In a fiber-optic link, the continuous semiclassical and discrete photon statistics models are widely used to characterize the output probability distribution of an optical amplifier. The semiclassical model describes the output as a chi-square distribution while the discrete photon statistics model describes it through the noncentral negative binomial (NNB) distribution. In this paper, we present a way of understanding the discrete photon statistical distribution at the output of the amplifier and the convergence of the same to its semiclassical counter part. We highlight this convergence through the relation between the NNB and chi-square distribution demonstrated through deviation analysis and simulations. The penalty incurred in bit error rate analysis due to semiclassical approximation over NNB is also discussed. © 2011 Optical Society of America.