A method of determining an optimum position for a replacement intraocular lens based on a position of the natural lens, said natural lens having an anterior surface and a posterior surface, the method comprising: modelling a topography of the anterior surface and a topography of a posterior surface of the natural lens; and determining the position of the intraocular lens based on the modelled topography wherein modelling the topography of the anterior surface and the topography of the posterior surface of the natural lens comprises: measuring the anterior shape of the eye's cornea; determining direct optical measurements of at least one parameter of the cornea of the eye and at least one parameter of the lens of the eye; determining the refractive index of the cornea; characterized in that the method further comprises: correcting the optical measurements to account for the effect of the refractive index of the cornea on the direct optical measurements; measuring the aberration of the eye; calculating the refractive index of the lens by combining the corrected measurements and the aberration; and further correcting the optical measurements of the lens to account the effect of the refractive index of the lens on the direct optical measurements and combining corrected values to model the topography of the anterior surface and the topography of the posterior surface of the natural lens. modelling a topography of the anterior surface and a topography of a posterior surface of the natural lens; and determining the position of the intraocular lens based on the modelled topography wherein modelling the topography of the anterior surface and the topography of the posterior surface of the natural lens comprises: measuring the anterior shape of the eye's cornea; determining direct optical measurements of at least one parameter of the cornea of the eye and at least one parameter of the lens of the eye; determining the refractive index of the cornea; characterized in that the method further comprises: correcting the optical measurements to account for the effect of the refractive index of the cornea on the direct optical measurements; measuring the aberration of the eye; calculating the refractive index of the lens by combining the corrected measurements and the aberration; and further correcting the optical measurements of the lens to account the effect of the refractive index of the lens on the direct optical measurements and combining corrected values to model the topography of the anterior surface and the topography of the posterior surface of the natural lens. measuring the anterior shape of the eye's cornea; determining direct optical measurements of at least one parameter of the cornea of the eye and at least one parameter of the lens of the eye; determining the refractive index of the cornea; characterized in that the method further comprises: correcting the optical measurements to account for the effect of the refractive index of the cornea on the direct optical measurements; measuring the aberration of the eye; calculating the refractive index of the lens by combining the corrected measurements and the aberration; and further correcting the optical measurements of the lens to account the effect of the refractive index of the lens on the direct optical measurements and combining corrected values to model the topography of the anterior surface and the topography of the posterior surface of the natural lens. correcting the optical measurements to account for the effect of the refractive index of the cornea on the direct optical measurements; measuring the aberration of the eye; calculating the refractive index of the lens by combining the corrected measurements and the aberration; and further correcting the optical measurements of the lens to account the effect of the refractive index of the lens on the direct optical measurements and combining corrected values to model the topography of the anterior surface and the topography of the posterior surface of the natural lens.