Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2001-08-23
2003-08-05
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S422000
Reexamination Certificate
active
06603608
ABSTRACT:
This application claims benefit of Japanese Application Nos. 2000-251869 and 2001-7199 filed in Japan on Aug. 23, 2000 and Jan. 16, 2001, respectively, the contents of which are incorporated by this reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a variable focal length optical element and an optical system using the same. More particularly, the present invention relates to a variable focal length optical element for use in an image pickup optical system having a zooming mechanism and a focusing mechanism and also relates to an optical system using such a variable focal length optical element.
2. Discussion of Related Art
Variable-magnification image pickup optical systems using prisms with free-form surfaces are known from Japanese Patent Application Unexamined Publication Numbers [hereinafter referred to as “JP(A)”] 8-292372, 11-317894, 11-317895, etc.
However, JP(A) 8-292372 is arranged to perform zooming by moving a plurality of prisms. Therefore, it is necessary to ensure a space for moving the prisms in the apparatus in advance, which leads to an increase in the size of the apparatus. Further, because the prism moving mechanism needs to move the prisms rectilinearly with high accuracy, the structure becomes complicated. This causes costs to increase and hinders assembling considerably.
Similarly, JP(A) 11-317894 and 11-317895 have a zooming mechanism in which a transmission lens optical element is moved, and hence suffer from the problem that the structure becomes complicated as in the case of the above.
Thus, the above-described conventional techniques need to ensure a space for moving an optical element in advance. That is, it is necessary to ensure a useless space. Therefore, it is impossible with the conventional techniques to construct a compact variable-magnification optical system. In addition, the optical element per se produces aberrations. Therefore, it cannot be used to change optical parameters to a considerable extent.
Common zooming and focus adjustment are generally effected by moving some optical elements in the optical system along the optical axis direction. However, this method has the disadvantage that it is difficult to move the optical elements completely parallel to the optical axis. Tilt or decentration of the optical elements causes the image quality to be degraded. In particular, it has heretofore been impossible to construct a variable-magnification optical system for use with a small-sized image pickup device because the requirement for the accuracy of movement is very strict.
SUMMARY OF THE INVENTION
The present invention was made in view of the above-described problems with the prior art.
An object of the present invention is to provide an image-forming optical system or the like capable of focal length adjustment, focus adjustment, etc. with a simple arrangement in which a desired portion of an optical element is selectively utilized by moving an aperture in a direction approximately perpendicular to the optical axis.
To attain the above-described object, the present invention provides a variable focal length optical element having at least one continuous rotationally asymmetric surface. The optical power of the optical element is variable by selecting an optical path through an aperture.
In this case, it is possible to obtain an image-forming optical system capable of adjusting the focal length, the image-formation position, etc. with a simple arrangement in which a desired portion of the optical element is selectively utilized by moving the aperture in a direction approximately perpendicular to the optical axis.
It is desirable that the relationship A between the size D of the aperture and the amount of movement d of the aperture, i.e. A=D/d, should satisfy the following condition:
0.01<A<1 (1)
The reasons for adopting the above-described arrangements in the present invention, together with the functions thereof, will be described below.
FIG. 1
is a diagram schematically showing light rays in the meridional section of an image-forming optical system (optical element) S substantially corrected for spherical aberration, curvature of field, astigmatism, distortion, axial chromatic aberration and lateral chromatic aberration but allowed to generate large comatic aberration. Because of the large comatic aberration, light rays {circle around (1)}, {circle around (2)} and {circle around (3)} emanating from an object O at different field angles are incident on different positions on an image plane I. Accordingly, if the position of the aperture of the image-forming optical system S is moved from P
1
through P
2
to P
3
in a direction approximately perpendicular to the optical axis (herein assumed to be an axial principal ray passing through the center of each of the apertures P
1
, P
2
and P
3
and reaching the center of the image plane, although the optical axis is defined by an axial principal ray passing through the center of the entrance pupil and reaching the center of the image plane), light rays passing through the image-forming optical system S change their passing position and angle, i.e. {circle around (1)}→{circle around (2)}→{circle around (3)}, and are incident on the image plane I at different heights H
1
, H
2
and H
3
. Therefore, the image-formation magnification of the object O on the image plane I varies according to the position of the aperture. Accordingly, the magnification can be varied by moving a stop constituting each aperture in a direction approximately perpendicular to the optical axis. On the same principle, the image-formation position and the principal point position can be adjusted by moving the pupil position in a direction approximately perpendicular to the optical axis. It should be noted that the aperture position can also be moved in a direction perpendicular to the plane of the figure. Therefore, it is possible to effect zooming, for example, by movement in the plane of the figure and to perform focusing, for example, by movement in a direction perpendicular to the plane of the figure.
It is preferable that the relationship A between the size D of the aperture, which limits the cross-sectional area of a beam of light, and the amount of movement d of the aperture, i.e. A=D/d, should satisfy the following condition:
0.01<A< (1)
It should be noted that when the aperture is projected within the variable focal length optical element by an optical path selecting device for selecting an optical path, D and d correspond to the size of the aperture and the amount of movement thereof within the variable focal length optical element.
If A is not larger than the lower limit of the condition (1), i.e. 0.01, the amount of movement of the aperture becomes excessively large in comparison to the size of the aperture. Consequently, it becomes impossible to construct a compact optical element, which is one object to be attained by the variable focal length optical element according to the present invention. If A is not smaller than the upper limit of the condition (1), i.e. 1, the required amount of movement of the aperture cannot satisfactorily be ensured. Consequently, it becomes impossible to make the focal length variable.
It should be noted that in Example 4 (described later), D, d and A are as follows:
D
d
A
1.15
5.34
0.22
In addition, the present invention provides an optical system having at least two optical elements. The optical system includes a first unit and a second unit. The first unit is positioned at the object side of the optical system to form a first image. The second unit projects the first image onto an image plane. An optical path selecting device for selecting an optical path is positioned in the vicinity of the first image. Either the first or second unit includes a variable focal length optical element having at least two continuous rotationally asymmetric surfaces.
In this case, it is desirable that an optical path in the variable focal length optical element should be selected
Olympus Optical Co,. Ltd.
Pillsbury & Winthrop LLP
Spector David N.
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