Ocular spherical aberration and theoretical through-focus modulation transfer functions calculated for eyes fitted with two types of varifocal presbyopic contact lens.
Through focus modulation transfer functions (MTFs) are compared for six different cases: an aberration free eye, an eye suffering from typical amounts of undercorrected spherical aberration, and each of such eyes wearing either a Hydrocurve II or PS45 varifocal contact lens. It is assumed that the measured power profiles of the varifocal lenses, which are typical of centre distance and centre-near designs, can be approximated to primary spherical aberration, that the lenses are well centred, and that the spherical aberrations of each lens and eye are additive. It is found that in general, with a 4 mm pupil, reasonable modulation transfer is maintained over a greater focal range (by about 1 dioptre) with the varifocal-wearing eyes. This implies that useful vision will be given over a greater range of object distances, as required by the presbyope. However, the maximal modulation transfer with a varifocal is degraded compared with that of the eye alone. Since the amount and sign of ocular spherical aberration may vary with the individual, it would be expected that any particular design of varifocal would be more successful with some wearers than with others.
Plakitsi
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Optometry and Vision Sciences, UMIST, PO Box 88, Manchester M60 1QD, UK.
Charman
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