It has long been recognised that chromatic aberration can introduce luminance artifacts into nominally isoluminant colour stimuli. In this study the effects of chromatic aberration (along with those of defocus and stimulus spatial frequency) on the chromaticity of the retinal image are considered. Such optical effects have important methodological and functional implications for visual physiology. The "Silent Substitution" principle is a fundamental feature of modern colorimetry, being employed in both psychophysical and electrophysiological approaches to the visual system. The theoretical colour spaces introduced by MacLeod and Boynton (1979) and Derrington et al. (1984) are also ultimately based on this principle. All such applications of the silent substitution principle are sensitive to the optical effects of chromatic aberration, defocus, spatial frequency and stimulus chromaticity. The spatial acuity of the mechanisms of colour vision are appreciably lower than those of the luminance system (Mullen, 1985). In addition chromatic aberration has been shown to be a cue to ocular accommodation (Fincham, 1951). The analysis presented in this study suggests a possible explanation for these findings in terms of the ecological and computational constraints placed on the visual system by chromatic aberration.