THE RELATION BETWEEN CORNEAL AND TOTAL ASTIGMATISM

Using computer-assisted videokeratoscopy we measured corneal astigmati sm and compared these results over a range of corneal zone diameters w ith total ocular astigmatism derived by subjective refraction. Videoke ratoscopes permit a more detailed analysis of the power distribution w ithin a given corneal surface area, enabling comparison to the total a stigmatism for equivalent aperture sizes. Although there were signific ant individual variations, the group average data supports the traditi onal view of a linear relation between corneal and total astigmatism. This was true across the range of apertures tested from 2 to 7 mm, wit h the coordinates of the relation being consistent with that of the mo dified Javal's rule; namely a slope of 1 and an intercept of approxima tely 0.50 D against-the-rule residual astigmatism.