The eye resolves less detail from far away. That is why billboards use low DPI while photo books aim near 300.
Rule of thumb
Closer than arm’s length → aim high (≈240–300 DPI). Wall / poster distance → often 100–150 DPI. Street / billboard distance → much lower is normal.
Always verify with pixels
Multiply finished width (inches) by target DPI. If your image is narrower than that, soft results are likely at that size.
FAQ
Why don’t billboards need 300 DPI?
Viewers stand far away, so lower pixel density still looks continuous. 300 DPI would make files impractically huge.