1. Field of the Invention
The present invention relates to a taking lens comprising two lens groups each of which consists of a single lens element suitable for fixed-focus camera such as a low price camera and a single-use lens-fitted film unit.
2. Description of the Prior Art
A variety of photographic cameras that meet various uses and functions have been on the market. In recent years, there are lens-fitted film units on the market. Such a lens-fitted film unit provides persons with enjoyment from easily taking photographs. The lens-fitted film unit, which comprises a light-tight film casing with an exposure mechanism including a taking lens and a shutter incorporated therein and a roll of film factory loaded therein, is widely used because of such convenience that taking pictures is possible right after purchase of it and it is only given as it is to a photofinisher for development and printing when the maximum number of exposures available on the film are partly or completely exposed.
Because the lens-fitted film unit has the feature to offer low prices and is demanded to be produced at low costs, it is made as simple in structure as possibly. For example, the taking lens is installed to a lens holder with a fixed diameter of aperture and has a fixed focus suitable for standard photography in quite many opportunities. A lens-fitted film unit factory loaded with a roll of conventional 35 mm film is provided with a semi-field angle (xcfx89) of approximately 34xc2x0, a taking lens with a focal length of approximately 32 mm and an F-number of approximately 9.5. A lens-fitted film unit factory loaded with a roll of conventional 24 mm film is provided with a semi-field angle (xcfx89) of approximately 35xc2x0, a taking lens with a focal length of approximately 24 mm and an F number of approximately 9.5.
Taking lenses for these lens-fitted film units and low-priced cameras typically comprise one or two lens elements, and, the recent general tendency for such a taking lens is to consist of two lens groups and a single element for each lens group in light of great advantage to corrections of various aberrations. Many two lens group-two lens element taking lenses comprise two meniscus lens elements arranges with their concave surfaces facing each other or comprise a positive power meniscus lens element and a bi-convex lens element arranged in order from the object side to the image side. In consideration of shaping performance and optical characteristics of taking lenses, it is advantageous to form a lens element spherical rather than aspheric. However, if providing a taking lens comprising two meniscus lens elements each of which consists of aspheric surfaces, the taking lens not only produces aggravation of spherical aberrations which is too serious for correction but also is limited to an F number of approximately 11 which significantly lowers brightness on an image plane.
Therefore, when utilizing a taking lens of this kind, it is typical to form the object side surface of a lens element arranged on the object side aspheric or to form both object side surface of a lens element arranged on the object side and image side surface of a lens element arranged on the image side aspheric. This taking lens is appropriately corrected in spherical aberration and improved in F number up to approximately 9. For example, as disclosed in Japanese Unexamined Patent Publication No. 1-307712, forming both object side surface of the lens element arranged on the object side and image side surface of the lens element arranged on the image side aspheric provides desirably balanced distribution of various aberrations. On the other hand, a taking lens including a positive power bi-convex lens element arranged on the object side is easily corrected in spherical aberration and, however, provides aggravation of field curvature which leads to significant aggravation of image sharpness at the periphery of an image. Accordingly, likely the taking lens disclosed in the above publication, the lens element arranged on the object side is provided with an aspheric surface at the object side so as to control aggravation of field curvature. Because forming the object side surface of the lens element arranged on the object side aspheric produces aggravation of field curvature in a sagittal plane too serious for correction, a camera with the conventional taking lens described above makes up for the aggravation of field curvature of the taking lens by making the image plane concave to the object side in a lengthwise direction in which a film is advanced.
Because the concave image plane causes distortion of an image on the image plane, it is necessary to leave the taking lens distortion with the intention of cancelling the distortion due to the curvature of the image plane, as a result of which than taking lens produces aggravation of chromatic difference of magnification to an extent difficult to correct. Furthermore, the taking lens is limited in semi-field angle (xcfx89) to approximately 30xc2x0, which makes it undesirable to incorporate the taking lens into fixed-focus cameras such as the lens-fitted film unit described above.
It is, therefore, an object of the present invention to provide a two lens group-two lens element taking lens which provides high brightness on an image plane and well balanced distribution of various aberrations without reducing a field angle.
It is another object of the present invention to provide a taking lens suitable for fixed-focus photographic devices such as lens-fitted film unit and low price conventional cameras which have an image plane less curved to make up for aggravation of field curvature of the taking lens.
The foregoing objects of the present invention are accomplished by a two lens group-two lens element taking lens comprising, in order from the object side, a first lens group which consists of a positive power first meniscus lens element convex to the object side, a fixed aperture and a second lens group which consists of a positive power second meniscus lens element concave to the object side. The positive power first meniscus lens element at an image side surface and the positive power second meniscus lens element at one of image and object the surfaces are defined by the following relationship:
Z=Ch2/[1+{square root over (1xe2x88x92+L (1+K+L )C2h2+L )}]+X1h4+X2h6+X3h8+X4h10
where Z is the surface sag at semi-aperture distance h from the axis of the 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, and X1, X2, X3 and X4 are aspheric constants. Further, the positive power first and second meniscus lens elements are designed and configured so as to satisfy the following conditions:
|f1 | greater than f2
R1 greater than R2
where f1 is the focal length of the positive power first meniscus lens element, f2 is the focal length of the positive power second meniscus lens element, R1 is the radius of curvature of the object side surface of the positive power first meniscus lens element, and R2 is the image side surface of the radius of curvature of the positive power first meniscus lens element.
Satisfaction of the conditions realizes high brightness on an image plane and well prevents aggravation of various aberrations such as spherical aberration and field curvature without reducing the field angle. Configuring the positive power first meniscus lens element with non-aspheric surface at the object side restrains aggravation of field curvature in the sagittal plane. In consequence, it is possible to alleviate field curvature of the taking lens with an effect of alleviation of chromatic difference of magnification due to curvature of the image plane even when the image plane is less curved.
The taking lens is favorably incorporated in a fixed-focus photographic device provided with an image plane which is bent concave in a direction of film movement to the object side and on which the taking lens forms an image of an object, such as a lens-fitted film unit and a low-price conventional camera.
The positive power first and second meniscus lens elements are preferably made of plastics with an effect of improvement of productivity.