We analyzed the aspheric and/or diffractive surface effects to the performance in the long wavelength infrared (8-12 $mu$m). Also we investigated the dependence of the NA values for the fixed effective focal length 100 mm when the field angle was varied from 5 degrees to 30 degrees stepped by 5 degrees. We chose the merit function as a criteria to compare the performance of the different lenses. Based on the analysis of the aspheric and/or diffractive surface effects, we designed the optical system of F/l.0 for the uncooled thermal imaging system. As for detector the pixel size was 45 $mu$m square and the number of pixels were a 320${ imes}$240 pixels.