Defocus correction in the optical system of the eye: unconventional degrees of freedom.

Despite the interest in developing improved formulas for intraocular lens power calculation, there are several sources of uncertainty that may well give rise to a significant residual refractive error. Those concerning the estimation of the corneal power are reviewed. In addition, we explore the possibility of introducing changes in some unconventional parameters of the eye to compensate for defocus and illustrate their effectiveness in two cases: a natural eye and an eye that has undergone previous surgical actions (anterior refractive surgery and cataract surgery with an intraocular lens implant). The results show that changes in the refractive index, thickness, or posterior radius of the cornea have relatively little effect on the overall refractive error. However, small changes in the refractive indexes of the aqueous or the vitreous humors are highly effective, much more so than a similar amount of change in the anterior curvature of the cornea. This fact opens new and attractive possibilities to compensate for refractive error through the introduction of changes in degrees of freedom so far considered unconventional.