{HEAD ~ The imaging lens system} 21, wherein the fourth lens element has {TAIL ~ a concave object side surface} 
{ HEAD ~ The imaging lens system } 21 , wherein the fourth lens element has { TAIL ~ a concave object side surface }
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL