With the development of science and technology over time,infrared related technologies are becoming increasingly mature,and many new technologies and theories have been widely used. Now the single f number zoom infrared optical system has been developed to the direction of multi F number. For the optical system with fixed F number,the aperture of through light needs to be changed when the focal length is changed,so the aperture of through light will be reduced when the field of view is large and the energy utilization rate is low. Variable F can solve this problem,several optical design in the case of keeping the aperture optical system can be wide field and inspects a switch,make full use of the system of the chase light aperture,improve energy utilization efficiency,this article introduces the F number of related design,and puts forward some design ideas.