Rods, Cones, and the Fovea

Figure 4. is a diagram of horizontal cross section of the right eye viewed from the top. The nose is towards the bottom of the figure.

For another diagram of the eye with links to definitions, click HERE

The retina contains the receptors for vision: about 100 million rods and 6 million cones. Because it contains these light detectors, the retina serves the same function as film in a camera does. But film in a camera passively records light. The retina actively changes what it receives. One thing the retina does is to adapt to the intensity of light reaching it. It can even adapt different areas separately to the different amounts of light they receive. This means that a part of the retina that receives a lot of light becomes less sensitive, whereas another part that receives only a little light can get more sensitive. This is why you can see things in brightly lit places and in shadows at the same time. A photograph can show clearly a brightly lit area or a shaded area, but not in the same picture, because the film cannot change its sensitivity.

Rods and cones convert light to neural signals which the nervous system can use. The 6 million cones are concentrated mostly in and near the fovea, a small dimple in the middle of the retina. The fovea receives light from the middle of the visual field, where you look directly. The remaining cones are scattered in the rest of the retina. The 100 million rods are locates in the retina away from the fovea, so they carry out peripheral vision ("side" vision towards the edge of the visual field).

Figure 6 shows the distribution of rods and cones on the retina.

The X axis shows the distance on the retina of the right eye from the fovea towards the middle (nose) and side (temple) of the head. The Y axis shows the density of the rods or cones. The olive-colored line for rods goes to zero at the fovea, where the red line for cones rises rapidly to its peak. The line for rods reaches maximum about 10o away from the fovea, and slopes downward further away.

For more information about how the retina works, click HERE

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