hereinafter , one or more exemplary embodiments will be described in detail with reference to accompanying drawings . in drawings , like reference numerals denote like elements to avoid repetition . fig1 is a schematic diagram of a zoom lens system according to an exemplary embodiment . a photographing apparatus according to an exemplary embodiment includes the zoom lens system of fig1 and an image pickup device having an image plane on which a subject image is formed by the zoom lens system . fig1 illustrates the zoom lens system with respect to numerical embodiment 1 described later . as shown in fig1 , the zoom lens system includes a first lens group g 1 having positive refractive power , a second lens group g 2 having negative refractive power , a third lens group g 3 having positive refractive power , and a fourth lens group g 4 having positive refractive power sequentially from an object side ( left of fig1 ), wherein an aperture sp is disposed at the third lens group g 3 closest to the object side . in fig1 , g denotes an optical block corresponding to an optical filter or a faceplate . ip denotes an image plane which corresponds to an imaging plane of a solid image pickup device ( photoelectric conversion device ), such as a charge coupled device ( ccd ) sensor or a complementary metal - oxide semiconductor ( cmos ) sensor , when the zoom lens system according to the current exemplary embodiment is used as a photographing optical system of a surveillance camera , a video camera , or a digital still camera , and which corresponds to a film plane when the zoom lens system according to the current exemplary embodiment is used in a silver salts film camera . the second lens group g 2 moves to an image plane side as shown by an arrow 2 a during variable magnification from a wide - angle position to a telephoto position . here , the first and third lens groups g 1 and g 3 are fixed in an optical axis direction , and the fourth lens group g 4 moves to compensate for a change of an image plane location according to the movement of the second lens group g 2 while moving to an object side during near - distance focusing . a solid curve 4 a and a broken curve 4 b of the fourth lens group g 4 shown in fig1 show moving trajectories for compensating for an image plane change according to the variable magnification from the wide - angle position to the telephoto position respectively during focusing on an infinite object and a near - distance object . the first lens group g 1 includes a cemented lens of a negative lens 11 having a meniscus shape having a convex surface facing the object side and a positive lens 12 having a convex surface facing the object side , and two positive lenses 13 and 14 having convex surfaces facing the object side . the second lens group g 2 includes two negative lenses 21 and 22 and a cemented lens of a positive lens 23 having a convex surface facing the object side and a negative lens 24 sequentially from the object side . the third lens group g 3 includes an independent positive lens 31 having an aspherical surface at the object side , and a cemented lens of a positive lens 32 and a negative lens 33 , sequentially from the object side . the fourth lens group g 4 includes a cemented lens of a positive lens 41 having an aspherical surface at the object side and a negative lens 42 , sequentially from the object side . the zoom lens system according to the current exemplary embodiment will now be described in detail . the zoom lens system according to the current embodiment includes , sequentially from the object side , the first lens group g 1 having positive refractive power , the second lens group g 2 having negative refractive power , the aperture sp fixed with respect to the image plane , the third lens group g 3 having positive refractive power , and the fourth lens group g 4 having positive refractive power , wherein the first and third lens groups g 1 and g 3 are fixed in the optical axis direction during variable magnification from the wide - angle position to the telephoto position , the variable magnification is performed as the second lens group g 2 moves from the object side to an image side , and the fourth lens group g 4 moves in the optical axis direction to compensate for the change of the image plane location according to the movement of the second lens group g 2 while performing a focusing function . the first lens group g 1 includes one negative lens and three positive lenses sequentially from the object side , the second lens group g 2 includes at least three negative lenses and one positive lens , the third lens group g 3 includes an independent positive lens and a cemented lens of a positive lens and a negative lens sequentially from the object side , the fourth lens group g 4 includes at least one of a positive lens and a negative lens , and following conditions are satisfied . here , f t denotes a focal length of the first lens group g 1 , f 2 denotes a focal length of the second lens group g 2 , and f t denotes a focal length of the zoom lens system in a telephoto position . positive refractive power is distributed by disposing at least three positive lenses in the first lens group g 1 , and specifically , aspherical aberration in the telephoto position is easily compensated for . also , by using a glass material having an abbe number greater than or equal to 80 for a positive lens , axial chromatic aberration and lateral chromatic aberration are specifically compensated for in the telephoto position . by disposing at least three negative lenses and one positive lens in the second lens group g 2 , negative refractive power is distributed and comma aberration generated according to a change of a viewing angle in the wide - angle position is compensated for , thereby promoting high performance . in detail , by disposing a cemented lens of a positive lens and a negative lens , aberration , such as chromatic aberration , may be satisfactorily compensated for and in addition , an adhesion error during manufacture may be reduced , thereby obtaining stable optical quality . the third lens group g 3 is capable of satisfactorily compensating for off - axis aberration generated during the variable magnification by including at least one surface having an aspherical surface shape . also , since the third lens group g 3 includes two positive lenses and one negative lens , chromatic aberration generated in the third lens group g 3 may be satisfactorily compensated for . moreover , since principal points of the third lens group g 3 may be disposed at the object side , a distance from the third lens group g 3 to the imaging plane may be reduced , and thus the zoom lens system may be miniaturized . in addition , the negative lens in the third lens group g 3 may have a meniscus shape having a high curvature at the image plane side such that spherical aberration is easily compensated for , or / and the negative lens in the third lens group g 3 may form a cemented lens with a positive lens such that an interval of principal points with a positive lens of the third lens group g 3 nearest to the object may be increased , thereby obtaining a stable optical performance during manufacture . the fourth lens group g 4 may satisfactorily compensate for axial aberration generated during the variable magnification by including a cemented lens of a positive lens and a negative lens . also , the fourth lens group g 4 that is a moving lens group may be light - weighted according to a simplified structure of the fourth lens group g 4 via cementing , and may quickly perform a focusing operation . in detail , the fourth lens group g 4 may include at least one aspherical surface to easily and satisfactorily compensate for spherical aberration generated in the fourth lens group g 4 . the conditional expression 1 is an expression defining a focal length of the second lens group g 2 and a focal length of the zoom lens system in a telephoto position . if the value of | f 2 / f t | is greater than upper limit of the conditional expression 1 , refractive power of the second lens group g 2 is weakened , a moving amount during variable magnification is increased , and an overall length of zoom lens system is increased and thus it is difficult to miniaturize the zoom lens system . also , if the value of | f 2 / f t | is lower than lower limit of the conditional expression 1 , refractive power of the second lens group g 2 is strengthened , and it is difficult to satisfactorily compensate for an aberration change during the variable magnification from the wide - angle position state to the telephoto position state . in detail , a numerical range of the conditional expression 1 may be set to satisfy conditional expression 1a below . the conditional expression 2 is an expression defining a focal length of the first lens group g 1 and a focal length of the second lens group g 2 . if the value | f 1 / f 2 | is greater than upper limit of the conditional expression 2 , refractive power of the first lens group g 1 is weakened , an overall length of the zoom lens system is increased and moreover , a lens diameter of the first lens group g 2 should is increased , and thus it is difficult to miniaturize the zoom lens system . also , if the value | f 1 / f 2 | is lower than lower limit of the conditional expression 2 , refractive power of the first lens group g 1 is strengthened and it is difficult to compensate for spherical aberration in a telephoto position and to promote a high performance . in detail , a numerical range of the conditional expression 2 may be set to satisfy conditional expression 2 a below . in detail , the third lens group g 3 may satisfy following conditions . here , f 3 denotes a focal length of the third lens group g 3 , f 31 denotes a focal length of a positive lens closest to the object side of the third lens group g 3 , and f t denotes a focal length of the zoom lens system in a telephoto position . the conditional expression 3 is an expression defining the focal length of the third lens group g 3 and the focal length of the zoom lens system in the telephoto position . if | f 3 / f t | is greater than an upper limit of the conditional expression 3 , refractive power of the third lens group g 3 is weakened , and in order to maintain refractive power in the entire third and fourth lens groups g 3 and g 4 , refractive power of the fourth lens group g 4 needs to be strengthened . accordingly , it is difficult to satisfactorily compensate for aberration , such as astigmatism or comma aberration . if | f 3 / f t | is lower than a lower limit of the conditional expression 3 , refractive power of the third lens group g 3 is too strengthened , and thus aberration , such as spherical aberration , is increased . in detail , a numerical range of the conditional expression 3 may be set to satisfy conditional expression 3a below . the conditional expression 4 is an expression defining the focal length of the positive lens of the third lens group g 3 closest to the object side and the focal length of the third lens group g 3 . if | f 31 / f 3 | is greater than an upper limit of the conditional expression 4 , refractive power of the positive lens closest to the object side in the third lens group g 3 is too weakened and an overall length of the zoom lens system is lengthened . if | f 31 / f 3 | is lower than a lower limit of the conditional expression 4 , refractive power of the positive lens closest to the object side in the third lens group g 3 is too strengthened and aberration , such as spherical aberration , is increased . in detail , a numerical range of the conditional expression 4 may be set to satisfy conditional expression 4 a below . in detail , the fourth lens group g 4 may satisfy a following condition . here , f 4 denotes a focal length of the fourth lens group g 4 and f t denotes a focal length of the zoom lens system in a telephoto position . the conditional expression 5 is an expression defining the focal length of the fourth lens group g 4 and the focal length of the zoom lens system in the telephoto position . if | f 4 / f t | is greater than an upper limit of the conditional expression 5 , refractive power of the fourth lens group g 4 is weakened , it is difficult to miniaturize the zoom lens system because a moving distance ( i . e ., a moving range of the fourth lens group g 4 to the object side or the image side ) during focusing of the fourth lens group g 4 is increased and it is difficult to reduce an interval between the third lens group g 3 and the fourth lens group g 4 . if | f 4 / f t | is lower than a lower limit of the conditional expression 5 , refractive power of the fourth lens group g 4 is strengthened , and , it is difficult to satisfactorily compensate for an aberration change during variable magnification from a wide - angle position state to a telephoto position state . in detail , a numerical range of the conditional expression 5 may be set to satisfy conditional expression 5a below . as described above , by suitably arranging the first through fourth lens groups g 1 through g 4 , the zoom lens system according to the current exemplary embodiment may have about 30 times variable magnification , maintain an optical performance corresponding to a high pixel image pickup device , and be miniaturized . according to the current exemplary embodiment , the zoom lens system that has about 30 times variable magnification , is small , and maintains a satisfactory optical performance corresponding to a high pixel image pickup device from a wide - angle position to a telephoto position and from a near - distance to an infinity may be obtained in an image device , such as a surveillance camera , a video camera , or a digital still camera . examples of the current exemplary embodiment will now be described in detail . numerical embodiments 1 through 5 below are detailed examples suitable to the conditional expressions 1 through 5 above . in each of numerical embodiments 1 through 5 , a surface number i denotes an order of an optical surface from the object side . in each of numerical embodiments 1 through 5 , r i denotes a curvature of an ( i ) th optical surface , d i denotes a surface interval between the ( i ) th optical surface and an ( i + 1 ) th optical surface , and nd i and v d i respectively denote a refractive index and an abbe number of a material of an ( i ) th optical member with respect to a d - line . a back focus bf is a distance from a lens surface of a lens closest to the object to an image plane . an overall length is obtained by adding bf to a distance from a center of a lens surface of a lens closest to the object to a center of a lens surface of a lens closest to the image plane . also , a unit of length is in mm . an aspherical surface may be represented by equation 1 below when k denotes a conic constant , a4 , a6 , a8 , and a10 denote aspherical surface coefficients , and x denotes a displacement in an optical axis direction at a location of height h from an optical axis based on a surface vertex . here , r denotes a radius of curvature . also , for example , “ e - z ” denotes “ 10 - z ”. f denotes a focal length , f no denotes a f - number , and ω denotes a half viewing angle . table 11 below is a correspondence table of the numerical embodiments 1 through 5 and the conditional expressions 1 through 5 . each numerical value shows a value defined in each of the conditional expressions 1 through 5 . as such , all of the numerical embodiments 1 through 5 satisfy conditional expressions 1 through 5 . according to one or more exemplary embodiments , it is possible to achieve miniaturization and about 30 times variable magnification in a positive - negative - positive - positive four group zoom lens system . while exemplary embodiments have been particularly shown and described above , it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present inventive concept as defined by the following claims .