From Academy Bylaws:
Article III, Section 1. Membership shall be by invitation of the Board of Governors. Invitations to active membership shall be limited to those persons active in the motion picture arts and sciences, or credited with screen achievements, or who have otherwise achieved distinction in the motion picture arts and sciences and who, in the opinion of the Board, are qualified for membership.
To be considered for invitation to membership in the Cinematographers Branch of the Academy, an individual must:
(a) have two theatrical feature film credits, at least one within the last three years, as Director of Photography of a caliber which, in the opinion of the executive committee, reflect the high standards of the Academy,
(b) have screen credit as Director of Photography on a picture nominated for the Academy Cinematography Award,
(c) have, in the judgment of the Cinematographers Branch Executive Committee, otherwise achieved unique distinction, earned special merit or made an outstanding contribution as a motion picture cinematographer.