As in most of industrial cases, in Spark-Ignition (SI) engines, the expansion of premixed flames is strongly affected by turbulence flow fields. However, as the flame radius and curvature are non-negligible during the combustion development, global stretch effect can drastically change the turbulent flame propagation speed. In this paper, spherical turbulent premixed flames are studied inside a constant-volume vessel for a wide range of air-isooctane mixture and initial turbulence by using simultaneously Mie scattering tomography and 2-views Schlieren techniques. With the 2-views Schlieren diagnostic, the flame volume can be reconstructed and a more 'real' flame radius can be determined. The comparison between this radius, which is more usually determined by only 1-view Schlieren, and the one obtained by tomography was possible, and enabled to select non-conveyed flames with a global spherical shape. In the first part, the values of the correction factor for Schlieren images, first introduced by Bradley et al. in 2003, is discussed as function of the turbulent intensity and initial pressure. In the second part, the effect of turbulent intensity and initial pressure for isooctane/air mixtures on flame propagation speed evolution is evaluated as function of the stretch rate.