A CARJ CREW; ON A VISUAL APCH TO EWR; DEPARTED ASSIGNED ALT ONCE FIELD WAS IN SIGHT; WITHOUT FIRST GETTING APPROVAL FROM ATC.

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UNFAMILIAR (RELATIVELY) WITH THE AREA; WE WERE TOLD TO EXPECT A VISUAL APCH. WE HAD BEEN TO THE ARPT MULTIPLE TIMES THIS MONTH SO THOUGHT WE WOULD BE ABLE TO SEE IT. AS WE GOT CLOSER TO THE OM FOR RWY 22R; APCH ASKED IF WE HAD THE ARPT. WE TOLD HIM 'NO' AND HE ASKED IF WE WANTED THE ILS OR CONTINUE FOR THE VISUAL. KNOWING WE SHOULD BE ABLE TO SEE THE ARPT AS WE GOT CLOSER TO LINING UP WITH THE RWYS; THE CAPT MADE THE DECISION TO CONTINUE WITH THE VISUAL. I RELAYED THIS TO APCH. THEY THEN SAID; '...DSND TO 1900 FT FOR THE VISUAL.' WE HAD BEEN NAVING BY THE FMS; AND SWITCHED TO NAV BY NAVAIDS (ILS WAS TUNED IN). NORMALLY THE INBOUND COURSE WILL AUTO-TUNE ON THE HSI; MINE DID AND THE CAPT'S DID NOT. AT THIS POINT THE CAPT HAD THE AIRFIELD IN SIGHT AND DISCONNECTED THE AUTOPLT. I LOOKED AT MY COMMERCIAL CHARTS AND REACHED TO DIAL IN HIS INBOUND COURSE ON HIS HSI. WHILE I WAS DOING THIS; HE HAD APPARENTLY STARTED A VISUAL APCH THINKING WE HAD CLRNC. I DIDN'T NOTICE THIS OCCURRING SINCE I WAS TUNING IN HIS HSI. APCH CAME OVER THE RADIO AND SAID '...ALT ALERT; DON'T DSND ANY FURTHER UNTIL YOU HAVE THE FIELD IN SIGHT (APPROX 1600 FT). I IMMEDIATELY TOLD HIM '...FIELD IN SIGHT.' WE WERE THEN CLRED FOR THE VISUAL APCH INTO NEWARK INTL AND LANDED WITHOUT FURTHER INCIDENCE.