All about cameras, but our eye is not a camera.
A simple experiment:
Take Poser, set any focal distance, take the ground plane, set the Camera to look at the ground plane and apply any texture to the ground plane. Next render the scene.
What do you see? At medium distance the texture looks good, at long distance the texture is not so good and in short distance the texture is horrible and distorted.
You can argue that is the texture, well take a huge and with excellent quality texture. When you render it the result is better, but still is bad at short distance.
Now look through your window and at your desktop at the same time. Do you see that the texture of the desktop that if few centimeters fro your eye horrible and distorted? Nooo!
You can argue that is the focal distance, fine. Adjust Poser's camera to a focal distance until the texture at short distance become to look good. What do you see?, well most of the scene had disappeared remaing a big ground and any object that is far away when visible is deformed.
Meantime your eyes continue to see your desktop surface and the far away though your window continue to be normal, nothing disappear and nothing get deformed no matter where you set your focus.

Now the focal length in action. To adjust the focal distance the lens of a camera moves. Between focus on near and far objects the lens moves some millimeters and depending on the camera it can move some centimeters, also for near or far objects you need to adjust the aperture. For very near objects you need special cameras that doesn't work for long distance unless you change the lens.
Well,  beside cartoons, I never noticed the eyes of any person moving front-back, neither changing the eyes for looking at very near or far away objects

Stupidity also evolves!