A novel definition measurement based on correlation of the wavelet transform at adjacent scales is presented, which improves the sharpness function`s performance of unimodality and noise-immunity. It takes advantage of the property that wavelet coefficients magnitude of signals increase as scale increases and wavelet coefficients magnitude of noise decrease with increasing scale and calculates the multiplication of wavelet coefficients at the adjacent scales to construct the evaluation method. It is capable to amplify signals and suppress noise. Compared with traditional algorithms using Image Data acquired by CCD, the proposed method is found to be more robust and accurate to estimate the focusing extent.