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Human Eyesight & 4K Viewing

October 17, 2013

Resolution has been advancing continuously since the early days of analog television. The latest incarnation is 4K displays, but even higher resolution devices are possible. In this article, we take a closer look at the science of visual acuity, along with when higher resolutions are beneficial to the viewing experience.

BACKGROUND

Our perception of detail depends not only on the smallest features in a subject, but also on the distance at which these features are observed. Human vision is therefore typically described in terms of angular resolution:

Being able to resolve lines in characters that are separated by one sixtieth of a degree (an “arc minute”) is the goal when testing eyesight. Since this resolution is typically assessed using an eye chart at a distance of 20 feet, this level of performance is defined as 20/20 vision. Similarly, 20/50 means one can only resolve detail that someone with 20/20 vision could resolve from 50 feet away. Alternatively, with the metric system these are all standardized around 6 meters and referred to as 6/6 vision instead.

Although 20/20 is a reasonable capability for someone with good eyesight, this is by no means a limit. Each viewer fits in differently amongst the full spectrum of visual capabilities. Healthy eyes or someone with glasses may actually resolve substantially more. Furthermore, the visual capabilities required for discerning text are typically much higher than those required for perceiving additional detail.

RESOLUTION VS VIEWING ANGLE

With photography, fine art prints typically utilize 300 pixels per inch, which requires a viewing distance of one foot for someone with 20/20 vision to resolve. However, pixels do not always have to be this densely packed. With enlargements, the standard rule of thumb is that these will be viewed at a distance similar to the diagonal print dimension. This causes the print to encompass an angle of view spanning about 50° in its longest dimension:

Although these viewing angles are not always possible or practical, they are usually where someone will stand for an immersive viewing experience. At the same angle of view, one may therefore prefer being closer to a smaller screen, but further from a larger one:

Smaller Screen
(Closer Viewing)
Larger Screen
(Further Viewing)

This is primarily because this angle closely matches the 40-60° central angle of view of our own eyes:

Filling this angle helps the viewer feel more as though they are within a scene as opposed to looking at it inside a rectangle. In effect, higher resolution enhances the sense of detail, whereas wider viewing angles enhance the sense of “being there.” Both are needed to enhance the sense of realism.

The key is that as the viewing angle increases, either due to a larger image or a closer viewing distance, the resolvable pixels also increases. Applying similar viewing angles to home cinema, high definition displays are typically out-resolved and can appear pixelated. 4K resolution is required to produce maximally sharp and seemingly continuous pixels for a majority of viewers:

The Health and Nutrition Examination Survey of 1972 demonstrated that 72.8 percent of the civilian non-institutionalized population 4-74 years of age in the United States has distance visual acuity of at least 20/20 in their better eye “with usual correction” (using glasses and other visual aids).

With theatrical display, relative viewing distances are often a little closer, depending on which seats are chosen. Widescreen aspect ratios may also be intended to extend into the viewer’s peripheral vision. Seats near the middle to rear of most large-screen theaters create a 45-60° angle of view, but seats near the front can result in 90° or higher:

The diagram above depicts a typical large-screen theater with a 70 foot (21.3 meter) screen width. Although the IMAX, GSCA and other large-screen specifications recommend a minimum viewing distance of one screen width (53° viewing angle), the seating rows above extend out to a viewing angle of 45°. Even then, note how the majority of viewers can resolve more than 2K resolution from every seat in the theater.

OTHER CONSIDERATIONS

There will always be some viewers or viewing conditions that mean a given pixel count is not fully resolved. However, this does not necessarily mean those pixels were wasted, nor that they were not beneficial. A pixel density just beyond resolvability is important to prevent individual pixels from becoming visible. Furthermore, displays and projectors with higher resolutions typically also render coarser details with higher contrast, and these details are resolvable.

Regardless, imaging technology typically caters to optimal viewing, not necessarily to the least common denominator. When everything else is in place, display technology should maximize these conditions to ensure the highest fidelity possible. Tablets and notebooks now exceed 1080P, and even smart phones are beginning to have 1080P displays. Depicting more detail with cinema is a natural next step.

In addition, 4K content is only one of several factors contributing to our perception of detail. Cameras, lenses and focusing technique all need to meet a higher standard. Oversampled recordings are also helpful for making the most of a given pixel count. Only when all of these pieces are in place will the 4K viewing experience have its maximal effect.

FURTHER READING


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