We determine the Wigner distribution of a laser experimentally from intensity profiles obtained by moving slit technology. In order to find out whether the phase calculated from a measured Wigner distribution can be trusted, we add a known spherical aberration to a Helium Noen laser by a plan-convex collimating lens orientated "the wrong way." The magnitude of the aberration can be influenced by the beam diameter at the lens. The M<sup>2</sup>-values calculated from the Wigner distribution of the aberrated beam at different levels of aberration is in agreement with theory. The coefficient of spherical aberration obtained from the measurement agrees with the one predicted by aberration theory if the impact of the aberration on the beam profiles is large enough (M<sup>2</sup> > 1.2). The Wigner distribution of a tapered semiconductor laser is also examined. The measured phase at the semiconductor facet is aberrated. The aberration however can not be identified to origin from exit of the beam from high index semiconductor to air through a planar interface.