The present study introduces an experimental technique based on a Free Surface-Synthetic Schlieren (FS-SS) method in order to characterize free surfaces subjected to strong deformations. Current synthetic Schlieren methods are based on local image correlation and thus limited to rather weak image deformations, implying that they can only resolve rather large surface wavelengths and limited wave amplitude. The present method is a substantial improvement that allows to measure much stronger image deformations, providing access to shorter surface wavelengths and larger amplitudes (i.e. larger surface curvatures).