In solar adaptive optics system, the correlation algorithm is commonly used with low contrast extended objects to measure wave-front error. On the basis of theoretical analysis and comparison of measurement error of absolute difference function and absolute difference function-squared using parabolic interpolation, the solar granulation images collected from real scene are used to validate the two algorithms experimentally. The result indicates that the correlation function of absolute difference function-squared better accords with parabolic distribution than the absolute difference function. The measurement accuracy of absolute difference function-squared is better than absolute difference function when using parabolic interpolation.