Photography – Disposable or recyclable camera
Reexamination Certificate
1999-05-25
2001-02-13
Mathews, Alan A. (Department: 2851)
Photography
Disposable or recyclable camera
C396S097000, C359S820000
Reexamination Certificate
active
06188841
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a lens-fitted film unit with a taking lens comprising injection molded plastic lens elements.
2. Description of Related Art
Lens-fitted film units having been on the market are inexpensive and make one easily enjoy taking pictures. Such a lens-fitted film unit having a case in which a simple exposure mechanism is installed and a roll of film is pre-loaded. The lens-fitted film unit is widely spread because of the convenience that it can be gotten whenever and anywhere one wishes to take pictures and is put to a photo-shop as it is for processing and printing. There are variety of lens-fitted film units ranging from the most simplified unit which incorporates a fundamental exposure mechanism only to the most sophisticated unit which is equipped with a taking lens suitable for telephotography and/or a strobe unit for night photography and indoor photography. These lens-fitted film units give one a wide choice according to one's intentions.
One of advantages of the lens-fitted film units is low production costs. Since it is mandatory to provide the lens-fitted film units at as low costs as possible, such a lens-fitted film unit is under the necessity of having a quite simplified structure. For example, the taking lens is held in position by a lens holder formed with a fixed aperture so as to focus images of standard sharpness of as many objects as possible. The taking lens comprises one or two lens elements. The taking lenses have focal lengths from about 30 mm to 35 mm for lens-fitted film units with a 35 mm format film pre-loaded therein and a focal length of about 24 mm for lens-fitted film units with a 24 mm format film pre-loaded therein. Further, for lens-fitted film units suitable for telephotograph, the taking lenses have focal lengths increased to about 100 mm.
Typically, plastic lenses formed by injection molding are employed as component lens elements of the taking lens. While such an injection molded plastic lens is suitable to be mass-produced and produced at production costs lowered as compared with glass lens elements, the injection molded plastic lens has the drawback that it easily varies its refractive power due to a change in ambient temperature. In particular, the injection molded plastic lens expands to weaken its curvature with the result of becoming more apt to decrease the refractive power as the ambient temperature rises. If the refractive power of the taking lens becomes too small, the taking lens shifts its focal point behind a film strip situated in a predetermined film plane, forming a blurry image on the film strip.
Therefore, the two component taking lens for the lens-fitted film unit is typically comprised of two plastic lens elements having different thermal expansion coefficients so as to control a shift of the focal point of the taking lens due to a change in ambient temperature. The longer the focal length of the taking lens is, the grater a change in refraction power of taking lens caused due to a change in ambient temperature becomes, and when the taking lens has a focal length longer than 50 mm, it is difficult to control a change in refractive power of the taking lens only by using different materials for the two plastic lens elements. In particular, when the taking lens suitable for telephotograph has a focal length of approximately 100 mm, it is completely impossible to control a change in refractive power of the taking lens only by using different materials for the two plastic lens elements.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a lens-fitted film unit which can always form a sharp image without being affected by a change in ambient temperature even though it is equipped with a taking lens comprising an injection molded plastic lens element.
The foregoing object of the invention is accomplished by providing a lens-fitted film unit in which a taking lens comprising an injection molded plastic lens element is held by an axially movable lens holder accompanied by thermally expandable axial distance adjusting means disposed between the lens holder and a predetermined film plane in which an unexposed photographic film strip is situated. The thermally expandable axial distance adjusting means forces the lens holder along an axis of the taking lens to vary the axial distance between the lens holder from the predetermined film plane when expanding or contracting in accordance with a change in ambient temperature, so as thereby to shift the focal point of the taking lens with the result of compensating a variation of the focal length of the taking lens due to a change in refractive power of the plastic lens element which is caused by axial expansion or axial contraction due to the change in ambient temperature. In the case where the taking lens comprises two injection molded plastic lens elements, it is preferred to dispose the thermally expandable axial distance adjusting means between the two plastic lens elements so that the axial distance adjusting means forces either one of the two plastic lens elements along the axis of the taking lens to vary the axial distance between the two plastic lens elements when expanding or contracting in accordance with a change in ambient temperature. The focal point of the taking lens is shifted and adjusted to the predetermined film plane to compensate a variation of focal length of the taking lens due to changes in refractive power of the plastic lens elements which are caused by axial expansion or axial contraction due to the change in ambient temperature. The axial distance adjusting means may preferably take the form of a ring disposed in the optical axis. The axial distance adjusting ring disposed between two plastic lens elements is preferably formed as a shield ring operative to block off peripheral light rays from the outside of an aimed scene or a fixed aperture ring operative to control the amount of light rays reaching the film strip.
Further, the axial distance adjusting means may take the form of thermally extendable axially positioning rod accompanied by a drive spring. The positioning rod extends in accordance with a rise in ambient temperature to define an axial position to which the lens holder or the taking lens is permitted to shift along the axis of the taking lens, and then the drive spring forces the lens holder or the taking lens to the axial position.
In either case, the axial distance adjusting means is preferable to have a thermal expansion coefficient greater than the lens element of the taking lens and may be made of one of metals including zinc, selenium, aluminum, permaroy and alloys thereof or one of non-metallic materials including ebonite, plastics and woods.
The axial distance adjusting means may comprise a bimetal plate capable of being warped in a plane perpendicular to the optical axis to change an apparent axial thickness thereof in accordance with a change in ambient temperature so as thereby to change an axial distance between the lens holder from the predetermined film plane with the result of adjusting the focal point of the taking lens to the predetermined film plane. The bimetal plate may take the form of a bimetal ring formed with a plurality of sections which are defined by slits. The bimetal ring at the sections is bent in a circumferential direction to change an apparent axial thickness thereof in accordance with a change in ambient temperature so as thereby to change an axial distance between the lens holder from the predetermined film plane with the result of adjusting the focal point of the taking lens to the predetermined film plane.
According to another embodiment of the invention, the unit body at least between the taking lens and the predetermined film plane is made to be capable of thermally expanding and contracting in a direction of an axis of the taking lens to change an axial distance between the taking lens and the predetermined film plane in accordance with a change in ambient temperature so as thereby to shift a focal poin
Fuji Photo Film Co. , Ltd.
Mathews Alan A.
Young & Thompson
LandOfFree
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