LOS ANGELES — Academy Awards voters are going to have to put extra effort into their choices for best picture.

Oscar overseers are adopting a preferential-voting system in which academy members rank their best-picture favorites from 1 to 10.

The change was made because of the academy’s decision to expand the best-picture field from five nominees to 10 starting with the awards next March.

Under the old system with five nominees, voters simply chose one film, making the winner the one that received the most votes. The new method adds weight to voters’ No. 2 and 3 picks.

If the academy had maintained the old system, the best-picture prize conceivably could have gone to a film with barely 10 percent of the vote.