Minus cylinder lenses are prescribed because of
Aniseikonia is defined as a condition wherein, images projected to visual coex from the two retinae are abnormally unequal in size and/or shape. Up to 5% aniseikonia is well tolerated. Meridional aniseikonia - refractive differences only occur in one meridian In a plus cylinder the lens is ground on the anterior surface of spectacle and thus there is more scattering and distoion of image as compared to a minus cylinder which is ground on posterior surface. Thus a minus cylinder is more comfoable to the patient and hence all prescriptions are written with a minus cylinder. Extra edge: - In pt with monocular aphakia wearing contact lens, % of aniseikonia is approximately 8% - In aphakia, Aniseikonia is decreased with contact lens than glasses. Hence, IOL implantation is preferred management NOTE- Aneisokonia with IOL- (1-3%)
Cheaper to produce and distribute
Preferred due to meridional aniseikonia
Reduces minification
Causes less coma and trefoil
Optics and Errors of Refraction