The entire backside of your eyeball contains cells that detect light, so why do your eyes dart back and forth as you focus on these words? Because one tiny region, the fovea, has more photoreceptors — and therefore sharper vision — than the rest of the eye.
In this micro-landscape (a cross section of the retina), the fovea appears as a valley just one quarter as wide as a grain of sand. In this depression, information processing cells known as ganglion cells get out of the way so light can more easily hit the photoreceptors below — appearing here under the fovea as dark purple spheres, tightly packed at the center and thinning out on the sides. They lie at the bottom of the retina just above the blood vessels that supply their energy and oxygen, the band of sharp white and navy blobs at the image’s bottom.
Yet despite its diminutive size, this spot contains nearly all of your 6 million or so “cone” cells — the cells that sense color in bright conditions. These cells excel at resolving fine details in bright light but are less capable than other parts of the eye in the dark. That’s why you can sometimes see dim stars out of the corner of your eye that then disappear when you look straight at them with your fovea instead.
Nave, C. R. (2017). Rods and Cones. Retrieved April 29, 2020, from HyperPhysics website: http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html
Rehman, I., Mahabadi, N., Motlagh, M., & Ali, T. (2020). Anatomy, Head and Neck, Eye Fovea. In StatPearls. Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK482301/