A contact lens has a central region and an annular region surrounding the central region, the central region having a first optical power and the annular region having a second optical power. In a method of measuring the add power, the wavefront aberration of the contact lens is measured; a polynomial is fitted to the measured wavefront aberration. Terms of the polynomial correspond to types of aberration comprising defocus and astigmatism and residual aberrations. The add power is calculated as a defocus corresponding to the residual aberrations.