Functional Differences

• Rods are the more sensitive photoreceptors and are capable of detecting light at lower levels, but they are not capable of detecting different colors. For this reason, rods feature most prominently in night vision and peripheral vision. Cones are not as light-sensitive, but they provide better resolution and spatial acuity and are also responsible for color vision. Cones come in three varieties, with pigments that can detect short, medium and long wavelengths of light, respectively, allowing for color perception.
  
  

Structural Differences

• Both types of cells have an outer segment consisting of stacked membranes connected to the cell body and a synoptic fiber. The outer membranes of the rod cell form a cylinder, while these membranes form a cone shape in the cone cell. The rod cell's circular membranes, however, are free-floating disks, whereas the cone cell has disks that attach directly to the outer membrane. Additionally, the rods' greater photoreceptivity comes from the fact that they contain a different pigment called rhodopsin.
  
  

Location

• Cones are located throughout the retina but are concentrated most heavily in a spot known as the fovea centralis, which lies at the center of the visual axis, where the most light enters the eye. Rods, by contrast, are not found directly at the fovea; they are distributed in large numbers throughout the rest of the retina. Both types of cells are absent at the point where the optic nerve attaches to the retina, an area known as the blind spot.
  
  

Number and Density

• Rods are far more numerous than cones. The typical human eye contains approximately 120 million rods and between 6 million and 7 million cones. The distribution of rods and cones also differs substantially. The vast majority of cones are packed into the fovea centralis, where the density of cones can approach 150,000 per square millimeter. Moving just 10 to 15 degrees away from the fovea, the number of cones drops off dramatically. The density of rods is greatest 15 to 20 degrees from the fovea and drops off much more gradually toward the outer portion of the retina.