sentences:  When the photographer pressed either of two AF buttons on the barrel of the lens, the computer would signal the lens' motor to turn the lens' focusing helical until it sensed equal contrast between the sensor's two rows.This meant that each row was reading the subject as equally out of focus.Since one row was mounted slightly in front of the film plane, while the other was an equal distance behind, this equated to highest subject contrast in between the rows and a sharp, in-focus subject on the film.