Determine defocus image sharpness through image sharpness evaluation function is the key to realize camera auto-focusing

using sum of optimal discrete cosine transform（ DCT） coefficients as method for evaluating image sharpness

and do correlation analysis using this method with the image modulation transfer function（ MTF）

in order to verify the effectiveness of evaluation method

the results show that evaluation method based on discrete cosine transform coincide with the MTF. After normalization

the best image values are 1

the function decrease with the image becomes hazing.