In the direct drive, since the target is becoming smaller in compression process, the initial focal spot will no longer match the compressed target, leading to large losses of energy from target edge. Optical zooming means making the focal spot smaller when the target compresses. In this way, the coupling efficiency between the beam and target can be improved. It has great significance. The primary problem of solving the optical zooming is the qualification of small focal spot. According to the full-band optical transmission rules, in this paper constructed are low and medium-high frequency wavefront distortion sources under different smoothing conditions, and then given is a scaling relation between wavefront and focal spot size. The scaling relation can be regarded as a wavefront criterion of small focal spot. Using this wavefront criterion, the correction range of low and medium-high frequency wavefront distortion can be known to achieve a small focal spot.