use, the telephoto lens is corrected for chromatic aberrations by making some other provisions, as in Japanese Laid-Open Patent Application No. Hei 6-324262. In this document, there is disclosed a telephoto lens that includes at least one diffractive optical element having a positive refractive power, at least one refractive optical element having a positive refractive power and at least one refractive optical element having a negative refractive power. That telephoto lens has an F-number of about 2.8 and is relatively corrected well for chromatic aberrations.
However, in that telephoto lens, the diffractive optical element is positioned in a front section of the optical system where both the paraxial on-axial ray and the pupil paraxial ray enter at respective heights (from the optical axis) both of which are relatively large. So, the diffractive optical element gets a large diameter and the production cost thereof becomes high.
For example, to manufacture diffraction gratings at a relatively excellent mass productivity, there is a method of stamping glass by a metallic mold or like means, while melting at a high temperature. Another method is to apply a layer of ultraviolet-ray setting resin to the surface of a glass substrate. A pattern is then formed by a stamping press and then the layer is exposed to ultraviolet rays to harden. Another method is to mold plastic resin itself by a die. In any case, as the diameter of the diffractive optical element increases, the pattern transferability and the die releaseability deteriorate, so that the desired performance or sufficient diffraction efficiency cannot be obtained.
Also, the method of directly cutting glass to form the diffraction grating and the method of wet or dry etching a flat substrate of SiO
2
or others to form the grating grooves in stepwise patterns are usable. However, as the diameter of the diffractive optical element increases, the mass productivity becomes poor and the production cost increases.
BRIEF SUMMARY OF THE INVENTION
It is an object of the invention to provide an optical system which can make the diffractive optical element smaller in size than heretofore possible.
In accordance with a first aspect of the invention, there is provided an optical system, which comprises positive and negative refractive optical elements, and positive and negative diffractive optical elements for diffracting light from the positive and negative refractive optical elements.
In accordance with a second aspect of the invention, there is provided an optical system, which comprises, positive and negative refractive optical elements, and positive and negative diffractive optical elements for diffracting light from the positive and negative refractive optical elements, wherein the positive diffractive optical element is located between the positive and negative refractive optical elements and the negative diffractive optical element.
In accordance with a third aspect of the invention, there is provided an image forming optical system including a plurality of optical units, which comprises a positive optical unit disposed closest to an object side, the positive optical unit having at least one positive refractive optical element and at least one negative refractive optical element, a positive diffractive optical element disposed closer to an image side than the positive optical unit, and a negative diffractive optical element disposed closer to the image side than the positive diffractive optical element. In the image forming optical system, a diffraction grating of each of the positive and negative diffractive optical elements has a form of revolution symmetry with respect to an optical axis. Further, the image forming optical system comprises a negative optical unit having a negative refractive optical element and disposed between the positive optical unit and the positive diffractive optical element. Then, the negative refractive optical element in the negative optical unit is one or plural in number, and focusing is effected by moving the negative optical unit along the optical axis. Further, the image forming optical system satisfies the following conditions:
|h
B
/h
A
<1
|H
A
/H
B
<0
−1
<H
B
/H
1
<0
where
h
A
is a height of a paraxial on-axial ray incident on the positive diffractive optical element,
h
B
is a height of a paraxial on-axial ray incident on the negative diffractive optical element,
H
A
is a height of a pupil paraxial ray incident on the positive diffractive optical element,
H
B
is a height of a pupil paraxial ray incident on the negative diffractive optical element, and
H
1
is a height from the optical axis of a pupil paraxial ray incident on a surface on the object side of a lens disposed closest to the object side included in the positive optical unit.
More preferably, the image forming optical system satisfies the following conditions:
|h
B
/h
A
|<0.95
|H
A
/H
B
|<0.95
−0.95
<H
B
/H
l
<−0.01.
In the optical system according to each of the first to third aspects, there is an aspect in which the positive and negative diffractive optical elements are formed respectively on a surface on the object side and a surface on the image side of a single element.
Further, in the optical system according to each of the first to third aspects, it is preferred to satisfy the following conditions:
0.05<&phgr;
A
/&phgr;<2
−2 <&phgr;
B
/&phgr;<0.05
where
&phgr; is a refractive power of the entire optical system,
&phgr;
A
is a refractive power for a first-order diffracted ray of the positive
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Sugarman Scott J.
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