system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do our eyes "lock on" to objects, keeping them in constant focus despite movement, to the point where un-focusing takes conscious effort?
gpt
Our evolutionary needs required us to be able to narrowly focus on specific objects at specific distances, be it a branch we're jumping to grab, a predator hiding in the bushes, or a prey animal we're hunting. We developed the ability to control our eyes independent of our head (birds, for example, lack this and move their heads to see, not their eyes). Our forward facing eyes increase the overlap of vision from each eye and are adapted to increase depth perception, to better understand distance and relative motion of the things we're focusing on. 

We have the ability to change the shape of the lens in our eyes which adjusts the beam of light we're "seeing", allowing us to track the image and keep in focus. 

In short, our ancestors who lacked this focusing abilities missed the branch they were swinging for, didn't see the tiger in the grass, and didn't kill the food they needed to survive.