The numerical results show that the appearance of focal switch rests not only on the distance of aperture and lens but also on truncation parameter ; focal switch possibly occurs only when truncation parameter is between its critical maximum and minimum truncation parameters 數值結果表明:焦開關的出現不僅與光闌透鏡分離量有關,還與截斷參數值密切相關;僅當截斷參數取某一上、下限之間的值時,才可能出現焦開關。