A GA INST RATED PLT BECOMES CONFUSED ABOUT THE NEED TO USE AN INST APCH WHEN THE TWR RPTS IFR AND YET HE CAN SEE THE RWY.

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WE WANTED TO MAKE A VFR FLT FROM OAKLAND TO SACRAMENTO MATHER; DUE TO A PAX'S DISCOMFORT WITH IMC. WE HAD CHKED WX 3 TIMES BEFORE DEP; AND MHR CONSISTENTLY RPTED 5 MI AND CLR. APCHING SACRAMENTO AIRSPACE FROM THE SW; WE OBTAINED MATHER ATIS; WHICH WAS AGAIN 5 MI AND CLR. WE WERE RECEIVING FLT FOLLOWING FROM SACRAMENTO APCH; AND WHEN WE WERE 4 MI FROM THE FIELD; SACRAMENTO APCH ADVISED THAT MATHER HAD JUST ISSUED A SPECIAL 1/2 MI VISIBILITY; CEILING 100 FT WITH FOG. QUITE A SURPRISE! WE REQUESTED AND WERE CLRED FOR THE ILS TO RWY 22L. AS WE APCHED THE ARPT; WE COULD CLRLY SEE RWY 22L; BUT THE EDGE OF THE FOG BANK HAD ENVELOPED THE TWR. WE ADVISED APCH THAT THE RWY WAS VFR; AND WE'D LIKE TO CANCEL IFR; AND JUST TURN BASE AND LAND. APCH REPLIED THAT HE DIDN'T THINK WE COULD CANCEL IFR; BECAUSE WE DIDN'T HAVE 1 MI OF VISIBILITY. WE ADVISED THAT WE HAD AT LEAST 5 MI OF FLT VISIBILITY; AND DIDN'T THAT GOVERN? APCH THEN HANDED US OFF TO THE TWR. TWR CLRED US TO LAND; WHICH WE DID IN VFR CONDITIONS. AFTER CHKING WITH MY INSTRUCTOR; I REALIZE I DIDN'T HANDLE THIS PROPERLY. WITH THE TWR RPTING IFR AND LESS THAN 1 MI VISIBILITY; I HAD NO OPTION BESIDES AN INST APCH. I FOCUSED ON THE VFR RWY AND FORGOT THAT RPTED VISIBILITY CTLED THE REQUIREMENT FOR A CLRNC.