Viewing distance is the distance between the viewer and the screen, fixed for every subject and reported, because it determines which artefacts are visible and so a floating distance silently adds noise to every score. ITU-R BT.500-15 expresses it as a multiple of picture height rather than an absolute length, and offers two choices. The preferred viewing distance (PVD) is where viewers naturally choose to sit, used when the question is how real viewers experience the content. The design viewing distance (DVD) is the geometric distance at which two adjacent pixels just merge - the eye at the limit of the display's resolution - the hardest test of resolution. For a 4K display BT.500 places the resolution-critical distance at about 1.6 to 3.2 picture heights, the lower end used when the test is about resolution. It is one of the controlled viewing conditions, alongside calibrated display luminance and room lighting; mark it on the floor and seat every viewer there, or you measure the room rather than the codec.
