The observed emission-line ratios of the extended emission-line regions of Cen A and 3C 227 are compared with the theoretical results from photoionization models, assuming a power-law spectrum for the nuclear ionizing radiation. We show that a variation of the optical depth of the emitting gas can mimic a variation of the ionization parameter. Most of the observational data are explained if each subregion in the extended emission-line gas is characterized by a different value of the optical depth and shows a small density contrast between a 'cloud' and a more dilute gas. However, the observed forbidden O III line ratio cannot be reproduced by photoionization, and an additional heating mechanisms is required. Solar abundances give the best fit to the observations. However, for most subregions the (N/O) ratio is between solar and half solar.