some embodiments of the image pickup system according to the invention are now explained . fig1 is illustrative of the construction of a digital camera that is one embodiment of the image pickup system according to the invention . referring to fig1 , reference numeral 10 indicates an image pickup system or a digital camera comprising an image pickup optical system 1 , a filter 2 , an image pickup device 3 , a controller 4 , a built - in memory 5 , an electronic view finder 6 , and an interface 7 . in the aforesaid image pickup system , light emanating from an object point is focused through the image pickup system 1 comprising optical elements ( such as lenses ) on the light - sensing surface of the image pickup device 3 such as a ccd to form an object image thereon . the image pickup device 3 is an array of regularly arranged photoelectric converters , and between the image pickup system 1 and the image pickup device 3 there is located a filter 2 having a low - pass effect on prevention of a moiré phenomenon due to such an array . in some cases , an infrared cut filter for cutting off infrared light may be located . a light beam incident on the image pickup device 3 is converted by the photoelectric converters to electric signals which are then inputted into the controller 4 , where they are subjected to image processing such as gamma correction and image compression and then sent to a personal computer 9 or the like via the built - in memory 5 and interface 7 . the resultant signals are then transmitted to a reflection type image display device ( not shown in fig1 ), from which they are fed to the electronic view finder 6 comprising an illumination system , a reflection type image display device , a viewing optical system or the like , so that the image to be picked up or the picked up image can be observed by an observer . image data may be sent from the built - in memory 5 to an auxiliary memory 8 , while the same image data may be sent from the interface 7 to the personal computer 9 . fig2 is illustrative of the construction of a silver - salt camera to which the image pickup system according to the invention is applied . as shown in fig2 , a silver - salt camera 20 comprising the image pickup system of the invention is provided with an image pickup optical system 11 , a film 12 , an objective lens 13 , an image pickup device 14 such as a ccd , a first controller 15 and another or a second controller 16 as well as , as in the case of the fig1 digital camera , a built - in memory 5 and an electronic view finder 6 . in the silver - salt camera 20 shown in fig2 , a light beam from an object point is focused through the image pickup optical system 11 on the film 12 to form an object image thereon . a light beam from the object point is also focused through the objective lens 13 separate from the image pickup optical system 11 on the image pickup device 14 such as a ccd to form an object image thereon . the light beam incident on the image pickup device 14 is converted by photoelectric converters forming the image pickup device 14 to electric signals , which are then inputted into the first controller 15 , where they are subjected to image processing such as gamma correction and image compression and then sent to a reflection type image display device and fed to the electronic view finder 6 constructed of an illumination system , a reflection type image display device , a viewing optical system , etc ., so that the image to be picked up can be observed by an observer . on the other hand , if information , etc . stored by the controller 15 in the built - in memory 5 are used , it is then possible for the user ( observer ) to view the picked - up image . the second controller 16 is provided to control the image pickup optical system 11 . on the basis of signals from the second controller 16 , the first controller 15 may recognize information on the zooming , focusing , etc . of the image pickup optical system 11 , so that adjustments are made depending on the image pickup angle of view of the image to be displayed on the reflection type image display device . alternatively , the second controller 16 may recognize information on the focusing , etc . of the image pickup optical system 11 , so that the range of the image displayed on the reflection type display device is corrected ( for parallax ). still alternatively , signals may be sent from the first controller 15 to the built - in memory 5 or an interface ( not shown ), thereby producing output on a personal computer or the like . further , the objective lens 13 may be dispensed with . for instance , a light beam from the image pickup optical system 11 may be split into two or more beams , one of which is used for finder purposes . this finder light beam is used to form an image on the image pickup device 14 such as a ccd for viewing purposes . fig3 is illustrative of the construction of a typical electronic view finder used herein . reference numerals 21 r , 21 g and 21 b represent a red light source , a green light source and a blue light source , respectively . for instance , light - emitting diodes are used . reference numeral 22 stands for an illumination optical system , 23 a viewing optical system , 24 the optical axis of the viewing optical system 23 , 25 a viewer &# 39 ; s eye , 26 a reflection type image display device , and 27 a plane - parallel plate with a polarizing half - silvered mirror 28 mounted thereon . in the thus constructed finder , illumination light from the light sources 21 r , 21 g and 21 b is reflected by the illumination optical system 22 comprising a reflecting mirror in one direction ( upwardly in fig3 ). the optical axis 24 of the viewing optical system 23 is designed to intersect vertically the substantial center of the image display device 26 . leaving the light sources 21 r , 21 g and 21 b and reflected by the illumination optical system 22 constructed of a reflecting mirror in one direction , illumination light propagates with its center intersecting almost vertically the optical axis 24 of the viewing optical system 23 , and is then reflected at the half - silvered mirror 28 toward the reflection type image display device 26 . the reflection type image display device 26 used is a reflection type twisted nematic liquid crystal display device with its twist angle set at 45 °. an image displayed on this reflection type display device 26 is viewed by an observer via the viewing optical system 23 through the plane - parallel plate 27 with the polarizing half - silvered mirror 28 mounted thereon . in the electronic view finder of such construction , when the illumination light emanating from the light sources 21 r , 21 b and 21 b is in a randomly polarized state , it is linearly polarized by the polarizing half - silvered mirror 28 in a certain direction for illuminating the liquid crystal display device 26 . for instance , when the polarizing half - silvered mirror 28 is designed in such a way as to reflect s waves and transmit p waves , the illumination light reflected at the half - silvered mirror 28 is defined by s waves . reflected at the half - silvered mirror 28 to illuminate the liquid crystal display device 26 that is an image display device , the illumination light passes through voltage - applied pixels and a liquid crystal layer , at the bottom of which it is reflected , leaving with the polarizing direction turned through 90 °. thus , the illumination light , which has been entered as s waves into the image pickup display device 26 and modulated thereat , leaves in the form of p waves . upon re - incidence on the plane plate 27 , nearly all this p - wave light transmits through the polarizing half - silvered mirror 28 , arriving at a viewer &# 39 ; s eye 25 via the viewing optical system 23 . the light sources 21 r , 21 g and 21 b are put on in order , so that red , green and blue light rays are successively guided to the liquid crystal display device 26 . in turn , the liquid crystal display device 26 displays successively images corresponding to the thus guided light rays , so that color images are formed . as mentioned above , the electronic view finder used herein can be a compact , light - weight finder that is simplified in construction , and makes effective use of light , because of no substantial losses in the quantity of light emanating from the light sources 21 r , 21 g and 21 b . the action of the viewing optical system 23 enables the observer to perceive images on the image display device 26 as virtual images on an enlarged scale . preferably in this case , the electronic view finder should be designed such that an illumination optical path where light beams leaving the light sources 21 r , 21 g and 21 b enter the image display device 26 upon reflection at the polarizing half - silvered mirror 28 and a viewing optical path where light beams reflected at the image display device 26 are guided to the viewer &# 39 ; s eye upon transmission through the polarizing half - silvered mirror 28 form a reciprocating optical path between the polarizing half - silvered mirror 28 and the image display device 26 . with this arrangement , the optical path through the viewing optical system can be used as a combined forward and backward optical path , so that wasted optical elements ( transmitting surfaces or reflecting surfaces ) and space can be eliminated unlike an optical system having separate two optical paths , thereby making the image pickup system compact . this arrangement is also helpful for prevention of flare light . in the finder shown in fig3 , it is noted that a curved surface such as a rotationally symmetric paraboloid may be used instead of the half - silvered mirror 28 . it is also noted that the illumination optical system 22 may be located at an optical path passing through the half - silvered mirror 28 and the viewing optical system 23 may be positioned at an optical path for reflecting the half - silvered mirror 26 . in the case , the distance a from the display screen of the image display device 26 to the surface of the viewing optical system 23 located nearest to the image display device 26 side is understood to mean the length of that optical path . the viewing optical system 23 in the electronic view finder shown in fig3 may be constructed as in the following examples . in the following examples 1 to 5 , the display screen is in a rectangular form having a length of 3 . 84 mm in the horizontal direction and a length of 2 . 88 mm in the vertical ( short - side ) direction with a diagonal length of 4 . 8 mm . in the following example 6 , the display screen is in a rectangular form having a length of 8 . 96 mm in the horizontal direction and a length of 6 . 66 mm in the vertical ( short - side ) direction with a diagonal length of 11 . 164 mm . fig4 ( a ) through 4 ( d ) are sectional views including the optical axes of the viewing optical systems according to examples 1 to 4 , and fig5 ( a ) through 5 ( c ) are sectional views including the optical axes of the viewing optical systems according to examples 5 to 7 . numerical data on these examples will be enumerated later . in each example , “ lcd ” represents a liquid crystal display device forming part of the image display device , “ ep ” an eye point ”, and “ l ” the diagonal length of the image display device . as shown in fig4 ( a ), the viewing optical system of example 1 is composed of , in order from the image display device side , a double - convex positive lens and a doublet consisting of a double - convex positive lens and a negative meniscus lens concave on its object side , while the surface nearest to the eye point side is formed of an aspheric surface . the values for conditions ( 1 ) to ( 3 ) in this example and the angle of field , 2ω , in the diagonal direction of an image under observation are as follows : as shown in fig4 ( b ), the viewing optical system of example 2 is composed of , in order from the display device side , a doublet consisting of a double - convex positive lens and a negative meniscus lens concave on its object side and a double - convex positive lens , while the object side - surface of the double - convex positive lens located on an eye point side is formed of an aspheric surface . the values for conditions ( 1 ) to ( 3 ) in this example and the angle of field , 2ω , in the diagonal direction of an image under observation are as follows : as shown in fig4 ( c ), the viewing optical system of example 3 is composed of , in order from a display device side , a double - convex positive lens and a doublet consisting of a double - convex positive lens and a negative meniscus lens concave on its object side , while the surface , on an eye point side , of the double - convex positive lens on the object side is formed of an aspheric surface . the values for conditions ( 1 ) to ( 3 ) in this example and the angle of field , 2ω , in the diagonal direction of an image under observation are as follows : as shown in fig4 ( d ), the viewing optical system of example 4 is composed of , in order from a display device side , a negative meniscus lens convex o its object side and a double - convex positive lens while the object - side surface of the double - convex positive lens is formed of an aspheric surface . the values for conditions ( 1 ) to ( 3 ) in this example and the angle of field , 2ω , in the diagonal direction of an image under observation are as follows : as shown in fig5 ( a ), the viewing optical system of example 5 is composed of , in order from a display device side , a double - concave negative lens and a double - convex positive lens , while the eye point - side surface of the double - convex positive lens is formed of an aspheric surface . the values for conditions ( 1 ) to ( 3 ) in this example and the angle of field , 2ω , in the diagonal direction of an image under observation are as follows : as shown in fig5 ( b ), the viewing optical system of example 6 is made up of , in order from a display device side , a double - concave negative lens and a double - convex positive lens while the object - side surface of the double - convex positive lens is formed of an aspheric surface . the values for conditions ( 1 ) to ( 3 ) in this example and the angle of field , 2ω , in the diagonal direction of an image under observation are as follows : as shown in fig5 ( c ), the viewing optical system of example 7 is made up of , in order from a display device side , a negative meniscus lens convex on the display device side and a double - convex positive lens while the object - side surface of the double - convex positive lens is formed of an aspheric surface . in this example , there is used a transmission type liquid crystal display device wherein light sources are located on the back side of a display screen so that an image is formed by light transmitting through the transmission type liquid crystal display device . the values for conditions ( 1 ) to ( 3 ) in this example and the angle of field , 2ω , in the diagonal direction of an image under observation are as follows : in what follows , numerical data on each example will be given . it is noted that r 1 , r 2 , . . . represent the radius of curvature of each lens surface , d 1 , d 2 , . . . represent the spacing between lens surfaces , n d1 , n d2 , . . . represent the d - line refractive index of each lens , and ν d1 , ν d2 , . . . represent the abbe number of each lens . it is also noted that ro stands for the radius of curvature of the display screen of “ lcd ”, d 0 indicates the spacing between the display screen of “ lcd ” and the first lens surface , r 6 in examples 1 - 3 and r 5 in examples 4 - 6 each show the radius of curvature of the “ ep ” surface , and d 5 in examples 1 - 3 and d 4 in examples 4 - 6 each represent an eye relief . length is given in mm . here let x stand for an optical path provided that the direction of propagation of light is positive and y indicate a direction perpendicular to the optical axis . aspheric surface shape is given by x =( y 2 / r )/[ 1 +{ 1 −( k + 1 )( y / r ) 2 } 1 / 2 ]+ a 4 y 4 + a 6 y 6 + a 8 y 8 here r is a paraxial radius of curvature , k is a conical coefficient , and a 4 , a 6 and a 8 are the 4th , 6th and 8th aspheric coefficients , respectively . r 0 = ∞ ( lcd ) d 0 = 6 . 59 r 1 = 52 . 055 d 1 = 6 . 86 n d1 = 1 . 58913 ν d1 = 61 . 14 r 2 = − 11 . 570 d 2 = 1 . 09 r 3 = 54 . 042 d 3 = 6 . 06 n d2 = 1 . 56384 ν d2 = 60 . 67 r 4 = − 9 . 258 d 4 = 2 . 05 n d3 = 1 . 80518 ν d3 = 25 . 42 r 5 = − 17 . 680 ( aspheric ) d 5 = 17 . 00 r 6 = ∞ ( ep ) r 0 = ∞ ( lcd ) d 0 = 7 . 00 r 1 = 10 . 449 d 1 = 7 . 54 n d1 = 1 . 56384 ν d1 = 60 . 67 r 2 = − 14 . 467 d 2 = 1 . 64 n d2 = 1 . 80518 ν d2 = 25 . 42 r 3 = − 114 . 013 d 3 = 0 . 50 r 4 = 20 . 456 ( aspheric ) d 4 = 3 . 45 n d3 = 1 . 58913 ν d3 = 61 . 14 r 5 = − 18 . 079 d 5 = 17 . 00 r 6 = ∞ ( ep ) r 0 = ∞ ( lcd ) d 0 = 6 . 52 r 1 = 52 . 055 d 1 = 8 . 25 n d1 = 1 . 58913 ν d1 = 61 . 14 r 2 = − 10 . 708 ( aspheric ) d 2 = 0 . 30 r 3 = 34 . 600 d 3 = 5 . 78 n d2 = 1 . 56384 ν d2 = 60 . 67 r 4 = − 9 . 258 d 4 = 0 . 80 n d3 = 1 . 80518 ν d3 = 25 . 42 r 5 = − 23 . 115 d 5 = 17 . 00 r 6 = ∞ ( ep ) r 0 = ∞ ( lcd ) d 0 = 12 . 28 r 1 = 50 . 414 d 1 = 1 . 06 n d1 = 1 . 58423 ν d1 = 30 . 49 r 2 = 9 . 124 d 2 = 0 . 55 r 3 = 9 . 908 ( aspheric ) d 3 = 6 . 42 n d2 = 1 . 49236 ν d2 = 57 . 86 r 4 = − 9 . 014 d 4 = 17 . 00 r 5 = ∞ ( ep ) r 0 = ∞ ( lcd ) d 0 = 10 . 91 r 1 = − 26 . 234 d 1 = 1 . 06 n d1 = 1 . 58423 ν d1 = 30 . 49 r 2 = 24 . 828 d 2 = 1 . 51 r 3 = 13 . 612 d 3 = 5 . 92 n d2 = 1 . 49236 ν d2 = 57 . 86 r 4 = − 8 . 868 ( aspheric ) d 4 = 17 . 00 r 5 = ∞ ( ep ) r 0 = ∞ ( lcd ) d 0 = 14 . 64 r 1 = − 37 . 022 d 1 = 1 . 02 n d1 = 1 . 58423 ν d1 = 30 . 49 r 2 = 52 . 882 d 2 = 1 . 32 r 3 = 15 . 833 ( aspheric ) d 3 = 6 . 82 n d2 = 1 . 52542 ν d2 = 55 . 78 r 4 = − 13 . 482 d 4 = 17 . 00 r 5 = ∞ ( ep ) r 0 = ∞ ( lcd ) d 0 = 15 . 04 r 1 = 68 . 309 d 1 = 1 . 77 n d1 = 1 . 58423 ν d1 = 30 . 49 r 2 = 17 . 414 d 2 = 0 . 71 r 3 = 13 . 379 ( aspheric ) d 3 = 7 . 90 n d2 = 1 . 52542 ν d2 = 55 . 78 r 4 = − 15 . 234 d 4 = 17 . 00 r 5 = ∞ ( ep ) fig6 to 12 are aberration diagrams for examples 1 to 7 , in which “ sa ”, “ as ” and “ cc ” represent spherical aberrations , astigmatism and chromatic aberration of magnification , respectively . as can be appreciated from the foregoing , the present invention can provide such image pickup systems as summarized below : an image pickup system having an electronic view finder suitable for achieving compactness ; an image pickup system that enables an observer to have an easy grasp of the image pickup range ; an image pickup system that gets hold of a sufficient angle of field and satisfactory optical performance even when using an image display device provided with a display screen having a short diagonal length ; an image pickup system having an electronic view finder with well - corrected chromatic aberration of magnification ; an image pickup system provided with an electronic view finder wherein dust , etc . deposited on a viewing optical system are unnoticeable ; and an image pickup system provided with an electronic view finder that enables appropriate optical elements to be located even when a reflection type image display device is used as an image display device .