exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings . the invention may , however , be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein . rather , these embodiments are provided so that this disclosure will be thorough and complete , and will fully convey the scope of the invention to those skilled in the art . in the drawings , the shapes and dimensions may be exaggerated for clarity . fig1 is a lens configuration view illustrating an imaging optical system according to a first embodiment of the present invention . in the following lens configuration views , the thicknesses , sizes and shapes of the lenses may be exaggerated for clarity . in particular , the shapes of spherical or aspherical surfaces shown in the lens configuration views are only exemplary and should not be construed as being limiting thereto . as shown in fig1 , the imaging optical system according to this embodiment may include first , second , third and fourth lenses l 1 , l 2 , l 3 and l 4 . the first lens l 1 has positive refractive power and two convex refractive surfaces 2 and 3 . the second lens l 2 has negative refractive power and two concave refractive surfaces 4 and 5 . the third lens l 3 has positive refractive power and a meniscus shape . the fourth lens l 4 has a concave object - side refractive surface 8 . this imaging optical system may further include an aperture stop s disposed in front of the object - side surface of the first lens l 1 according to this embodiment , the first to fourth lenses l 1 to l 4 may be made of plastic and formed of aspherical lenses . since a lens made of plastic is manufactured by injection molding , even though the lens has a small radius of curvature or external diameter , it can be easily mass produced at low cost . also , since a press temperature is adjusted to be reduced , the abrasion of a mold is prevented and the number of exchanges and repairs , as well as the amount of maintenance is reduced , thereby promoting cost reduction as compared with a lens made of glass requiring a grinding process . here , the lens made of plastic may include a lens subject to surface coating for the purpose of preventing reflection or improving surface hardening by using plastic as a basic material . also , in order to restrict the refractivity change of a plastic lens according to temperature change , the plastic may be mixed with inorganic particles . meanwhile , an optical filter of , such as an infrared ray filter , a cover glass and the like , is provided between the fourth lens l 4 and an image plane ip . also , the image plane ip is an image formation surface of an image sensor such as a charge coupled device ( ccd ) or a complementary metal oxide semiconductor ( cmos ). in the imaging optical system of the present embodiment , the aperture stop s is disposed in front of the object - side surface of the first lens l 1 . also , the lenses of the optical system have refractive power distributed in such a way that the lenses , sequentially from the object side , have positive , negative and positive refractive powers . such a distribution of refractive power enables the imaging optical system to be superior in field curvature characteristics . also , these four sheets of lenses are configured to have aspherical surfaces to thereby enhance resolution while reducing distortion and spherical aberration . this produces a compact optical system superior in optical characteristics . in addition , the aperture stop s is provided in front of the object - side surface of the first lens l 1 to thereby restrict an effective aperture of the lenses disposed behind the aperture stop . furthermore , an exit pupil may be located far toward the object side from a last image - side surface . this lowers a light emitting angle and beneficially shortens a total length of the optical system . with this overall configuration , operational effects of following conditions 1 to 4 will be examined hereafter . where r 8 is the refractivity of the object - side surface 8 of the fourth lens l 4 , and f is an overall focal length of the optical system . condition 1 governs a ratio of a radius r 8 of curvature of the object - side refractive surface 8 of the fourth lens l 4 to the overall focal length f of the optical system . that is , condition 1 prescribes the shape of the fourth lens l 4 . deviation from lower and upper limits of condition 1 deteriorates telecentric characteristics and distortion characteristics of the optical system . where f 1 is a focal length of the first lens l 1 , and f is an overall focal length of the optical system . condition 2 governs a ratio between the focal length f 1 of the first lens l 1 and the overall focal length f of the optical system . that is , condition 2 prescribes the refractive power of the first lens l 1 . deviation from a lower limit of condition 2 increases the refractive power of the first lens l 1 , thus making it difficult to correct spherical aberration . deviation from an upper limit of condition 2 increases chromatic aberration . where f 2 is a focal length of the second lens l 2 , and f is an overall focal length of the optical system . condition 3 governs a ratio between the focal length f 2 of the second lens l 2 and the overall focal length f of the optical system . that is , condition 3 prescribes the refractive power of the second lens l 2 . similar to condition 2 , deviation from a lower limit of condition 3 increases the refractive power of the second lens l 2 , thus making it difficult to correct spherical aberration . deviation from an upper limit of condition 3 increases chromatic aberration , posing a difficulty to correct axial chromatic aberration . where f 4 is a focal length of the fourth lens l 4 , and f is an overall focal length of the optical system . condition 4 governs a ratio between the focal length f 4 of the fourth lens l 4 and the overall focal length f of the optical system . that is , condition 4 prescribes the refractive power of the fourth lens l 4 . deviation from a lower limit of condition 4 deteriorates telecentric characteristics , thus making it difficult to correct distortion . deviation from an upper limit of condition 4 decreases the refractive power of the fourth lens l 4 , thus not ensuring the miniaturization of the optical system . now , the present invention will be examined in greater detail through specific numerical examples . as described above , in the following first to fourth embodiments , an imaging optical system includes a first lens l 1 having positive refractive power and two convex surfaces , a second lens l 2 having negative refractive power and two concave surfaces , a third lens l 3 having positive refractive power and a meniscus shape , and a fourth lens l 4 having a concave object - side surface , and an aperture stop s is provided in front of the object - side surface of the first lens l 1 . also , an optical filter of such as an infrared ray filter , a cover glass and the like may be provided between the fourth lens l 4 and an image plane ip . here , the image plane ip is an image formation surface of an image sensor such as a ccd or a cmos . meanwhile , aspherical coefficients used in each of the embodiments herein are obtained from following equation 1 , and “ e and a number following the e ” used in a conic constant k and aspherical coefficients a , b , c , d , e , f represent powers of 10 . for example , e + 01 and e - 02 represent 10 1 and 10 − 2 , respectively . where z is a distance from a vertex of a lens in an optical axis , y is a distance in a direction perpendicular to the optical axis , c is a reciprocal number of a radius r of curvature at the vertex of the lens , k is a conic constant , and a , b , c , d , e , f are aspherical coefficients . also , mtf ( modulation transfer function ) depends on a spatial frequency of a cycle per millimeter in an mtf graph of each of the embodiments and is defined by the following equation 2 between a maximum intensity ( max ) and a minimum intensity ( min ) of light . that is , mtf is most ideal when 1 and a smaller mtf deteriorates resolution . table 1 below shows numerical values of an imaging optical system according to a first embodiment of the present invention . fig1 is a lens configuration view illustrating the imaging optical system according to the first embodiment of the present invention , and fig2 is an mtf graph of the imaging optical system shown in table 1 and fig1 . fig3 a through 3c show aberrations of the imaging optical system shown in table 1 and fig1 . in the first embodiment , a total length tl from the object - side surface 2 of the first lens l 1 to an image plane 12 is 4 . 45 mm , and an overall focal length f of the optical system is 3 . 790 mm . table 3 below shows numerical values of an imaging optical system according to a second embodiment of the present invention . fig4 is a lens configuration view illustrating the imaging optical system according to the second embodiment of the present invention , and fig5 is an mtf graph of the imaging optical system shown in table 3 and fig4 . fig6 a through 6c show aberrations of the imaging optical system shown in table 3 and fig4 . in the second embodiment , a total length tl from the object - side surface 2 of the first lens l 1 to the image plane 12 is 4 . 45 mm , and an overall focal length f of the optical system is 3 . 775 mm . table 5 below shows numerical values of an imaging optical system according to a third embodiment of the present invention . fig7 is a lens configuration view illustrating the imaging optical system according to the third embodiment of the present invention , and fig8 is an mtf graph of the imaging optical system shown in table 5 and fig7 . fig9 a through 9c show aberrations of the imaging optical system shown in table 5 and fig7 . in the third embodiment , a total length tl from the object - side surface 2 of the first lens l 1 to the image plane 12 is 4 . 43 mm , and an overall focal length f of the optical system is 3 . 800 mm . table 7 below shows numerical values of an imaging optical system according to a fourth embodiment of the present invention . fig1 is a lens configuration view illustrating the imaging optical system according to the fourth embodiment of the present invention , and fig1 is an mtf graph of the imaging optical system shown in table 7 and fig1 . fig1 a through 12c show aberrations of the imaging optical system shown in table 7 and fig1 . in the fourth embodiment , a total length tl from the object - side surface 2 of the first lens l 1 to the image plane 12 is 4 . 45 mm , and an overall focal length f of the optical system is 3 . 790 mm . it is understood through the above embodiments that the imaging optical system having high resolution as shown in fig2 , 5 , 8 and 11 and being superior in aberrational characteristics as shown in fig3 , 6 , 9 and 12 can be obtained . in the meantime , values of conditions 1 to 3 for the above first to fourth embodiments are noted in table 9 . as set forth above , according to exemplary embodiments of the invention , the imaging optical system is suitable for subminiature optical devices such as a mobile phone camera using an image sensor such as a ccd or a cmos and is capable of minimizing various aberrations and obtaining an image having high resolution and high definition by adjusting a radius of curvature of refractive surfaces of each of the lenses and using aspherical surfaces . furthermore , by using a plurality of plastic lenses , it is easy to mass produce a lighter and lower - cost imaging optical system . while the present invention has been shown and described in connection with the exemplary embodiments , it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims .