Abstract:
An inverted Galilean finder is composed of an objective lens of a negative power and an eyepiece of a positive power, which are formed from a polystyrene resin by injection molding. The objective lens has a concave surface oriented to the eyepiece, and the eyepiece has a convex surface oriented to the objective lens. The inverted Galilean finder satisfies the following conditions:  
     −0.75≦ f 1/ f 2≦−0.60  
       f 1≦−22  
     −3.2≦ f 1/d≦−1.5  
     wherein f1 is a focal length of the objective lens, f2 is a focal length of the eyepiece, and “d” is an on-axis surface distance between the concave surface of the objective lens and the convex surface of the eyepiece. The inverted Galilean finder of the present invention provides a magnification of about 0.6 to 0.8.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an inverted Galilean finder that can be mounted in a small space and has a high magnification. The present invention relates also to a camera, especially a lens-fitted photo film unit, that is provided with such an inverted Galilean finder.  
           [0003]    2. Background Arts  
           [0004]    Lens-fitted photo film units are widely used as a kind of single-use economy cameras. The lens-fitted photo film unit, hereinafter referred to as the film unit, is preloaded with a roll of unexposed photo filmstrip, so the user can take photographs instantly at the purchase of it. After the completion of exposure, the user has only to forward the film unit to a photofinisher, to obtain the developed photo filmstrip and photo-prints made from the photographed pictures. Considering the above nature, being low-price and compact is important for the film unit.  
           [0005]    As an optical finder of the film unit, the inverted Galilean finder consisting of a concave objective lens and a convex eyepiece has been used because of its simple structure and compactness. The Galilean finder provides comparatively good performances at a low cost.  
           [0006]    However, because of the limit in the mounting space, the Galilean finders used in the conventional film units merely have a finder magnification of about 0.4 to 0.6, which is too small for many users. So it has been desired to provide the film unit with a finder having a larger magnification.  
           [0007]    In order to enlarge the magnification of the finder, it is usual using larger objective and eyepiece lenses. But this solution has problems that it needs a larger mounting space and a higher material cost for the finder, so the compactness and cheapness of the film unit are deteriorated.  
         SUMMARY OF THE INVENTION  
         [0008]    In view of the foregoing, an object of the present invention is to provide an inverted Galilean finder that has a high magnification, can be manufactured at a low cost and does not need a larger mounting space. The present invention also has an object to provide a lens-fitted photo film unit provided with such an inverted Galilean finder.  
           [0009]    To achieve the above and other objects, an inverted Galilean finder of the present invention is composed of an objective lens of a negative power and an eyepiece of a positive power, the objective lens having a concave surface oriented to the eyepiece, the eyepiece having a convex surface oriented to the objective lens, wherein the inverted Galilean finder satisfies the following conditions:  
           −0.75 ≦f 1 /f 2≦−0.60  (1)  
             f 1≦−22  (2)  
           −3.2 ≦f 1 /d≦− 1.5  (3)  
           [0010]    wherein f1 is a focal length of the objective lens, f2 is a focal length of the eyepiece, and “d” is a surface distance between the concave surface of the objective lens and the convex surface of the eyepiece on an optical axis of the Galilean finder.  
           [0011]    The inverted Galilean finder of the present invention provides a magnification of about 0.6 to 0.8. The objective lens and the eyepiece are preferably formed from a polystyrene resin by injection molding. Mounting the inverted Galilean finder satisfying the above conditions provides a compact camera or a lens-fitted photo film unit that has a sufficiently large finder magnification of about 0.6 and 0.8.  
           [0012]    If the value f1/f2 is above the upper limit of the first condition (1), the finder magnification cannot be so high, so that the remarkably superior performances to the conventional finder is not achieved. Below the lower limit of the first condition, indeed the finder magnification becomes higher, it is necessary to enlarge the lens size. Thus, the mounting space for the finder would increase, deteriorating the compactness of the film unit.  
           [0013]    The second condition: f1≦−22 defines an upper limit of the focal length of the objective lens having the negative power. When designing a lens system that satisfies the above first condition, power arrangement of the lens system is improved using an objective lens that satisfies the second condition, so it becomes easy to correct aberrations of the lens system. The second condition also makes it easy to define such an air space between the lenses that is suitable for use as a finder mounted in a lens-fitted photo film unit.  
           [0014]    The third condition: −3.2≦f1/d≦−1.5 defines a condition relating to the air space between the concave surface of the objective lens and the convex surface of the eyepiece on the optical axis, on the premise that these lenses constitute a lens system that satisfies the above first and second conditions. Above the upper limit of the third condition, the surface distance “d” and thus the mounting space for the finder become so large that the lens-fitted photo film unit would loose the compactness. Below the lower limit of the third condition, the refractive power of the objective lens becomes so small that it would be necessary to enlarge the finder mounting space. Otherwise, the surface distance “d” becomes so small that it would be hard to design composition of the two lenses. In either case, it becomes disadvantageous for the lens system to serve as a finder. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The above and other objects and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments when read in association with the accompanying drawings, which are given by way of illustration only and thus are not limiting the present invention. In the drawings, like reference numerals designate like or corresponding parts throughout the several views, and wherein:  
         [0016]    [0016]FIG. 1 shows a perspective view of a lens-fitted photo film unit according to an embodiment of the present invention;  
         [0017]    [0017]FIG. 2 shows an exploded perspective view of the lens-fitted photo film unit of FIG. 1;  
         [0018]    [0018]FIGS. 3, 6,  9  and  12  schematically show lens systems according to Examples 1, 2, 3 and 4 of the present invention;  
         [0019]    [0019]FIGS. 4, 7,  10  and  13  are graphs showing longitudinal aberrations of Examples 1 to 4 respectively; and  
         [0020]    [0020]FIGS. 5, 8,  11  and  14  are graphs showing transverse aberrations of Examples 1 to 4 respectively. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0021]    In FIG. 1, a lens-fitted photo film unit  1  has a camera body  2  that is substantially parallelepiped. The camera body  2  has a shutter button  3  and a counter window  4  showing the available number of exposures on its top side. The camera body  2  is wrapped with an ornamental label, and a taking lens  6  is exposed to the front side. A finder objective window  7  is formed above the taking lens  6 . A flash projector  8  projects a flash light at a flash photography.  
         [0022]    As shown in FIG. 2, the film unit  1  has a unit main body portion  13  whose front and rear are covered with a front cover  11  and a rear cover  12  respectively. The front and rear cover  11  and  12  and the main body portion  13  are formed from plastics. A flash device  14 , which has various circuit elements mounted on a circuit board  15 , is attached to the unit main body portion  13 .  
         [0023]    The unit main body portion  13  is integrally formed with a cartridge chamber  16  and a film chamber  17 . The cartridge chamber  16  holds a film cartridge  19  from which an unexposed photo filmstrip  18  is pulled out, whereas the film chamber  17  holds the unexposed photo filmstrip  18  in the form of a roll  20 . Open bottoms of the cartridge chamber  16  and the film chamber  17  are covered with bottom lids  21  and  22  respectively. A film winding wheel  23  is rotatably mounted on top of the cartridge chamber  16 , and is engaged with a spool  24  of the film cartridge  19 . Rotating the film winding wheel  23  drives the spool  24  to rewind the photo filmstrip  18  into the film cartridge  19 .  
         [0024]    An exposure chamber is also formed integrally with the unit main body  13  between the cartridge chamber  16  and the film chamber  17 , and an exposure unit  25  is mounted in front of the exposure chamber. The taking lens  6  is placed in front of a not-shown shutter aperture that is formed through the exposure unit  25 . A not-shown shutter blade is placed behind the shutter aperture and the taking lens  6 . A not-shown exposure opening is formed on the rear side of the exposure chamber. The exposure opening defines an exposure area of one picture frame on the photo filmstrip  18  that is held between the exposure opening and a film backing surface  26  that is formed on the rear cover  12 . While the shutter blade opens the shutter aperture, an image of a subject is formed on the photo filmstrip  18  through the taking lens  6 .  
         [0025]    The exposure unit  25  is provided with a well-known shutter mechanism that actuates the shutter blade to open and close the shutter opening, and a finder frame  27 . The shutter mechanism is charged as the photo filmstrip  18  is wound up by one frame by rotating the film winding wheel  23 , and is released in response to the shutter button  3  being pressed, causing the shutter blade to make an open-close operation for an exposure.  
         [0026]    An objective lens  30  and an eyepiece  40  are fitted in the finder frame  27 . The objective lens  30  and the eyepiece  40  are formed from a plastic material, e.g. a polystyrene resin, by injection-molding. The objective lens  30  is a concave lens, and the eyepiece  40  is a convex lens, so these lenses constitute an inverted Galilean finder optical system. The following description shows several examples of the inverted Galilean finder optical system of the present invention, wherein the lens elements are formed from a polystyrene resin. The eye point is located 10˜18 mm from the eyepiece, and the air space between the eyepiece and the objective lens is about 14 mm. In order to obtain a sharp field of view, aspherical lenses suppressing aberrations are used as the lens elements.  
       EXAMPLE 1  
       [0027]    [0027]FIG. 3 shows a composition of Example 1 of the inverted Galilean finder optical system of the present invention. Lens data of the objective lens  30  and the eyepiece  40  of Example 1 is shown in Table 1.  
         [0028]    In Table 1 and other tables, “No.” represents a surface serial number of a respective curved surface of the lens elements in the order from the object side, “R” represents a radius of curvature of the respective curved surface, expressed in the unit of millimeter. The surface distance means a thickness of a respective lens on the optical axis or an air space between adjacent lenses on the optical axis, expressed in the unit of millimeter. “Nd” represents a the refractive index of each lens with respect to d-line (wavelength of 587.6 nm). The lens surface allocated with “*” is an aspherical surface that satisfies the following condition:  
           Z=ch   2 /[1+{1−(1+ k ) c   2   h   2 } ½   ]+Ah   2   +Bh   6   +Ch   8   +Dh   10    
         [0029]    wherein “c”=1/R (R=radius of curvature), “h” is the height from the optical axis, and “k”, “A”, “B”, “C” and “D” are aspherical coefficients. The aspherical coefficients of the second surface of Example 1 are shown in Table 2.  
                                                     TABLE 1                       No.   R   SURFACE DISTANCE   Nd   Abbe Number                                1   INFINITY   1.35   1.59000   30.9       2   *15.679   13.95       3   23.289   2.5   1.59000   30.9       4   364.183                  
 
         [0030]    [0030]                           TABLE 2                                   k   0                           A   −0.152561E-04           B   −0.757461E-07           C   −0.330640E-10           D   −0.122694E-11                        
         [0031]    In Example 1, the focal length f1 of the objective lens  30  and the focal length f2 of the eyepiece  40  are:  
         f1=−26.57 mm  
         f2=40.06 mm.  
         [0032]    As shown in Table 1, the surface distance “d” between the second surface and the third surface, i.e. the air space between the concave surface of the objective lens and the convex surface of the eyepiece is:  
         d=13.95 mm.  
         [0033]    Therefore, in Example 1, f1/f2=−0.63, f1/d=−1.90.  
         [0034]    Accordingly, the characteristic values of the present invention satisfy all the three conditions:  
         −0.75≦ f 1 /f 2≦−0.60  (1)  
           f 1≦−22  (2)  
         −3.2≦ f 1 /d ≦−1.5  (3)  
         [0035]    Various aberrations of Example 1 are illustrated in FIGS. 4 and 5. In FIG. 4A, a curve S shows an astigmatism with respect to the sagittal image surface, and a curve T shows an astigmatism with respect to the tangential image surface. FIGS. 5A, 5B,  5 C and  5 D respectively show transverse aberrations at relative field heights of 1.00, 0.84, 0.54 and 0.00.  
         [0036]    Example 1 provides an inverted Galilean finder having a magnification of 0.65 and a sharp field of view where the aberrations are well compensated as shown in FIGS. 4 and 5.  
       EXAMPLE  2   
       [0037]    [0037]FIG. 6 shows a composition of Example 2 of the inverted Galilean finder optical system of the present invention. Lens data of the objective lens  50  and the eyepiece  60  of Example 2 is shown in Table 3.  
                                                     TABLE 3                       No.   R   SURFACE DISTANCE   Nd   Abbe Number                                1   INFINITY   1.2   1.59000   30.9       2   *16.165   13.8       3   *16.370   2.5   1.59000   30.9       4   45.551                  
 
         [0038]    The aspherical coefficients of the second and third surfaces of Example 2 are shown in Table 4.  
                                         TABLE 4                                   SECOND SURFACE   THIRD SURFACE                                    k   0   0       A   −0.359154E-04   −0.249269E-04       B   −0.590935E-07   0       C   0   0       D   0   0                  
 
         [0039]    In Example 2, the focal length f1 of the objective lens  50  and the focal length f2 of the eyepiece  60  are:  
         f1=−27.40 mm  
         f2=41.98 mm.  
         [0040]    As shown in Table 3, the surface distance “d” between the second surface and the third surface, i.e. the air space between the concave surface of the objective lens and the convex surface of the eyepiece is:  
         d=13.8 mm.  
         [0041]    Therefore, in Example 2, f1/f2=−0.65,  f 1/d=−1.99.  
         [0042]    Accordingly, the characteristic values of the present invention satisfy all the above three conditions.  
         [0043]    Various aberrations of Example 2 are illustrated in FIGS. 7 and 8. In FIG. 7A, a curve S shows an astigmatism with respect to the sagittal image surface, and a curve T shows an astigmatism with respect to the tangential image surface. FIGS. 8A, 8B,  8 C and  8 D respectively show transverse aberrations at relative field heights of 1.00, 0.83, 0.55 and 0.00.  
         [0044]    Example 2 provides an inverted Galilean finder having a magnification of 0.67 and a sharp field of view where the aberrations are well compensated as shown in FIGS. 7 and 8.  
       EXAMPLE 3  
       [0045]    [0045]FIG. 9 shows a composition of Example 3 of the inverted Galilean finder optical system of the present invention. Lens data of the objective lens  70  and the eyepiece  80  of Example 3 is shown in Table 5.  
                                                     TABLE 5                       No.   R   SURFACE DISTANCE   Nd   Abbe Number                                1   INFINITY   2.0   1.59000   30.9       2   *18.164   12.8       3   24.258   3.0   1.59000   30.9       4   251.609                  
 
         [0046]    The aspherical coefficients of the second surface of Example 3 are shown in Table 6.  
                                         TABLE 6                                   k   0                                        A   −0.916806E-05           B   −0.151759E-07           C   −0.198005E-09           D   0.564390E-12                      
 
         [0047]    In Example 3, the focal length f1 of the objective lens  70  and the focal length f2 of the eyepiece  80  are:  
         f1=−30.79 mm  
         f2=45.28 mm.  
         [0048]    As shown in Table 5, the surface distance “d” between the second surface and the third surface, i.e. the air space between the concave surface of the objective lens and the convex surface of the eyepiece on the optical axis is:  
         d=12.8 mm.  
         [0049]    Therefore, in Example 3, f1/f2=−0.68, f1/d=−2.41.  
         [0050]    Accordingly, the characteristic values of the present invention satisfy all the above three conditions (1) to (3).  
         [0051]    Various aberrations of Example 3 are illustrated in FIGS. 10 and 11. In FIG. 10A, a curve S shows an astigmatism with respect to the sagittal image surface, and a curve T shows an astigmatism with respect to the tangential image surface. FIGS. 11A, 11B,  11 C and  11 D respectively show transverse aberrations at relative field heights of 1.00, 0.83, 0.55, and 0.00.  
         [0052]    Example 3 provides an inverted Galilean finder having a magnification of 0.70 and a sharp field of view where the aberrations are well compensated as shown in FIGS. 7 and 8.  
       EXAMPLE 4  
       [0053]    [0053]FIG. 12 shows a composition of Example 4 of the inverted Galilean finder optical system of the present invention. Lens data of the objective lens  90  and the eyepiece  100  of Example 4 is shown in Table 7.  
                                                     TABLE 7                       No.   R   SURFACE DISTANCE   Nd   Abbe Number                                1   80.0   1.4   1.59000   30.9       2   *18.293   13.8       3   *25.217   2.2   1.59000   30.9       4   100.0                  
 
         [0054]    The aspherical coefficients of the second and third surfaces of Example 4 are shown in Table 8.  
                                         TABLE 8                                   SECOND SURFACE   THIRD SURFACE                                    k   0   0       A   −0.100414E-04   −0.516066E-05       B   −0.502635E-08   0.801960E-07       C   0.695539E-10   0       D   0   0                  
 
         [0055]    In Example 4, the focal length f1 of the objective lens  90  and the focal length f2 of the eyepiece  100  are:  
         f1=−40.54 mm  
         f2=56.54 mm.  
         [0056]    As shown in Table 7, the surface distance “d” between the second surface and the third surface, i.e. the air space between the concave surface of the objective lens  90  and the convex surface of the eyepiece  100  on the optical axis is:  
         d=13.8 mm.  
         [0057]    Therefore, in Example 4, f1/f2=−0.72, f1/d=−2.94.  
         [0058]    Accordingly, the characteristic values of the present invention satisfy all the above three conditions (1) to (3).  
         [0059]    Various aberrations of Example 4 are illustrated in FIGS. 13 and 14. In FIG. 13A, a curve S shows an astigmatism with respect to the sagittal image surface, and a curve T shows an astigmatism with respect to the tangential image surface. FIGS. 14A, 14B,  14 C and  14 D respectively show transverse aberrations at relative field heights of 1.00, 0.82, 0.56 and 0.00.  
         [0060]    Example 4 provides an inverted Galilean finder having a magnification of 0.74 and a sharp field of view where the aberrations are well compensated as shown in FIGS. 13 and 14.  
         [0061]    Although the present invention has been described with respect to the lens-fitted photo film unit, the inverted Galilean finder of the present invention is applicable to those cameras which allow the user to load a film cartridge repeatedly, and which are required to be economy and compact.  
         [0062]    The present invention is not to be limited to the above embodiment and examples but, on the contrary, various modifications will be possible to those skilled in the art without departing from the scope of claims appended hereto.