Abstract:
A low-cost wide-angle lens assembly being comprised of a decreased number of lenses yet capable of properly correcting optical aberrations. The second lens in each lens group from the minimum to the maximum horizontal viewing angle is commonly used, and the first lens in each lens group from a reference angle to the maximum horizontal viewing angle is commonly used, thus reducing the total number of required lenses. Aberrations are favorably corrected by satisfying predetermined conditions as to focal distances and back focuses of the lenses.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates to a wide-angle lens assembly for use in a monitor camera or a vehicle-mounted camera. 
     2. Description of Related Art 
     Wide-angle lens assemblies used in a monitor camera or a vehicle-mounted camera have been desired to be less in cost and size while keeping high performance. 
     A conventional compact type wide-angle lens system having a 70 to 90 degrees horizontal angle of view consists of two optical lenses. Also, another compact wide-angle lens system having a horizontal viewing angle of 90 to 120 degrees consists of three optical lenses. 
     In order to construct six different lens groups respectively having a viewing angle of 70°, 80°, 90°, 100°, 110°, and 120° in such conventional wide-angle lens systems, it is necessary to provide a total of 15 lenses even with the use of aspherical resin lenses; two lenses for each of the viewing angles from 70° to 90° and three lenses for each of the viewing angles from 100° to 120°. The lenses have to be fabricated with different sets of molds, thus increasing the cost of production. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a low-cost wide-angle lens assembly having fewer lenses by commonly using the same lenses yet being capable of properly correcting optical aberrations in each varying angle of view. 
     To accomplish the object, the present invention has a second lens commonly used in each lens group from the minimum to the maximum horizontal angle of view, and a first lens commonly used from a reference angle to the maximum horizontal viewing angle. Optical aberrations are corrected by satisfying predetermined conditions as follows: 
     (1)0.65&lt;bf/f2&lt;0.8; 
     (2) -1.8&lt;f1(minimum)/f2&lt;-1.1; 
     (3) -1.8&lt;f1(reference angle)/f2&lt;-1.1; 
     (4) -14.0&lt;f0(maximum)/f2&lt;-5.5; 
     (5) the second lens is commonly used in all lens groups; and 
     (6) the first lens is commonly used in lens groups of horizontal viewing angles from the reference angle to the maximum angle; where 
     f1(minimum) is a focal distance of the first lens in the lens group of the minimum horizontal viewing angle, f1(reference angle) is a focal distance of the first lens in the lens group of a reference horizontal viewing angle, f0(maximum) is a focal distance of the zeroth lens in the lens group of the maximum horizontal viewing angle, f2 is a focal length of the second lens, and bf is a back focus of each lens group. 
     The condition (1) is set to maintain spherical aberrations to be within a permissive range in each combination of the two-group two-lens system and the three-group three lens system. 
     The conditions (2) to (4) permit both spherical aberration and astigmatism to stay within their permissive range in each combination of the two-group two-lens system and the three-group three-lens system. 
     The condition (5) contributes to reduction of the cost for implementing both the two-group two-lens system and the three-group three-lens system. Similarly, the condition (6) contributes to reduction of the cost for implementing both the two-group two-lens system and the three-group three-lens system. 
     Accordingly, the wide-angle lens assembly of the present invention requires a less number of lenses yet being capable of correcting optical aberrations by satisfying these conditions (1) to (6). 
     Other and further objects, features and advantages of the invention will appear more fully from the following description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view showing one embodiment of a wide-angle lens assembly according to the present invention; 
     FIG. 2 is an explanatory view showing an arrangement of wide-angle lens assemblies of the viewing angles of 70° to 90°; 
     FIG. 3 is an explanatory view showing another arrangement of wide-angle lens assemblies of the viewing angle of 100° to 120°; 
     FIG. 4a-4c are diagrams showing aberrations in the wide-angle lens assembly of the viewing angle of 70°; 
     FIG. 5a-5c are diagrams showing aberrations in the wide-angle lens assembly of the viewing angle of 80°; 
     FIG. 6a-6c are diagrams showing aberrations in the wide-angle lens assembly of the viewing angle of 90°; 
     FIG. 7a-7c are diagrams showing aberrations in the wide-angle lens assembly of the viewing angle of 100°; 
     FIG. 8a-8c are diagrams showing aberrations in the wide-angle lens assembly of the viewing angle of 110°; and 
     FIG. 9a-9c are diagrams showing aberrations in the wide-angle lens assembly of the viewing angle of 120°. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A wide-angle lens assembly for a video camera according to one embodiment of the present invention will be described referring to FIGS. 1 to 9 and Tables 1 to 6. 
     The wide-angle lens assembly of this embodiment is designed to have different viewing angles of 70°, 80°, 90°, 100°, 110°, and 120° as shown in FIG. 1. In FIG. 1 a lens 1 serves as a second lens which is commonly used in each arrangement of the different viewing angles. A lens 4 is commonly used as a first lens in each lens group of the viewing angles of 90°, 100°, 110°, and 120°. 
     FIG. 2 shows the arrangement for each wide-angle lens assembly of the viewing angle of 70°, 80°, and 90°. As shown, the assembly comprises two groups of two lenses, including the first lens 2 (3,4) of a negative bi-concave lens having a convex side facing to an object side and the second lens 1 of a positive lens having a convex side facing to the object side. 
     Similarly, FIG. 3 shows the arrangement for each wide-angle lens assembly of the viewing angle of 100°, 110°, and 120°. As shown, the assembly comprises three groups of three lenses, including a zeroth lens 5 (6,7) of a negative meniscus lens, the first lens 4 of a negative bi-concove lens, and the second lens 1 of a positive lens, all having their convex sides facing to the object side. 
     In FIGS. 1 to 3, the reference numeral 8 denotes a flat parallel lens located on the back side of the last lens 1, since this embodiment is designed for use in a video camera. Exemplary values of optical parameters for each wide-angle lens assembly are shown in FIGS. 2 and 3 and Tables 1 to 6, in which f is a focal distance of each lens group and F represents an F number of the lens group. 
     Referring to FIG. 2 and Tables 1 to 3, Ri (i=1 to 6) is the curvature radius of a lens surface of an i-th lens from the object side, Di (i=1 to 6) is the distance between the surfaces of the i-th lens and of an (i+1)th lens, and Nj (j=1 to 3) is the index of refraction of aj-th lens. 
     
                       TABLE 1______________________________________f = 1.3082, F = 2.8,Horizontal Viewing Angle = 70°, bf/f2 = 0.7852, f1/f2______________________________________= -1.4647R1 = -17.10151 *1          D1 =0.526200 N1 = 1.526400R2 = 1.57860          D2 = 2.411752R3 = 1.55668 *2              D3 =  1.512826                              N2 = 1.526400R4 = -1.74523 *3              D4 = 0.438500R5 = INFINITY            D5 = 0.350800                              N3 = 1.517120R6 = INFINITY           D6 = 1.457646______________________________________ *1 Aspherical Surface K = 0.0, A = 0.154182E 02, B =  0.166538E 03, C = -0.256626E 04 *2 Aspherical Surface K = 0.0, A = -0.379525E 01, B = -0.462606E 01, C = 0.962347E 02 *3 Aspherical Surface K = 0.0, A = 0.557427E 01, B = 0.185043E 01, C = 0.320782E 01 
    
     
                       TABLE 2______________________________________f = 1.1398, F = 2.8,Horizontal Viewing Angle = 80°, bf/f2 = 0.7233, f1/f2______________________________________= -1.4647R1 = -17.10151 *1          D1 =0.526200 N1 = 1.526400R2 = 1.57860          D2 = 2.981802R3 = 1.55668 *2              D3 =  1.512826                              N2 = 1.526400R4 = -1.74523 *3              D4 = 0.438500R5 = INFINITY            D5 = 0.350800                              N3 = 1.517120R6 = INFINITY           D6 = 1.342667______________________________________ *1 Aspherical Surface K = 0.0, A = 0.3913852E 02, B =  0.616809E 04, C = -0.320782E 05 *2 Aspherical Surface K = 0.0, A = -0.379525E 01, B = -0.462606E 01, C = 0.962347E 02 *3 Aspherical Surface K = 0.0, A = 0.557427E 01, B = 0.185043E 01, C = 0.320782E 01 
    
     
                       TABLE 3______________________________________f = 1.0000, F = 2.8,Horizontal Viewing Angle = 90°, bf/f2 = 0.6821, f1/f2______________________________________= -1.4248    R1 = -13.15501 *1           D1 =0.526200                       N1 = 1.526400R2 = 1.57860          D2 = 3.529928R3 = 1.55668 *2              D3 =  1.512826                              N2 = 1.526400R4 = -1.74523 *3              D4 = 0.438500R5 = INFINITY            D5 = 0.350800                              N3 = 1.517120R6 = INFINITY           D6 = 1.266319______________________________________ *1 Aspherical Surface K = 0.0, A = 0.699749E 02, B =  -0.191211E 03, C = -0.121897E 04 *2 Aspherical Surface K = 0.0, A = -0.379525E 01, B = -0.462606E 01, C = 0.962347E 02 *3 Aspherical Surface K = 0.0, A = 0.557427E 01, B = 0.185043E 01, C = 0.320782E 01 
    
     Referring to FIG. 3 and Table 4 to 6, Ri (i=1 to 8) is the curvature radius of a lens surface of an i-th lens from the object side, Di (i=1 to 8) is the distance between the surfaces of the i-th lens and of an (i+1)th lens, and Nj (j=0 to 3) is the index of refraction of a j-th lens. 
     
                       TABLE 4______________________________________f = 0.9113, F = 2.8,Horizontal Viewing Angle = 100°, bf/f2 = 0.7042, f0/f2______________________________________= -12.0122    R1 = 10.08551              D1 =0.877001                             N1 = 1.526400R2 = 5.26200          D2 = 1.315501R3 = -13.15501 *1              D3 =  0.526200                              N2 = 1.526400R4 = 1.57860       D4 = 3.529928R5 = 1.55668             D5 = 1.512826                              N3 = 1.526400R6 = -1.74523 *3                   D6 = 0.438500R7 = INFINITY  D7 = 0.350800                       N4 = 1.517120R8 = INFINITY  D8 = 1.266319______________________________________ *1 Aspherical Surface K = 0.0, A = 0.699749E 02, B =  -0.191211E 03, C = -0.121897E 04 *2 Aspherical Surface K = 0.0, A = -0.379525E 01, B = -0.462606E 01, C = 0.962347E 02 *3 Aspherical Surface K = 0.0, A = 0.557427E 01, B = 0.185043E 01, C = 0.320782E 01 
    
     
                       TABLE 5______________________________________f = 0.8518, F = 2.8,Horizontal Viewing Angle = 110°, bf/f2 = 0.7132, f0/f2______________________________________= -7.9819R1 = 17.10151  D1 =0.877001 N1 = 1.526400R2 = 5.26200          D2 = 1.315501R3 = -13.15501 *1              D3 =  0.526200                              N2 = 1.526400R4 = 1.57860       D4 = 3.529928R5 = 1.55668 *2                    D5 = 1.512826                              N3 = 1.526400R6 = -1.74523 *3                   D6 = 0.438500R7 = INFINITY  D7 = 0.350800                       N4 = 1.517120R8 = INFINITY  D8 = 1.266319______________________________________ *1 Aspherical Surface K = 0.0, A = 0.699749E 02, B =  -0.191211E 03, C = -0.121897E 04 *2 Aspherical Surface K = 0.0, A = -0.379525E 01, B = -0.462606E 01, C = 0.962347E 02 *3 Aspherical Surface K = 0.0, A = 0.557427E 01, B = 0.185043E 01, C = 0.320782E 01 
    
     
                       TABLE 6______________________________________f = 0.8151, F = 2.8,Horizontal Viewing Angle = 120°, bf/f2 = 0.7186, f0/f2______________________________________= -6.4814    R1 = 32.88752          D1 =0.877001 N1 = 1.526400R2 = 5.26200          D2 = 1.315501R3 = -13.15501 *1              D3 =  0.526200                              N2 = 1.526400R4 = 1.57860       D4 = 3.529928R5 = 1.55668 *2                    D5 = 1.512826                              N3 = 1.526400R6 = -1.74523 *3                   D6 = 0.438500R7 = INFINITY  D7 = 0.350800                       N4 = 1.517120R8 = INFINITY  D8 = 1.266319______________________________________ *1 Aspherical Surface K = 0.0, A = 0.699749E 02, B =  -0.191211E 03, C = -0.121897E 04 *2 Aspherical Surface K = 0.0, A = -0.379525E 01, B = -0.462606E 01, C = 0.962347E 02 *3 Aspherical Surface K = 0.0, A = 0.557427E 01, B = 0.185043E 01, C = 0.320782E 01 
    
     Assuming that the optical axis extends along an x-axis, a y-axis is vertical to the x-axis, the paraxial radius of curvature is R, and the height from the optical axis is H, the displacement X of the optical axis from the reference vertex point O of an aspherical lens is expressed by: ##EQU1## where K is a conical constant and A, B, and C are aspherical coefficients. 
     FIGS. 4 to 9 show spherical aberration, astigmatism, and distortion in each different wide-angle lens assembly of the viewing angles from 70° to 120°. The reference numeral S and T represent a sagittal surface and a meridional surface, respectively. 
     It is understood that the horizontal viewing angle of the wide-angle lens assembly is arbitrarily determined within a range between the minimum and the maximum degree although its degrees in this embodiment are set at equal intervals of 10°. 
     As set forth above, the wide-angle lens assembly of the present invention permits the second lens to be commonly used in all lens groups from the minimum to the maximum horizontal viewing angle and the first lens to be commonly used in the lens groups from the reference angle to the maximum horizontal viewing angle, thus reducing the overall number of lenses yet being able to correct aberrations through satisfying the required conditions. 
     Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.