This paper clarifies the relationship between image quality and accommodation in human vision through two types of experiments. One examines how image-quality degradation influences the accommodation mechanism, and the other examines which type of information of quality-degraded images activates the accommodation mechanism. Actually, accommodative responses are measured using an infrared optometer while subjects are subjectively evaluating sharpness, noise, and pseudo-contours and while they are observing sine, square, and missing fundamental (MF) square waves. The following results were derived: (1) the accommodation lag increases as the degree of sharpness is degraded regardless of the tone-reproducing methods; (2) the accommodation lag decreases considerably in the existence of noise or pseudo-contours, whereas it increases for uniform or gently curved planes; (3) the spatial features of presented images activate the accommodation mechanism. These results suggest that accommodative responses influence human subjective judgments as well as being a human factor related closely to image quality and that the spatial features of quality-degraded images underlie human subjective judgments. In other words, they imply that objective image-quality evaluation metrics should satisfy the following two conditions: the incorporation of the accommodation characteristics into such metrics and the formulation of such metrics in the spatial region.