Barring contacts, a good lens designer doesn’t have many parameters that can be traded off to improve vision. The index has little effect on error. Note that, although chromatic aberration is often perceived as "blurry vision" in the lens periphery and gives the impression of power error, this is actually due to color shifting. Chromatic aberration can be improved by using a material with improved ABBE. The best way to combat lens induced power error is to limit the choice of corrective lens to one that is in the best spherical form. A lens designer determines the best-form spherical curve using the Oswalt curve on the Tscherning ellipse. This design gives the best achievable optical quality and least sensitivity to lens fitting. A flatter base-curve is sometimes selected for cosmetic reasons. Aspheric or atoric design can reduce errors induced by using a suboptimal flatter base-curve. They cannot surpass the optical quality of a spherical best-form lens but can reduce the error induced by using a flatter-than-optimal base curve. The improvement due to flattening is most evident for strong farsighted lenses. High myopes (-6D) may see a slight cosmetic benefit with larger lenses. Mild prescriptions will have no perceptible benefit (-2D). Even at high prescriptions, some high myope prescriptions with small lenses may not see any difference, since some aspheric lenses have a spherically designed center area for improved vision and fit.