In certain instances, the contrast of a photographic transparency needs to be altered to recognize some information from Lhe everpresent background of its image. For example, the contrast of aerial photographs often needs to be reduced,whereas the contrast of X-ray photographs requires an increase.A optical processing system with coherent feedback which was reported by D.P.Jablonowski and S.H.Lee [1,2] is an important device to realize contrast control. To accomplish optical feedback, partially reflecting mirrors and ordinary lenses have been utilized. However, it is difficult to manufacture the optical elements of high quality so that the sizes of the irages processed have been limited.Besides,the input photographs need to be put in a liquid gate to eliminate its phase noises, which increase the complexity of the process. In this paper,a new coherent optical system with phase conjuate feedback using four wave mixing (FWM) with photorefractive crystals is represented, which can be utilized to control the contrast of the optical images of input transparencies in real-time. The feedback can remove the effects of the phase noises of the system on output images. Some results obtained from computer-simulations and experiment are in acreement with our theoretical analyses.