What's the difference among screens with SAME DIMENSIONS but DIFFERENT PITCH?

The difference is the optimal viewing distance. The LED maxi screen with an higher definition (therefore a the smaller pitch and a greater number of LEDs per m2) has an higher image’s quality, ideal even for short viewing distance. The screen with a lower definition (therefore with a bigger pitch and a smaller number of LEDs per m2) has a lower image’s quality, ideal for greater viewing distance.
It is important to clarify that the definition is determined by the LED screen’s viewing distance once it will be installed: if a maxi screen will be observed from 300 meters, it doesn’t need an high definition because of a biological reason: the human eye can not discriminate small details from larger distances anyways. On the other side, a maxi screen observed from 30 meters needs a higher definition because from such a short distance the human eye is able to recognize even small details.
To provide a comparison, in the table below are represented two hypothetical screens of the same dimensions but with different definition: the first (fig. 1) has a higher definition and is therefore ideal even for short viewing distances. The second one (fig. 2) has a lower definition and it is ideal for greater viewing distances. To better understand the “viewing distance” concept, please look at the pictures from few centimeters and then from 3 meters: even if the images have a different definition, if observed from 3 meters distance, they seem to have the same quality.

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