Examinations for spectacles involve much more than ophthalmometric and retinoscopic measurements. The refractionist who relies too strictly on such measurements will make many mistakes. The ophthalmometer measures the paracentral and not the central area of the cornea. Away from the center the corneal curvature often tends to become parabolic or ellipsoid rather than spherical. Retinoscopy also suffers from inaccuracies. Refraction may vary through different parts of the pupil. The nonmacular portions of the retina are more readily illuminated than the macula. The area of illumination usually is as large as the macula and therefore much larger than the fovea, which is a depression within the macula and which may differ somewhat from the macula in refraction. This would explain, among other things, why some eyes require appreciably more minus spherical power than is indicated by the results of skiaskopic examination.
Objective measurements being subject to error, considerable reliance must be
EGGERS H. TWO NEW VISUAL TEST CHARTS. Arch Ophthalmol. 1941;26(1):71–78. doi:10.1001/archopht.1941.00870130087009