According to some studies, refractive error in adults is not significantly associated with crystalline lens power,
30,33 while others reported a negative correlation, with higher lens powers in myopes.
15,29 This is in contrast to the positive correlation found in children's eyes, with hypermetropes having higher refractive lens powers than emmetropes or myopes,
34,35 while the current study suggests that emmetropic eyes have significantly higher lens powers than in high myopes or hypermetropes. The positive correlation between lens power and spherical refraction in myopes also matches the known association between lower lens power and increasing axial length.
15,18,29,30,34,36 A myopic eye is usually longer, thus requiring a weaker lens to emmetropize, while a hypermetropic eye is typically shorter, requiring a stronger lens. At a certain age the correlation between spherical refraction and lens refractive power changes from positive to negative, which Iribarren
17 explained through the hypermetropic shift that occurs in adulthood. As the prevalence of hypermetropia increases with age
37 while the lens power decreases, the group of hypermetropes will consist of a mixture of newly developed hypermetropes with low lens power and a small portion of persistent hypermetropes with a higher lens power.
17 This new balance could reverse the correlation between lens power and refraction, and would explain why in the present study lens power is lower in the hypermetropic group in comparison with the emmetropic group. Still, our study shows higher lens powers in emmetropic subjects than in myopic subjects, similar to the correlation found in childhood. This could be explained by the fact that cataractous lenses were not included in this study, which would have increased lens power in older age due to a cataract-induced myopic shift. Through exclusion of cataractous lenses, the myopic subpopulation consisted largely of myopes since childhood, thus having a lower lens power. This is in contrast with the Reykjavik Eye Study
29 and the CIEMS study,
15 where lens power calculations were based on adults over the age of 50 and where cataractous lenses were not excluded.