Optimization of lithographic process plays a pivotal role in modern fabrication. Many parameters are engaged in optimizing optical systems used in lithographic process. In this paper, we show with a fixed rule for resolution enhancement techniques (RETs) we have an optimum value for image quality by definition of figure of merit (FOM). We find out how we are close to replicate a desired target mask on photoresist and derive an analytical formula for FOM versus numerical aperture (NA). The effect of variation in NA on quality of image that will be on photoresist is also shown.