The depth of focus of lenses employed in laser cutting can be extended using aspheric overcorrection. This allows the depth of focus to be enhanced without the need for longer working distance optics. However, as with conventional lenses, the performance of such lenses will depend on the mode quality of the laser machining system. Comparative trials on a range of zinc selenide aspheric lenses at CO2 wavelength, machining stainless steel under inert gas conditions, reveal that the surface finish of the cut edge, is strongly dependent on choice of laser system. The work underlines the urgent need for laser beam measurement techniques which can study the wavefront quality, as well as the beam intensity profile. A measurement technique which samples typically 1 X 10-4 of the main beam is described, which allows non invasive, in-line measurement of the beam intensity, and the wavefront. This provides sufficient information to predict and model the final image quality that is associated with any combination of lens design and laser source.