An ophthalmic refractor is described that provides a non-linear relationship between spherical power and refractive error by positioning a reference plane for a sensor system of the refractor in front of the cornea. The refractor may have a working distance and dynamic range that enable it to be mounted on a surgical microscope. Also described are computational methods for analysing the output from the ophthalmic refractor, utilising error minimisation, linear regression and Fourier Transform analysis.
