We are grateful for the important comments regarding magnification factors and ocular fundus measurements brought up by Dr Lempert. We would, however, like to correct the figures quoted by Dr Lempert; the mean areas were for controls 2.82 mm2 and for children with periventricular leukomalacia (PVL) 2.79 mm2. Magnification of fundus photographs may be assessed from (1) axial length, (2) refraction, or (3) refraction and the corneal curvature. In a telecentric system the magnification (M) is based on vergence of the internal axis, M=k × B (where k is a constant and B is the "reduzierte Axenlängenkonvergenz" as defined by Gullstrand1). The refraction and corneal curvature can be used to estimate the axis when axial length measurements are unavailable. In the correction techniques of Bengtsson and Krakau and Littman, B was estimated by formulae that included measurements of refraction and corneal curvature.
Jacobson L, Hellström A. Optic Nerve Hypoplasia Diagnosis Should Be Confirmed by Accurate Measurements—Reply. Arch Ophthalmol. 1998;116(6):831-832. doi: