Coupling ratio between an on-axis and off-axis source through an off-axis microlens as a function of microlens diameter for a multimode and single-mode fiber. The gain in contrast by using a single-mode fiber can readily be seen in the ratio of these two functions. For most microlens diameters this gain amounts to several orders of magnitude and reaches infinity at ∼1.28λ/D where the light is perfectly nulled on the fiber face by the single-mode fiber. This nulling is first order and is therefore very sensitive to wavelength and centering of the star around the central lenslet.