Looking at distant objects relaxes the eye lens, while looking at close objects requires effort.

“So, when you look far away, okay, when you see things far away, your lens actually relaxes. It can flatten out and you'll notice that it actually is relaxing to look at a horizon. Whereas, if I look at something up close to me, like this pen or my phone or a computer screen or this microphone, it takes effort. You'll sense the effort. [...] the lens actually gets thicker in order to bring the light to the retina and not to a location in front of it or behind it.”