At present the first-order parameter yni is used to quantify the focusing of retro-reflected rays on the detector plane, which causes narcissus effect. The shortage is that yni is not refrenced to the lens data and system data so it′s not useful to guide narcissus optimization. A new paramteroptical defocusing is defined to quantify the defocusing of retro-reflected rays, the relation between optical focusing and lens surface radius, aperture, position of intermediate image, focal length of lens before the surface and field of view is presented. Optical methods to minimize narcissus effect is summerized. The narcissus of a long-wavelength cooling infrared system is analysed with optical defocusing and the narcissus induced equivalent tempareture difference (NITD) is caculated. The major NITD contributing surfaces is optimised after which the NITD of zooming group drops by 50%, and that of fixed lens by 75%. The results demonstrate that optical defocusing can guide narcissus optimization effectively.