I found with the Pentax 6x7 Mk II and the extension tubes, the metering has to be done 'stopped down' method or it does not work properly -- I ruined some photos just after I got the MkII by relying on the built-in metering prism -- with my Mk I and a hand-held Weston Master V I had no trouble , just gave extra exposure than indicated to allow for the extension and photos came out perfectly !.