[ELI5]Why is it that sometimes things appear more clearly in our peripheral vision than when it is directly looked at?

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For example, when looking up the night sky, I can spot more stars in my peripheral vision. But some of them disappear when I look at that very spot directly.

In: 7

Peripheral vision is a pure anti predator survival tool which is why it’s very sensitive, it may just keep you alive..

Your eyes have two types of light sensitives cells, rods, and cones. Rods are more sensitive to light but only show the brightness, not the color. Cones are for color vision but are less light-sensitive than rods.

The central part of the vision contains more cones than the rods so you can see color bur require more light. The peripheral vision contains mostly rods so more light-sensitive but does not have a lot of colors.

The result is that dim light sources might be visible in the peripheral vision but not the central vision.

Evolutionary the function of peripheral vision is as a warning system to spot stuff that might be dangerous and color vision is not as important as low light vision. Detecting motion is often enough without really knowing what it is.

The central vision is used for stuff where color is more important like for example searching for fruit, something to hunt etc. We are mostly day active so good color vision in the day is an advantage.

The photoreceptor cells in our peripheral vision are spread out more, and so the image is blurred. The center of our vision is sharper, but our peripheral vision is better at detecting movement, shadows, and color.

Imagine you are standing in a field looking at a tree. If you focus your gaze directly on the tree, you will be able to see the fine details like the texture of the bark, the shape of the leaves, and the individual branches. If you look at the tree only from the corner of your eye, you will be able to detect movement from the branches swaying in the wind, but the details of the tree will be much less distinct.

I haven’t ready it here, so I might have one point to add:

The punctum caecum or blind spot ( https://en.m.wikipedia.org/wiki/Blind_spot_(vision) )
That’s the location where the optic nerve passes through the retina. Due to it beeing nervous tissue, there are no receptors present.

The wiki has a little test to demonstrate the blind spot.

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