Pixel Peeping – Visual Acuity and viewing distance
Different devices are meant to be viewed from different distances, thus the actual pixel size doesn’t have to be identical. You usually don’t hold a smartphone at arm’s-length to read the information on it, and you usually don’t watch TV from 3ft/1m away.
The optimal viewing distance of an image – either a physical print or a digital screen – is based on two factors; first is the diagonal image size and second are the pixels per inch required at that distance to give the impression of a sharp image.
- Firstly the rough rule of thumb is that the viewing distance should be 1.5 to 2 times the diagonal length. This will give you an optimal viewing distance for the overall image based on the human eye’s ideal viewing angle. For some cases this can be dropped to 1 times the frame’s diagonal, and as far as desktop screens go, this is usually a compromise we have to make, as with screens 24″ or bigger, it is not possible with the standard desk sizes to be seated 4ft/1.2m away as the rule of thumb would suggest. This is fine, but keep in mind that the further we move away from the RoT and closer to the screen, the more “panning” our eyes or even head has to do for us to control the action on the screen, simply because our eyes don’t have the viewing angle to see the displayed information from edge to edge.
- Secondly for the image to look good at the distance you choose, there need to be sufficient pixels per inch (ppi) to fool the eye into seeing a smooth image that isn’t pixelated. The minimum ppi needed for a print with acceptable quality is calculated by dividing the value 3438 by the viewing distance. Anything above this ppi will look good at the distance chosen.
So: minimum ppi for a 20/20 vision person to see a perfectly sharp image, has been determined to be: 3438 / Viewing Distance (in)
where 3438, a constant for human vision, was derived as follows:
1/ppi = 2 x Viewing Distance (in) x tan (0.000290888/2)
1/ppi = Viewing Distance (in) x tan (0.000290888)
ppi = 3438/Viewing Distance (in), where 0.000290888 radians (1 arc minute) is known as the ‘visual acuity angle‘ and represents how much resolution a human can see.
Thus for example the same high quality digital image can be printed as a full-cover magazine page, a large archival print for a gallery, or cover a huge billboard along the highways. PPI/DPI of the printed images will of course differ tremendously, but the intended viewing distance will be so different, that the human eye cannot tell.
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