MD83 CREW LANDED ON RWY 14 AT SGF. THEY FOUND THE RWY EDGE LIGHTS WERE OUTSIDE THE LOAD BEARING RWY PAVEMENT.

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AT 500 FT; SPD AND SINK WERE WITHIN TOLERANCES (STABILIZED). WE TOUCHED DOWN IN THE TOUCHDOWN ZONE ON THE CTRLINE WITH NO SIDE FORCES. I WENT INSIDE THE COCKPIT TO MONITOR THE INSTS. THE THRUST REVERSERS WERE IN EQUALLY AND NORMALLY. I THOUGHT THEY MIGHT GO ABOVE THE 1.6 EPR AND WAS ABOUT TO MAKE A CALL WHEN I NOTICED THE EPR DECREASING. I LOOKED OUTSIDE AND NOTICED WE WERE ANGLING TO THE L APPROX 10-15 DEGS. THE CAPT WAS CORRECTING SMOOTHLY TO RETURN TO THE CTRLINE. WE FELT A SLIGHT SHUDDER FOR APPROX 1 - 1 1/2 SECONDS. DURING THE TAXI IN; THERE WERE NO NOTICEABLE PROBS. AFTER SHUTDOWN; THE CAPT ELECTED TO PERFORM A POSTFLT INSPECTION AND NOTICED A LOT OF MUD AND GRASS ON THE L MAIN GEAR. ALTHOUGH WE HAD GONE TO THE L; I WAS SURE THE MAINS (L) HADN'T CROSSED THE RWY EDGE LIGHTS. AFTERWARDS; WE WERE INSPECTING THE RWY AND THE L EDGE WHEN WE NOTICED THAT THERE IS A GRASS STRIP BTWN THE ACTUAL RWY EDGE AND THE RWY LIGHTS. HAD WE KNOWN THIS I FEEL THE CAPT WOULD HAVE BEEN MORE AGGRESSIVE IN RETURNING TO THE CTRLINE. IN THE CURRENT SET-UP (GAP BTWN EDGE OF RWY AND LIGHTS) AT NIGHT IT IS EASY TO THINK YOU HAVE EXTRA RWY WIDTH. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: CREW LANDED ON RWY 14 AT SGF. THEY WERE USING THE RWY EDGE LIGHTING TO JUDGE THE DISTANCE TO THE EDGE OF THE RWY. THEY WERE SURPRISED TO FIND A GRASS AREA INSIDE THE RWY EDGE LIGHTS. THE CREW RPTED THE INCIDENT TO THE ACR. THE FO HAS BEEN SCHEDULED FOR A LINE CHK; AND THE CAPT WAS SENT FOR A SIMULATOR TRAINING SESSION. THE COMPANY DID NOT QUESTION WHY THE RWY LIGHTING WAS NOT AT THE LIMIT OF THE RWY LOAD BEARING AREA.