Image forming optical system having diffractive optical element

Details Image forming optical system having diffractive optical element Image forming optical system having diffractive optical element

1999-04-12

2001-07-31

Schwartz, Jordan M. (Department: 2873)

Optical: systems and elements

Lens

Telephoto

C359S755000, C359S757000

Reexamination Certificate

active

06268969

ABSTRACT:

BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to an image forming optical system having a diffractive optical element and, more particularly, to a photographic optical system of the telephoto type having a large relative aperture with a refractive optical element and a diffractive optical element used in combination to improve imaging performance, suited to silver-halide photographic cameras, video cameras, electronic still cameras, etc.
Description of Related Art
In general, in long-focal-length or telephoto lenses, as the focal length becomes longer, various aberrations, especially, longitudinal chromatic aberration and lateral chromatic aberration, tend to worsen. To correct these chromatic aberrations, there has been a previous proposal for combining a positive lens made of a low-dispersion material, which has an extraordinary partial dispersion, such as fluorite, with a negative lens made of a high dispersion material, so as to obtain achromatism. Such a method has found its use in many telephoto lenses.
Optical glasses of extraordinary partial dispersion such as fluorite are advantageous for correcting chromatic aberrations, but have a disadvantage that they are very expensive. In specific gravity, too, they are relatively greater than the other low-dispersion glasses whose partial dispersions are not extraordinary. Therefore, the use of fluorite leads also to a drawback that the whole lens system becomes heavier. (For example, fluorite is 3.18 and FK01 is 3.63 in specific gravity. In contrast, FK5 that has a small extraordinary partial dispersion is 2.46 and BK7 is 2.52 in specific gravity.) Moreover, the extraordinary partial dispersion glass surface is relatively susceptible to scratches. Further, FK01 or the like, when made in the form of large relative aperture lenses, gives rise to another drawback that these lenses are liable to crack when the ambient temperature changes rapidly.
Telephoto lenses in which chromatic aberration is corrected only by using a glass having no extraordinary partial dispersion have been proposed in Japanese Laid-Open Patent Applications No. Hei 6-324262 and No. Hei 6-331887.
The telephoto lens disclosed in the above Japanese Laid-Open Patent Application No. Hei 6-324262 is composed of 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, has an F-number of F2.8 or thereabout and is corrected relatively well for chromatic aberration. Also, the telephoto lens disclosed in the above Japanese Laid-Open Patent Application No. Hei 6-331887 is composed of a combination of a diffractive optical element and refractive optical elements, has an F-number of F2.8 or thereabout and is corrected relatively well for chromatic aberration.
In telephoto lenses adapted to be mounted on cameras having the automatic focusing function, which are extensively being developed in recent years, it has become general that focusing is effected by moving along an optical axis a lens unit disposed in a relatively rear portion, which is light in weight and provides little load on the lens barrel.
In a case where, in the photographic optical system disclosed in the above Japanese Laid-Open Patent Application No. Hei 6-324262, focusing is made to be effected by moving a rear lens unit, various aberrations including chromatic aberration become worse. Therefore, such a photographic optical system is difficult to use as a telephoto lens adapted for cameras having the automatic focusing function.
Further, in the optical system of the telephoto lens disclosed in the above Japanese Laid-Open Patent Application No. Hei 6-331887, as shown in
FIG. 10
or
FIG. 12
thereof, there is provided, as a lens unit that is relatively light in weight and has relatively little variation of monochromatic aberration during focusing, a lens unit disposed slightly in the rear (image side) of the middle point of the optical system and that consists of a cemented lens composed of a positive lens and a negative lens and having a negative refractive power as a whole. However, such a telephoto lens is not necessarily sufficient for the distribution of refractive power between the diffraction surface (diffractive optical element) and the refractive optical element and for setting of glass materials. Therefore, if the above negative lens unit is used for focusing, the variation of chromatic aberration becomes large.
BRIEF SUMMARY OF THE INVENTION
An object of the invention is to provide an image forming optical system, such as a telephoto lens, which is capable of making chromatic aberrations sufficiently small.
Another object of the invention is to provide an image forming optical system, such as a telephoto lens, whose chromatic aberrations vary to a small extent with focusing.
To attain the above objects, in accordance with an aspect of the invention, there is provided an optical system that comprises, in order from an object side to an image side, a first lens unit having a positive refractive power and a second lens unit having a negative refractive power, the second lens unit being moved during focusing, wherein the first lens unit has a diffractive optical element of positive refractive power and a first lens subunit of positive refractive power, and the first lens subunit comprsises at least one positive lens and at least one negative lens, the optical system satisfying the following conditions:
0.005<&phgr;
D
/&phgr;
1a
<0.0 5  (1)
50<&ngr;
1ap
<75  (2)
25<&ngr;
1an
<60  (3)
where
&phgr;
D
: the refractive power of the diffractive optical element of positive refractive power in the first lens unit,
&phgr;
1a
: the refractive power of the first lens subunit,
&ngr;
1ap
: the Abbe number of a material of the positive lens, included in the first lens subunit when there is only one positive lens in the first lens subunit or the mean value of Abbe numbers of materials of positive lenses, included in the first lens subunit included in the first lens subunit when there is more than one positive lens in the first lens subunit,
and
&ngr;
1an
: the Abbe number of a material of the negative lens included in the first lens subunit when there is only one negative lens in the firs lens subunit, or the mean value of Abbe numbers of materials of negative lenses, included in the first lens subunit when there is more than one negative lens in the first lens subunit.
In accordance with another aspect of the invention, there is provided an optical system that comprises, in order from an object side to an image side, a first lens unit having a positive refractive power and a second lens unit having a negative refractive power, the second lens unit being moved toward the image side along an optical axis during focusing from an infinitely distant object to a closest object, wherein the first lens unit has a diffractive optical element of positive refractive power composed of a diffraction grating of revolution symmetry with respect to the optical axis and a first lens subunit of positive refractive power, and the first lens subunit consists of at least one positive lens and at least one negative lens, the optical system satisfying the following conditions:
0.005<&phgr;
D
/&phgr;
1a
<0.05
50<&ngr;
1ap
<75
25<&ngr;
1an
<60
where
&phgr;
D
: the refractive power of the diffractive optical element of positive refractive power in the first lens unit,
&phgr;
1a
: the refractive power of the first lens subunit,
&ngr;
1ap
: then Abbe number of a material of the positive lens, or the mean value of Abbe numbers of materials of positive lenses, included in the first lens subunit included in the first lens subunit when there is more than one positive lens in the first lens subunit,
and
&ngr;
1an
: the Abbe number of a material of the negative lens included in the first lens subunit when there is only one negative lens in the first lens subunit, or the me

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