Binary fringe projection profilometry has experienced significant growth in recent years to eliminate the gamma effect of projectors. Determining the appropriate defocus amount is crucial for three-dimensional measurement in binary fringe defocus projection. Insufficient defocusing can result in fringes containing higher-order harmonics, while excessive defocusing can reduce fringe contrast, decrease phase resolution, and relatively increase noise impact. Both insufficient and excessive defocusing can affect the phase measurements accuracy. To address this problem, a method for determining the optimal defocus amount based on digital correlation is proposed. This method utilizes the correlation between fringes and their second-order derivatives to determine the optimal defocus amount and achieve quasi-sinusoidal fringes. In both under-defocus and over-defocus situations, the correlation coefficient is small, and it reaches its maximum value only when the fringes are quasi sinusoidal. The method was numerically simulated and experimentally validated. The experimental results show that the proposed method offers fast speed and high accuracy, enabling real-time tracking of the correlation coefficient during the binary fringe projection process and determining the optimal defocus amount.