Ringing Artifacts are the most challenging problem in image restoration. However, the source of ringing is still in suspense. This paper investigates the source of ringing by focusing on the mathematical principle of deconvolution with the degraded model. From continuous sampling to undersampling, the essential causes of ringing appearance are discussed through theoretical derivation and quantitative analysis. Based upon the discussion above, the optimal deconvolution for electro-optical imaging systems is put forward to achieve ideal balance between ringing suppression and sharpness enhancement. Comparison results show the superiority of the proposed optimal filters.