FR2723789A1 - OPTICAL SYSTEM FOR VIEWING APPARATUS - Google Patents

Abstract

The invention relates to a lens for a camera. According to the invention, the optical system of the camera consists of a single element of the converging meniscus type, the front face of which is aspherical, of a diaphragm located between the meniscus and the film and of a surface of Cylindrical exposure with a radius of curvature between 85 mm and 95 mm. Application to the production of a single-use camera.

Description

OPTICAL SYSTEM FOR VIEWING APPARATUS

  The present invention relates to single lenses formed of a single element and more particularly to horizontal or landscape lenses formed of a single element, which are particularly suitable for use as a lens (or lens) in inexpensive cameras such as

  single-use cameras.

  A horizontal format lens consists of a single lens element in the form of a converging meniscus and a diaphragm. The arrangement has been known for about 150 years and was originally used with photosensitive materials having such narrow spectral sensitivities in the blue field that satisfactory results were obtained with diaphragm values.

  high F. without color corrections.

  Current single-component lenses, which are used in simple cameras, such as single-use cameras, are usually made of materials with low dispersion and low refractive index.

  to reduce longitudinal chromatic aberration.

  Therefore, in order to reduce the difficulty of correcting chromatic aberration in a single-element lens system, lenses have been designed with the aim of reducing dispersion (using low-index and number-wise materials). High abbe (usually plastics)) to obtain the necessary power and reduce the numerical aperture (NA) of the lens.30 The diaphragm in a horizontal format lens is used to minimize coma aberration. It is well known that there is an optimum location for a diaphragm on the concave side of a meniscus lens element to reduce coma. It is also possible to obtain a useful correction of the curvature of the astigmatic field by "bending" the lens element. Therefore the meniscus form reduces another aberration. Spherical aberration can be slightly reduced by bending, but only at the expense of astignatic field curvature, since spherical aberration can be controlled by the limitation of the diaphragm, and this is what was done. Therefore, horizontal format lenses were usually limited to F / 11 and usually smaller apertures. Similarly, they were limited to half-field angles of 20 or less because of the deterioration of image quality at larger field angles;

  see Jacobs, Fundamentals of Optical Engineering, McGraw-

Hill, N.Y. (1943).

  The horizontally sized lens works well for small numerical apertures, for example ranging from F / 45 to F / 22. For higher apertures, for example from F / 8 to F / 11, the image towards the edge of the field is not as good as at the center of the field due to the presence of obliquely controlled light rays. inadequate. The conventional means for correcting the image towards the edge of the field is to use a small aperture or to add additional lens elements. The first solution is undesirable from the photographic point of view and the second is undesirable from the economic point of view. Thus, although some single-use cameras use a two-element lens, this second lens element creates additional mass and makes the lens system heavier and more expensive. These conditions are particularly important for single-use cameras, which must be light, compact and

cheap.

  Horizontal format lenses are simple and therefore cheap. Therefore, it would be desirable to use a horizontal format lens

  perfected in modern cameras.

  Attempts to improve this lens for modern cameras, lenses and films have been made frequently. Such attempts are described in US Patent No. 5,278,597, which will be described later hereinafter and in U.S. Patent Application 08 / 292,349, entitled HYBRID REFRACTIVE / DIFFRACTIVE ACHROMATIC CAMERA LENS AND CAMERA USING SUCH, which is an application of continuation in part of the US patent application 08 / 170,708 filed on December 29, 1993 in the name of Mark M. Meyers and which suggests a different solution for a camera having a single-element lens. The present invention also relates to improvements concerning a format lens

horizontal.

  An object of the present invention is to provide a camera having a horizontal format lens useful for an aperture between F / 8 and F / 16 and in particular for apertures between

F / 9 and F / 11.

  Another object of the present invention is to provide a horizontal format lens and a camera providing a useful image on a half-field angle

  at least 30 and preferably 33 or more.

  These and other objects of the invention are achieved with the aid of a camera comprising: (i) a camera body for holding a film, and (ii) an optical system consisting of a meniscus-shaped lens having an aspheric surface located on the front surface of the lens element, a diaphragm located between the lens element and the film, the film being curved to define a cylindrical curved segment of which the concave surface is turned towards this meniscus, and the film having a radius of

  curvature R such that 85 mm <IRI <95 mm.

  According to one aspect of the present invention, the lens is arranged to have a complete field

  observation w such that 65 <w <680.

  According to another aspect of the present invention, the lens element can operate with a diaphragm value F in a range 8 S F / # <12 and has a

  focal length f in the range 28 to 35 mm.

  Other features and benefits of this

  invention will emerge from the description given below

  Referring to the accompanying drawings, in which: - Figure 1 shows a schematic drawing of a camera 100 having a housing 5. a lens 10 according to the invention being shown at a distance from the photographic film 15 on which an image is formed, this lens having a curved front surface and a concave rear surface 2; FIGS. 2A-2D show plots of the aberration of the lens 10 of FIG. 1. FIG. 2A represents the plot of the longitudinal spherical aberration, FIG. 2B shows the trace of the lateral color, FIG. distortion and Figure 2D the sagiteal (S) and tangential (T) field curvature plots for three different wavelengths; FIG. 3 represents the plot of the polychrome MTF modulation transfer function of the lens 10 of FIG. 1; FIG. 4 is a plot of the polychromic MTF function corresponding to 0.7 times the field, i.e. at a half-field angle of 24.8 of the lens 10 of FIG. 1; FIG. 5 represents a plot of the polychrome MTF function corresponding to the total field, that is to say a half-angle of 33.40 of the lens 10 of FIG. 1. Referring to FIG. there is shown a horizontal format lens 10 for use in applications to cameras operating in visible light, and more particularly for use in a disposable camera 100 having a housing 5 , a diaphragm or aperture shutter ST, a photographic film 15 supported at a suitable location so that an image is produced by the lens on the film. The camera 100 comprises other known structures or means for advancing the film and a shutter to allow exposure of the film so as to capture an image of a scene to be photographed. The lens 10

  is a single element or singlet convex / concave (ie

  in the form of a meniscus) formed of a body having a first convex surface 1 on the side of the object and a concave surface 2 which constitutes the surface located opposite the first curved surface 1. It is basically a a lens in horizontal format. Both surfaces 1 and 2 are perpendicular to the optical axis of the lens. The refractive lens is formed by an optically transparent material having a refractive index of at least 1.45 and a V number greater than 50. In a preferred embodiment, the refractive index is equal to at 1.49 and the number V is 57.4. A suitable preferable material is any mouldable material such as an optical plastics material or a glass. If the lens element is molded, the plastic is preferable since it is cheaper and easier to mold. The value of the diaphragm F of the lens gives an opening of F / 10

  and the lens has a half-field angle of 33.40.

  The total thickness of the lens may be less than 4 mm and it is preferable that this thickness is about 1 to 3 mm. In this embodiment, it is equal to 1.645 mm. The focal length of the lenses in horizontal format according to the invention can be between 28 and

  mm and is preferably between 32 and 35 mm.

  According to one aspect of the invention, there is provided a horizontal format lens (i.e., a meniscus-shaped lens element) having a focal length in the range of 32 to 35 mm and diaphragm providing openings between F / 9 and F / 11. The focal length of the lens 10 is 32.84 mm. The center of curvature, the base radius of the convex curved surface 1, is located 6.33 mm to the right of this surface, along the optical axis. Surface 1 is aspherical and can be described by the following equation: x = Cy2 + DY4 + EY6 + FYS + GY '+ HY2 + IY'4 + JY'6 1+ l- (k + 1) C2Y2 The aspherical element weakens the surface power towards the edge (that is, the lens becomes more negative as one approaches the edge of the aperture). The degree of aspherical deformation is shown in Table 2. The Y / R value in Table 2 is the relative transparent aperture of

  the aspherical surface, Y / R = 1 representing half

  4 mm aperture for this embodiment.

  The meniscus shape of the lens and the spherical refraction surface serve primarily to reduce the astigmatic field curvature and spherical aberration in the actual image produced by the lens. The preferred material for the lens element is polymethyl methacrylate. It is easily moldable and simplifies the manufacture of the lens element with an aspherical surface. In addition, the high Abbe dispersion number (V = 57.4) of polymethyl methacrylate (PMMA) minimizes the introduction of chromatic aberrations into the image formed by the lens element. Exemplary dimensions and spacings are shown in Table 1, where the index n is located at the center of the range measured for the nominal wavelength x = 546.1 nm, the refractive index is equal at 1.49 and the number V is

equal to 57.4.

Table 1

  Area Radius Thickness Material Index Number 1 6,3336 (a) 1,645 PMMA 1,492 57,4 2 9,5006 (b) 0 Air 1,000 Air 3,223 Air 1,000 Diaphragm 26,367 Plan Image -90,00 (a) The surface profile is a aspherical element; the base radius of curvature is 6.3336. The aspheric profile of the surface is as described by equation 1 in which C = 0.1578881; k = 0; D = -O, 3-27619E;

  E = O, 55003E-4; F = -0,46081E-5; G = 0,13672E-6; H = O;

I = 0,84984E-10; and J = -0.58619E-11.

  (b) cylindrical shape; the radius of curvature (following

  the longitudinal dimension of the film) is -90 mm.

Table 2

  Y / R Deformity Sagittal sagittal total sag *

0.1 0.012642 0.012635

0.2 0.050721 0.050622

0.3 0.14704 0.114277

0.4 0.205402 0.204333

0.5 0.324026 0.322088

0.6 0.472269 0.469423

0.7 0.652451 0.648691

0.8 0.867728 0.862849

0.9 1.122449 1.16331

1.0 1.422748 1.416336

1,1 1,777628 1,768677

  * includes aspheric deformation.

  The lens of the first embodiment has a nominal or central wavelength λ = 546.1 nm. The lens is optimized and analyzed for a bandwidth of 440 to 650 nm around this central wavelength. The lens is designed to be used in a camera having appropriate conventional means or a conventional structure suitable for defining a curved shape of the film. The shape of the film is curved along a cylinder segment, the concave surface of the film being turned towards the surface 2 of the lens. Although in the prior art meniscal single lens elements, each used in connection with a curved configuration of the film, have been described, such film configurations have absolute radii of curvature of between 100 mm and 160 mm. . According to one aspect of the present invention, the best radius of

  curvature for the cylindrical film is about -

  mm, that is to say that we have 85 mm <IRI <95 mm, the

  concave surface being turned towards the lens.

  Although US Pat. No. 5,278,597 indicates that a radius of curvature R for the wire needs to be between -80 mm and -250 mm in order to obtain an acceptable performance, in all the described embodiments, the radii of curvature are between -100 mm and -200 mm. Most other uses have a radius of curvature of -140 mm. This is because the lens designers commonly know that the more the film is bent, the greater the variation between the image quality in the longitudinal direction and the image quality in the short direction (direction). parallel to

  the axis of the cylinder) of the cylindrical film is high.

  Therefore, since lens designers usually believed that the variation of the image increases gradually as the curvature of the film increases, they avoid radii of curvature less than 100 mm. However, we have found that for diaphragm values F providing apertures equal to F / 9 or higher (i.e. F / 11, ..., F / 16), the horizontal format lens has a sufficient depth of field to be used with a film arranged in a cylinder having a radius of curvature in the range of -85 to -95 mm, with no noticeable deterioration of image quality. In addition, we have found that this range of radii adapts to the field curvature and field distortion of the lens and therefore compensates for curvature and field distortion, so that when the film is flattened and is printed, the resulting image quality is superior. Since it is advantageous for the lens to collect as much light as is possible, the aperture value F of the lens should provide an aperture close to F / 9. Good results are obtained with lenses having diaphragm values providing openings in the range

F / 9 to F / l.

  Figures 2 to 5 are plots of aberrations and

  the MTF modulation transfer function of the lens.

  The performance analysis is performed along the spherical radius of best fit to the corner of the cylindrical film. For a 35 mm film format, this radius is -130 mm. The best fitting radius will be different for other formats. The -130 mm radius was used for analysis purposes only. The actual radius of the cylinder of the film is 90 mm. It is measured according to the longitudinal dimension of the film. In the small dimension of the film, the radius of curvature is infinite. It will be noted that on the axis, the MTF values of the lens for frequencies of 10 to lines / mm are equal to 0.45 or more, that at 0.7 of the field the average value NTF ((ST)) is greater than 0.3 for frequencies between 10 and 20 and the value of MTF is approximately 0.5 for 10 lines / mm and approximately 0.7

for 5 lines / mm.

  It is clear from the foregoing description

  that the invention provides a single-element lens for use in a camera lens such as inexpensive or single-use cameras. By using the characteristics of this lens, we obtain an extremely good control

of aberration.

  Finally, the lens according to the present invention is not limited to its use in a single-use camera, but can be used in any single-purpose camera or in any another image pickup device for focusing the light of the visible spectrum, and in particular in devices consisting of a single-lens component or element. Although in the previous embodiment, the focal length is 32.84 mm, the lens according to the present invention is particularly suitable for use in inexpensive cameras such as (but not exclusively) cameras. single-shot cameras having focal lengths of 32 to 35 mm and having an aperture value F providing apertures of between F / 8 and F / 12 and in particular such cameras having an aperture value F providing openings between F / 9

and F / 11.

  Changes and modifications within the scope of the invention will become apparent to those skilled in the art.

  the technique. This is why the previous description can

  be considered as purely illustrative, without any limiting character.

he

Claims (11)

  1. An optical system suitable for use in single-lens disposable cameras or other low-cost single-lens cameras operating in a range of the visible spectrum, including single meniscus-shaped element (10), which comprises a body of optically transparent material having a refractive index of at least 1.45, said body having first and second surfaces (1 and 2) on opposite sides , and a diaphragm (ST) located between said member and the film (15), characterized in that said member is adapted for exposing a bent film in a cylindrical shape, said film (15) having a radius of curvature between -85 and -95 mm, wherein the lens element is arranged to have the following angular coverage:     w 68.   w being the total field of observation.   Optical system according to claim 1, characterized in   that the refractive index is equal to 1.49.   An optical system according to claim 1, characterized in that said element (10) is operable at apertures in an 8 F / # 12 range and has a focal length f in the range of 28. at 35 mm.   Optical system according to claim 3, characterized in that said element (10) is operable for   openings in a range 9 F / # 11.   An optical system according to claim 1, characterized in that said element (10) is operable at apertures in a range 8 F / # 12 and has a focal length f in the range of 32. at 35 mm.   6. Optical system according to claim 5, characterized in that said element (10) has an opening such that that 9 <F / E 11.   Optical system according to claim 1, characterized in that a concave surface of said shaped element   meniscus (10) faces the film (15).   8. Optical system according to claim 1, characterized in that said film (15) has a radius of curvature such than 87 mm <R <95 mm.   Optical system according to claim 3, characterized in that a concave surface (2) of said shaped element   meniscus (10) faces the film (15).   10. Optical system according to claim 1, characterized in that said aspherical surface makes the vergence of the element (10) lower in the direction of the transparent opening. A single-use camera (100) comprising a) a camera housing (5); and b) a film (15) contained in said camera housing (5), said film (15) having a section to be exposed and having a cylindrical shape, characterized in that c) a lens ( 10) in the form of a meniscus consisting of a single element is mounted on said housing of the camera so as to allow exposure of said film (15), and wherein said section of film to be exposed has a radius of curvature R such that mm <IRI <95 mm.