| 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% |
