Unequal reduction in visual acuity with positive and negative defocusing lenses in myopes.

Myopes have a reduced accommodative response to negative lenses compared with nonmyopes. Mathematical models predict that the reduced accommodative response is due to a decrease in sensitivity to blur in myopes. We examined the effect of blur induced by positive and negative defocusing lenses on visual acuity in 12 myopes and 12 nonmyopes during cycloplegia for up to +/-3.00 D of defocus in 0.25 D steps. Although nonmyopes showed a symmetrical reduction in visual acuity with positive and negative lenses, the myopic group showed less acuity loss with negative lenses compared with positive lenses. The magnitude of visual acuity loss was lower with negative lenses in myopes compared with nonmyopes. No significant difference in visual acuity with positive lenses was found between myopes and nonmyopes. Residual accommodation (after cycloplegia) was about 0.20 D in both myopes and nonmyopes and was too small to explain the relatively good visual acuity through minus lenses in the myopic group. The reduced accommodative response known to occur in myopes may be due to the relatively small effect that negative lens blur has on their visual acuity.