To investigate the effects of phosphene array irregularity in visual prostheses, a model of phosphenes' positional uncertainty was devised and two different image down-sampling schemes, adapting and non-adapting, were proposed. Based on a simulation system, visual acuity tests were given to four normally sighted subjects under seven degrees of array irregularity and the two down-sampling schemes. With the irregularity index increasing from 0 to 0.6, visual acuity fell over a range of 0.22 logMAR (logarithm of minimum angular resolution) for the adapting scheme and 0.47 logMAR for the non-adapting scheme, both monotonously. Comparison between the two down-sampling schemes was made. For low array irregularity, non-adapting down-sampling afforded higher visual acuity, whereas for higher irregular the adapting approach did. Head movements were observed to play significant roles in the acuity tests.