Viewing Distance
The distance from which a print is normally seen, which determines how much resolution and sharpening it actually needs.
Formula / rule
Typical distance ≈ print diagonal
Measured in
m
In depth
Visual acuity is angular, so resolution requirements fall in proportion to distance. Because large prints are naturally viewed from further away, they can be printed at far lower ppi than a small one held at arm's length — which is why a billboard printed at 20 ppi looks perfectly sharp from the roadside.
Examples in practice
A two-metre print viewed from three metres reads as sharp at 100 ppi, where an A4 would not.