Before cameras, did artists grasp the concept of things being "out of focus"?

And this I think directly related to the OP’s question. As GuanoLad pointed out, there’s no real lack of “focus” in in the natural world as seen through human eyes; ignoring extreme edge cases, whatever you’re looking at is in focus. We think of distant things as getting fuzzier, but that’s not the same thing as focus (at least not in the lensing sense). The background of Mona Lisa shows the effects of haze and distance, not a lens effect (note that the colors wash out compared to the foreground, too, which doesn’t happen with misfocus).

Computer graphics started going through this phase about ten years ago where “photorealistic” images started incorporating a digital lack of focus at certain distances, to emulate focal length. It’s an interesting cultural curiosity that we consider an image to be MORE real if it contains the faults of a camera’s viewing system (focus) than if looks like the real world does to our eyes. We’ve grown up with photographs, and now we expect our digital images to look more like a photo than nature!

As I pointed out, there is – things that are too close to the human eye are out-of-focus. Aside from using a pinhole to restrict the edge rays, or pulling the object back, there’s nothing you can do about it – your eyes simply can’t focus on objects too close.
And people with myopia can’t focus on objects too far away. And people with presbyopia can’t focus on nearby objects. But it’s hard to get inside someone else’s head and know what they can or can’t see – that’s why I bring up nearby obkects. That lack of focus is common to and accessible by ALL people. The concept of “out of focus” is certainly available to everyone.

And anyone who opens their eyes underwater (assuming they’re in clear water) can experience the same thing, too.