Optimum HDTV viewing distance is the distance that provides the viewer with the optimum immersive visual HDTV experience.
Background HDTV is designed to provide an experience more realistic than the television system it is designed to replace. The "thrilling realism" HDTV attempts to offer arises from increased resolution (detail) and the typically large screen sizes. A larger display increases the visual angle at which content is viewed, both of which contribute to an increased feeling of presence. Thus, the correct viewing distance is critical to the enjoyment of HDTV as it is intended. While helping to define the HDTV standard, RCA engineer and member of the US delegation to the International Telecommunication Union-Radiocommunication Sector (ITU-R), Bernard J. Lechner, made an early analysis of viewing distance, deriving the so-called Lechner distance. This approach, based on the limits of the human eye, can be used for all resolutions (including future resolutions). This is the "optimal viewing distance" found in the ITU-R.
Presence The concept of presence has been described as the sensation of "reality", of "being there", and as "an illusion of non mediation". The concept of presence originated and was studied with regard to virtual reality (VR) and other 3D environments. It was later established that television viewers could also experience a feeling of presence. Presence is influenced by a number of factors, including video camera techniques, audio fidelity, visual and aural dimensionality, and most relevantly to this topic, image size (visual angle) and quality (angular resolution).
Visual angle The optimum viewing distance is affected by the horizontal angle of the camera capturing the image. One concept of an ideal viewing distance places the viewer where the horizontal angle subtended by the screen is the same as the horizontal angle captured by the camera. If this is the case, the angular relationships perceived by the viewer would be identical to those recorded by the camera. A mismatch in this regard is traditionally disregarded, but some rotating motions can make these distortions very noticeable as a pincushion effect. This is likely in 3D video games, so gamers are likely to adopt close viewing positions matched to a game's fixed field of view. If the camera's angle were always the same, an ideal viewing distance could be easily calculated. However, the camera's horizontal angle varies as the focal length of its lens changes. If the camera's sensor has fixed dimensions, a shorter focal length (wide angle) lens captures a wider angle of view, requiring the viewer to sit closer to the screen. Conversely, a longer focal length (telephoto) lens captures a narrower angle of view, demanding a more distant viewer position. Such opposing viewing distances would not only be impractical, but would negate the very purposes of telephoto shots (for example, to see a distant object in more detail, or minimize distortion in facial images) and wide-angle shots (causing the viewer to sit too close to the screen, where undesirable image artifacts would be visible). One compromise assumes the lens is "standard" (a 50 mm focal length, for a standard 35 mm format). A "standard" lens preserves the same spatial relationships perceived by a spectator at the camera location. For a "standard" lens image, viewing distance should be equal to the diagonal length of the screen.
It has been demonstrated that viewing a display that occupies a greater visual angle (also referred to as field of view) increases the feeling of presence. More importantly, the wider the visual angle (up to a plateau at approximately 80 degrees), the greater the feeling of presence.
Angular resolution
With printed graphics, resolution refers to the number of pixels (usually referred to as "dots") in a fixed linear measurement. With HDTV, resolution is measured in terms of the number of pixels in the physical display. When the resolution of a printed image is increased, the image is cleaner, crisper and more detailed. However, image quality does not improve if the increase in resolution exceeds the observer's visual capabilities. For an HDTV's image to noticeably improve, its resolution per degree of arc (or angular resolution) must increase as well as the pixel count of the display.
Recommendations To maximize the feeling of presence and thus provide a better viewing experience, the viewer would need to be situated at the theoretical spot where the HDTV occupies the widest view angle for that viewer. It is also important that the resolution of the display per degree of arc remain at a high quality level. Opinions regarding where the ideal position lies are numerous and varied. Recommendations on HDTV viewing distances fall into two general classes; a fixed distance based on HDTV display size, or a range of distances based on the display size. The most common recommendations from reasonably authoritative sources are presented below.
Fixed distance Fixed distance recommendations are the more common of the two types. For the most part, the majority of the fixed distance recommendations were issued before the end of 2007, when arguably HDTV displays were still in the early adoption phase. The concept of optimal viewing distance, which is based on physics and physiology, has become essential with the advent of UHD (Ultra High Definition) technology. This new technology allows for much shorter viewing distances, which requires greater precision.
Optimal viewing distance The "optimal viewing distance" is based on the limits of the human eye, i.e. its angle of resolution. This is its ability to distinguish between two pixels. For normal visual acuity (6/6 vision), this angle is 1 arcmin. To obtain a fixed distance for a given resolution, it must be expressed in picture heights (H). If a screen is 50 cm high and it is at a distance of 250 cm, then in picture heights, its distance is 5 H (250/50). Mathematically, this gives the distances shown in the following table:
This table can be used in a number of ways :
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