Table 1 Comparison of cones and photopic vision with rods and scotopic vision in humans

From: Why rods and cones?

  Cones and photopic vision Rods and scotopic vision
Overall contribution Cones underlie almost all our vision (under twenty-first century metropolitan light levels) Rods contribute only under restricted conditions: after a considerable time at very low light levels
Spatial acuity Extremely high (in the fovea) Very low
Speed of response Very fast (flicker detectable beyond 100 Hz) Slow
Operating range Enormous (twilight upwards, without limit) Restricted (twilight downwards several log units)
Saturation Cones do not saturate in steady light, no matter how bright Rods saturate at roughly twilight levels, and are unresponsive at daytime lighting levels
Light adaptation Cones adapt rapidly, exhibiting Weber Law desensitisation over a huge range of intensities Rods adapt more slowly and over a narrower range of intensities; at the lowest intensities scotopic adaption follows approximately a square-root law
Contrast sensitivity High (detect contrasts of 0.5%) Low (need contrasts of ~5%)
Recovery of photocurrent after full bleach Cones recover circulating current within 20 ms Rods take 20 min to recover circulating current; that is, ~60 000 × slower than cones
Dark adaptation of visual system after full bleach Photopic vision recovers full sensitivity in ~5 min Scotopic vision takes ~40 min to recover full sensitivity
Colour vision Trichromatic colour vision mediated by comparison of signals from three spectral classes of cone
Proportion of photoreceptors over entire retina 5% 95%
Proportion of photoreceptors in foveola 100% 0%