You will need to purchase one of these converters; Belkin AV360, Kanex HD, or Atlona HD620.
The imac accepts 720p and 1440p input.
Only the Atlona option will provide a 1080p image on the imac screen, this is done by upscaling a 1080p image up to 1440p (an input the Imac accepts).
The Belkin and Kanex will provide a 720p image on the imac screen.