The chick is commonly used as an animal model for human myopia and emmetropization. However one criticism of this model has been the very large refractive errors that are usually induced, either by visual deprivation or through the use of lenses to first impose focussing errors. This study sought to determine the threshold for compensatory responses to imposed defocus. We observed compensation in chick eyes fitted at hatching with +1D and -1D lenses (one over each eye). Consistent with the 2D interocular difference in refraction imposed, an average of 2.13D (SD 1.0D) of anisometropia and a 0.09 mm (SD 0.05 mm) interocular difference in vitreous chamber depth were recorded after five days of lens wear. The 1 D focusing errors imposed on individual eyes are not much greater in magnitude than the estimated depth of focus of the chick eye. The significance of these results for emmetropization are discussed.