Astigmatic dial technique. A, Conoid of Sturm and retinal image of an astigmatic dial as viewed by an eye with compound hyperopic astigmatism. B, Fogging to produce compound myopic astigmatism. C, The conoid of Sturm is collapsed to a single point. D, Minus sphere is added (or plus sphere subtracted) to produce a sharp image, and a visual acuity chart is used for viewing.