Myopes have an altered accommodation response compared to non-myopes. We examined the possible optical causes of this effect by developing theoretical models of modulation transfer functions in the presence of spherical aberration and spherical defocus. In addition, we compared the contrast sensitivity functions of myopes and non-myopes under conditions of defocus, and analysed the contrast sensitivity decrease. We found that the introduction of defocus led to a significantly different contrast sensitivity response in myopes compared to non-myopes. This result was compared to possible difference in levels of spherical aberration in these two groups. The possible interactions of spherical aberration and defocus will be discussed, with emphasis on the results of recent measurements of the aberrations of myopic and emmetropic eyes.