Depth Cues
The visual signals — overlap, relative size, convergence, haze and blur — that let a flat image suggest three dimensions.
In depth
With one eye and no motion, stereoscopic depth is unavailable, so a photograph relies entirely on learned monocular cues. Overlap is the strongest and most reliable: whatever hides part of something else is understood to be in front, regardless of size or focus. Combining several cues produces a far stronger impression than relying on any one.
Examples in practice
Overlapping headlands receding into haze read as clearly separated in distance even in a completely flat light.