Due to absence of quantitative and evolutionary view on flame-front's wrinkling morphonology, descriptions upon instability onset of premixed expanding spherical flames in previous literatures are implicit and inconsistent. This paper utilized shadowgraphy and high-magnification lens in the optical configuration, which clearly manifested cracks in wrinkled flame images and facilitated precise segmentation of them. A simple algorithm was developed to reconstruct real lengths of cracks from these planar projected images. Furthermore, a meaningful parameter, the averaged cell radius, was deducted from the total crack length. Application of this novel methodology on the near-equidiffusive C2H2-air flame reveals similar growth speeds of flame cracks under different pressures before the flame front transits into cellular appearance. Before this transition, the averaged cell radius kept fluctuating around a roughly stable level which was intimately dependent on initial disturbances. Once the transition started, the effect of inherent instability dominates over initial disturbances and averaged cell radii of repeated tests gradually converged to a similar value. The above methodology builds up an automatic way to quantitatively characterizing cracks and cells' developing history, which extracts more useful and precise information from image series than merely manual estimation of cell sizes at individual shoots in previous studies. (C) 2019 Elsevier Ltd. All rights reserved.