A method of determining a geometrical measurement of a retina of an eye, comprising obtaining a two dimensional representation of at least a portion of the retina of the eye (34), deriving a geometrical remapping which converts the two dimensional representation of the retinal portion to a three dimensional representation of the retinal portion (36), using one or more coordinates of the two dimensional representation of the retinal portion to define the geometrical measurement to be taken of the retina on the two dimensional representation (38), using the geometrical remapping to convert the or each coordinate of the two dimensional representation of the retinal portion to an equivalent coordinate of the three dimensional representation of the retinal portion (40), and using the or each equivalent coordinate of the three dimensional representation of the retinal portion to determine the geometrical measurement of the retina of the eye (42).