A method for reliably determining the axial length of an eye uses optical coherence tomography (OCT), where the eye is aligned with a fixation mark so that the optical axis of the measuring instrument coincides at least approximately with a visual axis of the eye. The axial length is determined from at least one B-scan taken in an initial AS mode (anterior segment mode) and the axial length is also determined from at least one B-scan taken in a second RS mode (retina segment mode). A resultant reliable axial length of the eye is determined using the axial lengths from the AS and RS modes, where available and where possible, or the process is ended without a resultant axial length.
