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 数值结果表明：焦开关的出现不仅与光阑透镜分离量有关，还与截断参数值密切相关；仅当截断参数取某一上、下限之间的值时，才可能出现焦开关。
