Accommodation and presbyopia in the human eye. 1: Evaluation of in vivo measurement techniques.

As part of a cross-sectional study of aging of the human accommodative apparatus, unaccommodated anterior chamber depth, unaccommodated lens thickness, and accommodative amplitude were each determined by two or more independent techniques in 100 normal emmetropic subjects, age 18-70 yr. For anterior chamber depth, the order of accuracy seemed to be: optical pachymetry > slit-lamp Scheimpflug photography > Ascan ultrasonography, although there was good agreement among all three methods. For lens thickness, slitlamp Scheimpflug photography seemed more accurate than A-scan ultrasonography when the generally accepted lenticular sound velocity of 1641 m . s(-1) was used for the ultrasonographic calculation. The agedependent divergence allowed calculation of a putative relationship between lenticular sound velocity and age, indicating that velocity decreased by ~3 m . s(-1) . yr(-1). Accommodative amplitude declined with age, reaching a minimum, by age 50 yr, of 0.5 diopters as measured by coincidence refractometry. This residual 0.5 diopters may represent the inherent focal depth of the eye's optical system, rather than active accommodation. Maximum accommodative amplitude based on the subject's ability to clearly visualize a target through minus lenses was always 1.50 diopters greater than the refractometrically measured maximum, regardless of age. This difference presumably represents increased depth of field provided by pupillary constriction, rather than active accommodation.