We analyses beam quality and compensation efficiency in the case ol finite correct-element size by using the deformable mirror as a spatial filter. Although the StrehPs ratio increases as the subaperture size reduces, it reduces very slowly when the size is small. Based on our theories the optimal size can be easily determined for high performan-ce-to-eost ratio. In order to compare with experimental results we have introduced concept of compensation efficiency. The calculations reveal that the compensation effciency is low for very weak turbulence. It increases at the beginning while turbulence becomes stonger. But it rapidly decreases after reaching the maximum value. Theese conclusions support the previous experimental facts.