Two-focus switchable lens

Photography – Having variable focal length of camera objective – Lens moves in or out of light path

Reexamination Certificate

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Details

C396S452000

Reexamination Certificate

active

06308012

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a lens system of a two-focus switching type; and, in particular, to a two-focus switchable lens which is suitable for a compact, low-cost still camera and the like.
2. Description of the Prior Art
There have recently been strong demands for taking pictures according to object positions while switching between telephoto and wide-angle modes even in compact, low-cost still cameras.
As techniques of such a kind, those disclosed in Japanese Unexamined Patent Publication Nos. 7-281092 and 9-021946 have been known. The former is of a type comprising two lenses of negative and positive ones, in which the positive lens is moved so as to change the focal length; whereas the latter is of a type in which a lens on the wide-angle side and a lens on the telephoto side are switched therebetween.
In the former, however, not only a mechanism for moving the lens is necessary, but both the total lens length and the outside diameter of the negative lens become greater, whereby it may be problematic from the viewpoint of achieving compactness.
In the latter, on the other hand, each of the wide-angle and telephoto lenses is constituted by two elements of positive and negative lenses, so that four lenses in total are necessary, thereby contradicting the demands for achieving lower cost and compactness.
Also, when a wide-angle-side lens and a telephoto-side lens are used in a switchable fashion, it is necessary to employ a configuration in which the difference between their respective aperture stop positions is reduced in order to yield a smaller shutter diameter in response to the demand for compactness. However, it has been difficult to increase the power ratio of telephoto/wide angle in a simple configuration while satisfying such a requirement.
SUMMARY OF THE INVENTION
In view of such circumstances, it is an object of the present invention to provide a simple, two-focus switchable lens which can enhance the power ratio of telephoto/wide angle even in a state with a smaller shutter diameter while satisfying demands for compactness and lower cost.
The two-focus switchable lens in accordance with a first aspect of the present invention is a two-focus switchable lens comprising a taking lens system selectively using a wide-angle-side lens and a telephoto-side lens having respective focal lengths different from each other; and a fixed shutter, disposed on an image surface side of the taking lens system, for determining an amount of exposure;
the wide-angle-side lens comprising one or two positive lenses including a positive meniscus lens having a convex surface directed onto the image surface side;
the telephoto-side lens comprising one positive meniscus lens having a convex surface directed onto an object side;
the two-focus switchable lens satisfying the following conditional expression (1):
1.2
<f
T
/f
W
<2.1  (1)
where f
W
is the focal length of the wide-angle-side lens, and f
T
is the focal length of the telephoto-side lens.
This configuration defines a basic condition of the two-focus switchable lens in accordance with the present invention. In order to yield an inexpensive, simple switchable lens system having practically sufficient performances, a fixed shutter is indispensable. Also, in view of the demand for compactness, it is necessary to decrease the shutter diameter, whereas the power ratio of telephoto/wide angle is required to increase. For satisfying these contradictory requests, it is important that the wide-angle-side lens have a lens configuration with as long a back focus as possible, and that the telephoto-side lens have a lens configuration with as short a back focus as possible. Consequently, the aperture stop of the telephoto-side lens is located closer to the fixed shutter, so that the shutter diameter can decrease, thus making it possible to attain compactness and prevent the number of shutter blades from increasing, whereby the cost can be lowered.
For increasing the back focus, the wide-angle-side lens may be configured as a so-called retro-focus type in which negative and positive lenses are combined. In this case, however, barrel distortion may increase so much that curvature of field becomes greater even when an aspheric surface is formed, whereby its performances may be problematic. Therefore, the wide-angle-side lens comprises one or two positive lenses including a positive meniscus lens having a convex surface directed onto the image surface side, thereby yielding favorable performances and allowing its back focus to increase.
The telephoto-side lens, on the other hand, is constituted by one positive meniscus lens having a convex surface directed onto the object side, so as to shorten its back focus and be able to attain practically sufficient imaging performances while in a very compact configuration.
The above-mentioned conditional expression (1) defines the ratio between the respective focal lengths of the wide-angle-side lens and telephoto-side lens. Namely, functions as a telephoto/wide-angle switchable lens system may not be achieved if f
T
/f
W
is less than the lower limit of conditional expression (1). If it exceeds the upper limit, on the other hand, then the position of the aperture stop with respect to the fixed shutter becomes farther as long as a lens system having a simple configuration is used, whereby the shutter diameter becomes so large that it may hinder the camera from achieving compactness, and a problem in terms of cost may occur.
Preferably, the wide-angle-side lens comprises an aperture stop for defining brightness, which is positioned between the wide-angle-side lens and the fixed shutter when the wide-angle-side lens is selected.
This feature is concerned with the wide-angle-side lens and specifies that the aperture stop for defining brightness is disposed behind the wide-angle-side lens. Thus, the lens system of the present invention constituted by one or two positive lenses including a positive meniscus lens having a convex surface directed onto the image surface side can effectively use an aspheric surface so as to easily prevent coma and curvature of field from deteriorating.
Preferably, in this case, the telephoto-side lens comprises an aperture stop for defining brightness, which is positioned between the telephoto-side lens and the fixed shutter when the telephoto-side lens is selected, and satisfies the following conditional expression (2):
0.15
<des/de<
0.36  (2)
where des is the distance between the fixed shutter and the aperture stop on an optical axis, and de is the distance between the aperture stop and the image surface on the optical axis.
This feature is concerned with the telephoto-side lens and specifies that the aperture stop for defining brightness is disposed behind the telephoto-side lens. As a consequence, it is possible to alleviate a characteristic of the telephoto-side lens that its back focus is so long that it is too far from the fixed shutter, and coma and curvature of field can be lowered, whereby imaging performances can be made favorable.
Here, the above-mentioned conditional expression (2) defines the positional relationship between the aperture stop of the telephoto-side lens and the fixed shutter. Namely, if the lower limit of conditional expression (2) is not fulfilled, then back focus may become too short, though it is advantageous for compactness since the shutter diameter can be reduced. As a consequence, the focal length may be so short that telephoto effects become insufficient, or the aperture stop position may be so close to the fixed shutter that the outside diameter of the lens system is too large, thereby hindering compactness from being attained. If the upper limit of conditional expression (2) is exceeded, on the other hand, the shutter diameter may be so large that compactness is hindered from being attained.
The two-focus switchable lens in accordance with a second aspect of the present invention is a two-focus switchable lens comprising a taking lens system selectively u

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