Editor,—Garway-Heath et al described a “keratometry and ametropia” method to correct measurements of optic disc size for ocular magnification.1The new method implies that the refraction, the power of the lens, and the power of the cornea are all independent (uncorrelated) variables. Table 2 on page 644 (Summary of ocular biometry) clearly demonstrates that this is not always the case; in fact, the variance of the lens power was almost the same as the variance of the total power of the eye. The explanation has to be that the power of the lens and cornea were negatively correlated. Measurements of the corneal curvature were therefore of little use for their purpose. Garway-Heathet al noted that the improvement over the use of uncorrected measurements was moderate, but they failed to draw the obvious conclusion: if correction is necessary, and correction based on spectacle refraction is considered unsatisfactory, correction based on measurements of the axial length of the eye is the only alternative—and quite feasible in these days when ultrasound biometry is used to predict intraocular lens power …