a first embodiment of the present invention will be described with reference to fig1 and 2 . a lens barrel of the present invention comprises : a plastic pressing ring 6 , a plastic lens chamber 7 , 1 st - 5 th lenses 1 - 5 held in the plastic lens chamber 7 , a compressed spring 8 disposed between the second lens 2 and the third lens 3 for urging in a direction such that the second lens 2 and the third lens 3 are separated from each other in opposite directions , a parallel spacer 9 , and spacers 10 for positioning two adjacent sets of lenses in the optical axis direction . the lens groups 1 - 5 and the spacers 10 are urged by the compressed spring 8 in the optical axis direction so as to receive appropriate load pressures . the plastic lens chamber 7 , the plastic pressing ring 6 , the lens groups 1 - 5 and the spacers 10 have different linear expansion coefficients . as the temperature changes , the size of each component changes to accumulate to form a difference value . under the action of the compressed spring 8 , the distance between the second lens 2 and the third lens 3 in the optical axis direction is changed . in the above - described lens barrel , the back focus a is most sensitive to the change of the distance between the second lens 2 and the third lens 3 in the optical axis direction . in other words , the above - described lens barrel can be divided into two parts : the 1 st - 2 nd lenses 1 - 2 and the 3 rd - 5 th lenses 3 - 5 , with the compressed spring 8 therebetween . as the temperature changes , the size of each component changes to accumulate to form a difference value , and the distance between the second lens 2 and the third lens 3 can be effectively increased or decreased under the action of the compressed spring 8 . the lens barrel of the present invention makes an adjustment to the back focus as the final assembling process at normal temperature , so as to obtain excellent optical property . a method of correcting back focus variation of the lens barrel caused by a temperature change with respect to the normal temperature will be described with reference to fig1 and 2 . the lens barrel and the image sensor are positioned and fixed with respect to each other by means of a prescribed adjustment means at normal temperature such that the image sensor surface 11 is within the range of the depth p of focus of the lens barrel . as shown in fig1 , at normal temperature , the distance between the second lens 2 and the third lens 3 in the optical axis direction is d , the length of the plastic lens chamber 7 in the optical axis direction is b , and the image sensor surface 11 is within the range of the depth p of focus of the lens barrel . in this state , once the temperature changes , the thermal expansion coefficients of the lenses 1 - 5 and the plastic lens chamber 7 and the refractive indexes of the lenses 1 - 5 will change , which causes a back focus a of the lens barrel to change . at this time , if no correction is made and the image sensor surface 11 deviates from the range of the depth p of focus of the lens barrel , the optical property of the lens barrel will deteriorate ; in other words , the performance of the lens barrel at normal temperature cannot be maintained . even if the temperature changes to a temperature different from the normal temperature , the lens barrel of the present invention will not be affected by the temperature change and its performance at normal temperature can be maintained . the lens barrel of the present invention corrects back focus variation of the lens barrel based on the linear expansion coefficient of the material . assuming that the size variation of the plastic lens chamber 7 is b = b * α lh * δt when the linear expansion coefficient of the plastic lens chamber 7 is α lh and the temperature variation of the plastic lens chamber 7 is δt . on the other hand , assuming that the size variation of the respective lenses 1 - 5 is c li = c li * α li * δt when the linear expansion coefficient of the respective lenses 1 - 5 is α li ( i = 1 , 2 , . . . ), the size of the respective lenses 1 - 5 in the optical axis direction is c li ( i = 1 , 2 , . . . ) and the temperature variation of the respective lenses 1 - 5 is δt , and assuming that the size variation of the respective spacers 10 is e si = e si * α si * δt when the linear expansion coefficient of the respective spacers 10 is α si ( i = 1 , 2 , . . . ), the size of the respective spacers 10 in the optical axis direction is e si ( i = 1 , 2 , . . . ) and the temperature variation of the respective spacers 10 is δt , the accumulated total of the above size variations is σ ( c li + e si ). if the relationship between the linear expansion coefficient of the plastic lens chamber 7 and the linear expansion coefficient of the lenses 1 - 5 is α lh α li and the relationship between the linear expansion coefficient of the plastic lens chamber 7 and the linear expansion coefficient of the spacers 10 is α lh α si , the relationship between the size variation of the plastic lens chamber 7 and the size variations of the lenses 1 - 5 and the spacers 10 is b & gt ; σ ( c li + e si ). in the lens barrel of the present invention , a variation tendency of the distance between the second lens 2 and the third lens 3 is contrary to a variation tendency of the back focus a of the lens barrel . that is , once the distance between the second lens 2 and the third lens 3 increases , the back focus a of the lens barrel is shortened ; and once the distance between the second lens 2 and the third lens 3 decreases , the back focus a of the lens barrel is lengthened . an explanation will be made as to the method of correcting the back focus of the lens barrel , in the case where the temperature changes from the normal temperature to a high temperature . in the lens barrel of the present invention , the second and the third lenses 2 and 3 are formed of plastic material . as the refractive index of the plastic lenses varies greatly at different temperatures , the temperature change exerts the greatest influence on the variation in the back focus . in this situation , once the temperature is high , the refractive index of the second and the third lenses 2 and 3 will change , causing the back focus a of the lens barrel to become longer . although the refractive index of other lenses ( glass lenses ) also changes at this time , because the refractive index change of glass lenses makes very little influence on the back focus a compared to the refractive index change of the plastic lenses , the explanation thereof is omitted . at a high temperature , the plastic lens chamber 7 , the lenses 1 - 5 and the spacers 10 of the lens barrel expand at the same time and respectively undergo the above - mentioned size variations b and σ ( c li + e si ). as stated above , the lens barrel of the present invention is divided into two parts : the 1 st - 2 nd lenses 1 - 2 and the 3 rd - 5 th lenses 3 - 5 , with the compressed spring 8 therebetween , the compressed spring 8 urges the second and the third lenses 2 and 3 in such a direction that these two lenses become distant from each other . thus , based on an accumulated size difference d = b − σ ( c li + e si ) generated by the expansion of the plastic lens chamber 7 , the lenses 1 - 5 and the spacers 10 , the distance between the second and the third lenses 2 and 3 is increased and the back focus a of the lens barrel is shortened . therefore , at a high temperature , the part of the back focus a of the lens barrel , which becomes longer due to a change of the refractive index of the second and the third lenses 2 and 3 , is corrected by increasing the distance between the second and the third lenses 2 and 3 on the basis of the accumulated size difference generated by the expansion of the plastic lens chamber 7 , the lenses 1 - 5 and the spacers 10 , thereby suppressing a variation in the back focus a . further , at a low temperature , the change of the refractive index of the second and the third lenses 2 and 3 and the change of the distance between the second and the third lenses 2 and 3 can achieve the same effect by means of an action reverse to the above action at the high temperature . to make a further generalization , the contraction or expansion amount of the size of each component caused by a temperature change is quite little , however , with respect to the change in the entire length b of the lens barrel , the variation in the back focus a of the lens barrel is dealt with by increasing or decreasing the space between the second and the third lenses 2 and 3 . the optical system is designed such that a variation in the back focus a can be realized by means of a change in the space between the second and the third lenses 2 and 3 , thereby , a variation in the back focus a caused by a temperature change can be effectively corrected . a brief explanation of the change of the refractive index of the lens will be made here . the material of the lens has a property that the refractive index of the lens becomes smaller when the temperature increases and becomes greater when the temperature decreases . the sensitivity of the refractive index about the temperature varies with the material of the lens . in particular , the refractive index of the plastic lens is very sensitive to temperature , so the influence of the temperature change on the back focus is great . the second and the third lenses 2 and 3 in the lens barrel of the present invention are plastic lenses . the second lens 2 has negative refractive power , so the back focus becomes shorter / longer upon an increase / a decrease in temperature . further , the third lens 3 has positive refractive power , so the back focus becomes longer / shorter upon an increase / a decrease in temperature . as an absolute value of the refractive power of the second lens 2 is smaller than that of the third lens 3 , the back focus a of the lens barrel of the present invention becomes longer as the temperature increases and becomes shorter as the temperature decreases . furthermore , as stated above , the sensitivity of the refractive index of the lens about the temperature varies with the material of the lens , so it is possible to reduce the influence of the temperature on the variation in the back focus a of the lens barrel by making the second lens 2 from a material which has a refractive index more sensitive to the temperature than the refractive index of the material of the third lens 3 . thus , if this method is combined with the aforementioned method in which the back focus variation is reduced by changing the space between the lenses using the thermal expansion or contraction of the lens chamber , it is possible to suppress the variation in the back focus very effectively regarding a temperature change of a wider range . the following is a description of a second embodiment of the present invention with reference to fig5 - 8 . for the components whose functions in the second embodiment are the same as those in the first embodiment , the same reference signs are used , and detailed descriptions of these components will not be given unless there is such a need . the second embodiment is the same as the first embodiment , expect that the compressed spring 8 is replaced by a circular plate spring 8 . by employing the plate spring 8 which is thinner than the compressed spring 8 ( coil spring ) in the first embodiment , the space in the optical axis direction can be reduced and thus downsizing of the lens barrel can be further achieved . the following is a description of a third embodiment of the present invention with reference to fig9 and 10 . for the components whose functions in the third embodiment are the same as those in the second embodiment , the same reference signs are used , and detailed descriptions of these components will not be given unless there is such a need . in the third embodiment , the second and the third lenses 2 and 3 , which are formed of plastic materials having the same or substantially the same linear expansion coefficient , have a structure of fitting into each other at an outer side of the plate spring 8 , such that the second and the third lenses 2 and 3 cannot be moved relative to each other in the radial direction thereof . when only the first and the second embodiments are adopted , although a variation in the focal position of the lens barrel can be effectively suppressed by increasing or decreasing the space between the second and the third lenses 2 and 3 in the optical axis direction , a core shift phenomenon between the lenses caused by a change in the sizes of the lenses in the radial direction due to a temperature change may be inevitable . however , with the fitting structure in the third embodiment , the second and the third lenses 2 and 3 are held in such a manner that they cannot be moved relative to each other in the radial direction thereof , so the above - mentioned core shift phenomenon can be effectively avoided . there is no particular limitation as to how the second and the third lenses 2 and 3 are fitted into each other , as long as it can be guaranteed that the second and the third lenses 2 and 3 cannot be moved relative to each other in the radial direction thereof . furthermore , in the third embodiment , the location of the parallel spacer 9 is changed and a sixth lens 12 is added , but these are mere modifications that are made according to the actual situation of the product and make no direct impact on the above effect of the third embodiment . to sum up , the lens barrel of the present invention does not need any mechanism that mechanically corrects the back focus of the lens barrel and a relative position of the image sensor , so it can effectively suppress a variation in the back focus of the lens barrel at different temperatures and can thereby maintain excellent optical property , without causing an increase in the size and the cost of the lens barrel . the present invention is a photographing lens barrel that can be used in cameras , video cameras and cell phones .