The quatum-statistical properties of an absorptive optical system driven by a partially coherent field are investigated. As the coherent component of the input field increases from zero, the intensity fluctuation of the output light may be slightly enhanced, i.e., the output field becomes less coherent as the input field becomes more coherent. This abnormal behaviour is caused by the phase fluctuation of the input light. In the low noise limit, the location,value of the maximum of the second-order correlation function for the input field are given analytically by means of the Gaussian approximation. In the Jiigh noise limit, an explicit expression of the second-order correlation function is also obtained.