 Also, one would be very hard pressed to find a laboratory that would be willing to acquire or shape custom eyeglass lenses from these materials, considering that such an order would most likely consist of just two different lenses that are specific to the wearer.In general, Vd values above that of Crown Glass and CR-39 are of dubious value, except in combinations of extreme prescriptions, very large lens sizes that cover a good portion of the face, high wearer sensitivity to dispersion, and occupations that involve work with very high contrast elements (e.g.reading dark print on very bright white paper, construction involving contrast of (dark) building elements against a cloudy white sky, a workplace with recessed can or other concentrated small area lighting shining on very bright white surfaces, etc.