When images are projected in competition the projector sets the size and aspect ratio of the image, typically most projectors use a 4:3 aspect ratio. Many digital SLR cameras capture images in a more landscape format with a 3:2 aspect ratio. Even portrait orientation images are shown on the same projector with black bars added …
Alan Taberer found this interesting article , about the judging of photographic competitions by the late Eddy Sethna FRPS AFIAP.