Let us clear the obvious case of three displays with same diagonal, placed next to each other: there is a certain distance at which you can tell whether they have different resolution images. Let us focus on the case of a single TV. To tell the difference between HD and Ultra HD, but also between SD and HD, an objective criterion is given by the minimum distance at which a viewer with normal visual acuity perceives the image on the TV without grain. This distance depends – of course – not only on the resolution, but also on the diagonal of the TV. There are some specific tables in this regard. Without having to resort to the tables right now, we can make a practical case that clarifies the idea. Imagine we have a 42inch set displaying a given image and let us put ourselves at such a distance so as to see the image without grain. Let us approach gradually: if when at a distance of about 150cm, the image begins to “shell”, we are faced with a standard definition image, otherwise it is high definition. If I get close at about 75 cm, and the image is still free from grain, we are faced with 4K image.
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