The working principle of curtain type shutter in an aerial camera is introduced and the relationship between sensitive characteristics and resolution for a film is analyzed. Then, expression of the best exposure is deduced by the parameters of photosensitive material. Finally, expression of the slit width is worked out by calculated expression of the exposure time. According to mapping EK3412, the best exposure is 0.073,0.098,0.116 and 0.136 lx·s ,when the image speed is 10,15,20 and 25 frames/s. After flying experimental verification, the resolution of the film can be increased by 10%~20%,which is closed to the theoretical resolution of 130 lpm. By controlling exposure amount to the mid-point on straight line of the characteristic curve, the resolution of photo be can significantly improved to meet the requirements of image interpretation.