Spatial resolution of the eye

Medical Physics | AQA A-Level Physics

Key Definition

Spatial resolution: The ability to distinguish two closely spaced objects as separate. Two point sources can only be resolved if their images fall on two separate photoreceptors with at least one unstimulated photoreceptor between them.

For two points of light to be seen as separate, their images must stimulate two different rods or cones with at least one unbleached (unstimulated) receptor in between.
Cones give better spatial resolution than rods because each cone has its own nerve fibre. Multiple rods share a single nerve fibre, so the brain cannot tell which rod was stimulated.
The fovea has the highest spatial resolution because it has the greatest density of cones.
In dim light, resolution is much worse because only rods function, and rods have poor individual resolution due to convergence.