We present real-time measurements of the wave front distortion induced by a variable focal lens. This lens, called Varioptic, is made of a transparent cell filled with twin liquids. We submit a 4.5mm in diameter lens upon a 110V voltage step inducing a optical power shift of about 25 dioptries (m-1) . Characteristic response time is shown to be of the order of a few 1/100s, the lens recovering its full quality after 5/100s. We present a scaling analysis of this response time versus lens size.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.