Photography – Disposable or recyclable camera
Reexamination Certificate
2000-06-05
2002-02-19
Mathews, Alan A. (Department: 2851)
Photography
Disposable or recyclable camera
C396S311000, C396S315000, C396S319000, C359S642000
Reexamination Certificate
active
06349171
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a taking lens and a lens-fitted film unit equipped with the taking lens.
2. Description Related to the Prior Art
A variety of photographic cameras have been on the market according to uses and functions. Further, in recent years, lens-fitted film units have been popular for the purpose of easily enjoying photography without using ordinary photographic cameras. Such a lens-fitted film unit comprises a plastic light-tight unit casing with a taking lens, an exposure mechanism including a shutter and an unexposed filmstrip that is pre-loaded. This lens-fitted film unit makes the photographer take picture immediately after purchase thereof and, after exposure of the maximum number of exposures available on a filmstrip thereof, is simply given to a photofinisher for processing and making prints. This convenience has popularized widely the lens-fitted film unit. Because one of the important advantages of the lens-fitted film unit is a low price, it is essential to employ a simple structure. Taking lenses for use with the lens-fitted film units or low price photographic cameras generally comprise one or two lens elements. Such a taking lens comprising one or two lens elements is too poor in optical performance to provide a high quality image. On this grounds, in order to remove the curvature of field of the taking lens, the lens-fitted film unit is configured such as to support a filmstrip on an image surface curved in a lengthwise direction of the image frame of a filmstrip or a horizontal direction and having a center of curvature on the object side. On the other hand, thanks to remarkable progress in processing speed, an image processing technique has been established for reading an image directly from negatives by an image scanner, and improving contrast and the tone of color of the image and correcting various aberrations through digital image processing so as to provide a high quality image.
When exposing an exposure frame of a film placed on a curved image surface, while an image is somewhat prevented from being blurred due to the curvature of field of the taking lens, it is accompanied by emphasized distortion due to the curvature of the film. When the image surface curves in the lengthwise direction of the image frame, the distortion takes barrel-shaped distortion in the lengthwise direction. In order to remove such barrel-shaped distortion, it is necessary to provide a gently curved image surface, or otherwise to design the taking lens with the intention of causing it to produce positive distortion. When providing the image surface with gentle curvature, the distortion is somewhat restrained, which is however realized at the cost for the effect of improvement of an out-of-focus image that occurs due to the curvature of field of the taking lens. Further, when the taking lens comprises a single lens element and an aperture stop disposed on the object side of the lens element, it inevitably produces positive distortion with the effect of improving the above mentioned problem. However, when employing two lens elements for the taking lens with the intention to improve optical performance, distortion that the taking lens produces tends to become negative with improvement of marginal image quality and, in particular, a modulation transfer function (MTF). Accordingly, designing the two-lens element taking lens with the intention to cause the taking lens to produce positive distortion leads to an occurrence of a reduction in MTF at marginal portions of an image and aggravation of longitudinal chromatic aberration of magnification, so that the effect of the two-lens element taking lens is made feeble. When providing the taking lens with positive distortion, pincushion-shaped distortion occurs in a transverse direction of the image frame or a vertical direction which is perpendicular to the lengthwise direction of the image surface.
As described above, an image formed by the conventional taking lens is accompanied by aggravation of image quality due to a complex effect of various aberrations and a decrease in contrast. On the grounds, if the digital image processing to correct and improve the image quality, it is necessary to implement various image processing, which needs an ample of time.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to provide a taking lens which forms an image most suitable for digital image processing.
It is another object of the present invention to provide a lens-fitted film unit equipped with a taking lens which forms an image most suitable for digital image processing.
The above objects of the present invention are achieved by a taking lens for forming an image on a filmstrip placed on an image surface which is curved in the lengthwise direction of the image frame of a filmstrip and has a center of curvature on the object end so as to correct the curvature of field for the taking lens in the image surface. The taking lens comprises in order from the object end to the image end a meniscus first lens element having a convex object side surface and a meniscus second lens element having a convex image side surface. Letting Ep, &dgr; and Db be the axial distance between the curved image surface and an exit pupil of the taking lens which takes a value in mm greater than zero, the axial distance of opposite lengthwise ends of the curved image surface from the center of the curved image surface which takes a value in mm greater than zero, and the distortion which is equivalent but opposite in direction to distortion D occurring due to the curvature of image surface and given by (&dgr;/Ep)×100%, respectively, the taking lens satisfies the following condition,
−10%≦
Ds≦Db−
0.5%
where
Ds is optical distortion in a plane including a paraxial focal point of light rays traveling from the exit pupil of the taking lens to a corners of the curved image surface.
The taking lens is desirably provided with an aperture stop disposed either on the image side of the meniscus second lens element or between the meniscus first lens element and the meniscus second lens element.
The taking lens preferably has aspheric surfaces at the convex object side surface of the meniscus first lens element and the convex image side surface of the meniscus second lens element. The aspheric surface may be defined by the following equation:
Z=ch
2
/[1+{1−(1
+K
)
c
2
h
2
}
½
]+Ah
4
+Bh
6
+Ch
8
+Dh
10
where
Z is the surface sag at a semi-aperture distance h from the optical axis of the taking lens;
c is the curvature of a lens surface at the optical axis equal to the reciprocal of the radius at the optical axis;
K is a conic constant;
A, B, C and D are aspheric coefficients.
The taking lens is installed to a lens-fitted film unit which comprises a light-tight unit housing to which the taking lens is fixedly attached, a film cartridge with an unexposed filmstrip which is preloaded in the unit housing and is recorded, optically or magnetically, with data relating to distortion of the taking lens for the purpose of compensating distortion of an image formed on the filmstrip by the taking lens during digital image processing.
According to the present invention, the zoom lens is structured with the design intention to produce aggravation of image quality due only to distortion and is, however, free from or significantly improved in aggravation of other aberrations, so that the digital image processing is allowed to compensate only distortion of an image for improvement of image quality.
Before describing the present invention in detail, reference is made to
FIG. 1
for the purpose of providing a brief background that will enhance an understanding of an occurrence of distortion.
As schematically shown in
FIG. 1
, a taking lens
60
that comprises a meniscus first lens element convex to the object side and a meniscus second lens element convex to the object side forming an image on an image surface,
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