Accommodation responses to sinusoidally modulated vertical gratings, at a vergence of -5.0 D, were measured over a 10 min period. Three spatial frequencies were employed: 1.67, 5.0 and 15 cycles degree-1. The results show that 1.67 and 5.0 cycles degree-1 elicit good accommodation responses which are temporally stable. All five subjects employed were unable to maintain an accurate accommodation response to a 15 cycles degree-1 grating. Consideration of the changes occurring in the retinal modulation with accommodative error indicates that spatial frequencies around 5.0 cycles degree-1 are the most useful for accommodation.