Optical: systems and elements – Lens – With reflecting element
Utility Patent
1999-05-04
2001-01-02
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Lens
With reflecting element
C359S731000, C359S732000, C359S799000
Utility Patent
active
06169637
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a catadioptric lens, and more particularly relates to a catadioptric lens suited for use as an imaging lens for a camera having a compact image pickup device.
BACKGROUND OF THE INVENTION
Catadioptric lenses have frequently been used in the past as long focal length lenses, particularly as super-telephoto lenses, for 35 mm format cameras. This is because long focal length dioptric lenses tend to be long in size. By adopting a catadioptric lens design, the overall lens length can be decreased while still providing a long focal length and controlling chromatic aberration (particularly secondary spectrum).
A conventional catadioptric photographic lens for 35 mm format film comprises, in order from the incoming direction of the light, an incident surface, a main mirror and a secondary mirror arranged in a manner that makes the lens compact. A convex incident surface is generally employed, a concave mirror is used as the main mirror, and a convex mirror is used as the secondary mirror. Accordingly, the power arrangement thereof is positive, positive, negative. In addition, an air lens is generally interposed in the optical path between the main mirror and the secondary mirror.
CCDs (charge-coupled devices) are often used as image pickup devices in digital cameras in combination with a conventional dioptric imaging lens. Nevertheless, CCDs have also become more compact as digital cameras have become more compact. Digital cameras are presently being developed that use compact CCDs having diagonal lengths of 16.76 mm (⅔ inch), 8.38 mm (⅓ inch), 6.35 mm (¼ inch), 5.08 mm (⅕ inch) and the like. With such compact digital cameras, the imaging lens must also be made sufficiently compact.
Therefore, the use of a catadioptric imaging lens originally developed as a photographic (i.e., imaging) lens for standard 35 mm format film may be considered for use as an imaging lens for use with compact image pickup devices like a CCD. However, applicants are unaware of prior art in which a catadioptric lens is used as an imaging lens in combination with a compact image pickup device, most likely for the difficulties discussed below.
The imaging performance of such a system can be studied from the reduced aberration values obtained by reducing the size of a super-telephoto catadioptric imaging lens for 35 mm format film in accordance with the dimensional ratio of the particular image pickup device to the 35 mm format film. However, simply reducing the size of a conventional super-telephoto catadioptric imaging lens for 35 mm format film and then using the lens in combination with a compact image pickup device is problematic for at least two reasons.
The first reason is that imagewise telecentricity cannot be sufficiently ensured. When using a CCD as an image pickup device, each ray must intersect the plane of the CCD at an angle that is within a certain number of degrees. Accordingly, the principal ray (i.e., the central ray of each ray bundle) requires a minimum fixed degree of telecentricity.
To keep the aperture of the main mirror small, a catadioptric lens for 35 mm format film generally uses a lens having positive power on the incident surface. This has the advantage that the stronger the positive power, the smaller the main mirror. On the other hand, since the incident surface has positive power, the diameter of the light beam extending to each image point widens at the incident surface. This results in an extreme drop in the incident height of the principal ray incident upon the incident surface as the image height increases. Thus, the angle of the imagewise principal ray with respect to the optical axis deviates from 0° as the image height increases, so that imagewise telecentricity cannot be sufficiently ensured over the image field. Accordingly, there is a risk that a sufficiently detailed picture can no longer be obtained by the CCD.
Furthermore, in a catadioptric lens provided with an incident surface, a main mirror and a secondary mirror, the cross-section of the light beam that contributes to imaging of image points is annular or alternatively, horseshoe-shaped, in which a portion of a ring is missing. Accordingly, the principal ray is actually blocked from reaching the image plane.
Imagewise telecentricity is low in a conventional catadioptric imaging lens for 35 mm format film. Consequently, the cross-section of the light beam that images at the maximum image height cannot maintain an annular shape. Thus, vignetting occurs in the upper part of some light beams, and the cross-section of those light beams exhibit a horseshoe shape.
The second reason why simply reducing the size of a super-telephoto catadioptric imaging lens for 35 mm format and using it with a CCD is problematic is that it is difficult to accurately manufacture the individual parts and to accurately assemble the parts. If a photographic catadioptric lens for 35 mm format film is used as an imaging lens for a CCD on the order of say ⅕ inch, the reduction magnification turns out to be approximately 0.084. Accordingly, the resolving power, which represents the required lens performance, is approximately 12X, the inverse of the reduction magnification. Thus, the more compact the image pickup device, the higher the resolving power of the imaging lens, and the greater the need to have a lens with parts made to a high tolerance and assembled with a high degree of accuracy.
Also, in a catadioptric imaging lens for 35 mm format film an air lens is generally interposed in the optical path between the main mirror and the secondary mirror. This makes high alignment precision of the main mirror and the secondary mirror difficult. Thus, an imaging lens used with a compact image pickup device requires a simplified construction is needed so that accurate alignment can be achieved when performing mechanical assembly.
SUMMARY OF THE INVENTION
The present invention relates to a catadioptric lens, and more particularly relates to a catadioptric lens suited for use as an imaging lens for a camera having a compact image pickup device. The present invention has a sufficiently high imagewise telecentricity and can be assembled with high precision.
Accordingly a first aspect of the invention is a catadioptric lens capable of forming an image of an object. The lens comprises along an optical axis, a first lens group having an annular incident surface that is objectwise concave and upon which light from the object is first incident, and a most imagewise lens surface. The lens further includes an annular main mirror arranged imagewise of the first lens group and having an objectwise concave reflective surface that reflects light objectwise. A secondary mirror is located objectwise of the annular main mirror and has an imagewise convex reflective surface that reflects light imagewise. This arrangement provides a glass optical path within the first lens group from the incident surface to the main mirror to the secondary mirror and to the most imagewise lens surface of the first lens group.
A second aspect of the invention is a catadioptric lens as described above, wherein the annular incident surface satisfies the condition:
0.5
<|r
a
/f|<
1
wherein r
a
is a radius of curvature of the annular incident surface, and f is an overall focal length of the catadioptric lens.
A third aspect of the invention is a catadioptric lens as described above, further satisfying the condition:
|&thgr;|<7°
wherein &thgr; is an angle, measured with respect to the optical axis, of a principal ray associated with a maximum image height of the image.
A fourth aspect of the invention is a catadioptric lens as described above, further satisfying the following conditions:
−0.4<(
r
a
−r
b
)/(
r
a
+r
b
)<−0.19
−0.3<(
r
b
−r
c
)/(
r
b
+r
c
)<0.3
0.2
<A/f<
0.6
wherein r
a
is a radius of curvature of the annular incident surface, r
b
is a radius of curvature of the main mirror, r
c
is a radiu
Lucas Michael A.
Nikon Corporation
Sugarman Scott J.
Vorys Sater Seymour and Pease LLP
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