Measuring fundus landmarks.

Three methods of determining the actual size of objects on the living fundus were utilized to measure the horizontal disc diameter (H) in 68 volunteers. H1 was calculated from fundus photographs based on the refractive power of the eye and according to the formula of Bengtsson and Krakau. H2 was derived from Littmann's trigonometric curves based on the anterior corneal curvature and the refractive power of the eye. H3 was derived from Littmann's trigonometric curves based on the axial length and the refractive power of the eye. H1 underestimated H2 by a mean of 0.91% (S.D. 2.19%) in 68 eyes. H3 underestimated H2 by a mean of 3.6% (S.D. 4.0%). We concluded that H1 was an acceptable approximation of H2 in a clinical setting and retrospective studies.