The problem of the second harmonic generation (SHG) is considered in the preset-field approximation for the scalar «ooe»-synchronism in a KDP crystal for the fundamental radiation at λ ≥ 0.53 μm. It is shown that the maximum of nonlinear conversion efficiency is reached at focusing the incident beam into the crystal using two crossed cylindrical lenses with the focal lengths being the unique optimal pair. In this case, wave detuning should be optimal as well. It is proved that the use of optimal cylindrical focusing increases the efficiency by approximately 25% as compared to the optical spherical focusing. Empirical expressions are proposed for approximate estimates of the optimal values of focal lengths and wave detuning. It has been revealed that the accuracy of these estimates keeps tolerable even if the initial parameters vary in quite a wide range.