Usually defocus is considered as the error signal in the negative feedback system of accommodation control. However it is difficult to estimate the amount of defocus from the contrast reduction of a retinal image because both defocus and the spatial structure of the target determine the contrast. To investigate whether the accommodation control system can perceive the amount of error signal (defocus) from the contrast reduction of a retinal image, step responses for various amplitudes of stimuli were measured. The traces were analysed using an exponential curve fitting method in which not only time constant, but also final value of the curve, could be estimated from a portion of the exponential curve. The near-to-far (or -intermediate) step responses have a common path and show that the curve initially goes towards a certain position around the dark focus of the subject and is suddenly clipped around the correct focus position. The time constant and the destination of the curve are independent from the stimulus step amplitude (amount of error). This fact suggests that the accommodative control system does not refer to the amount of error signal when error is large, and that the system monitors only whether retinal blur exists or not. When the system judges the retinal image to be no longer blurred during the preprogrammed fast movement, accommodative response is stopped suddenly and the control is transferred to the other mechanism to hold correct focusing.