I can do this very easily, and I’ve been able to as long as I can remember. I always assumed everyone could.
Then again, I have weird eyes in general. One has almost perfect vision, the other is really bad (don’t remember the exact prescription, but the big “E” on the vision chart is just a dark blur when I don’t have my single contact in).
Yes. Mentally, I move my point of focus from the monitor "backward" into space -- like an imaginary 3D cursor anchored on my screen, then moving away from me along the ray of my eyesight. Almost as if I was going to peer around the monitor and look for something behind it, except without actual movement. Imagine doing that, and it might help.
It's easier to do with one eye shut than with them both open.
I can do this. I've noticed that it's bringing my focus to a point closer to my face. I guess I can do it the other way because I can refocus, but I've noticed my eyes out of focus close and in focus at distance. I'm curious if this ability correlates with good vision, because my distance vision is ridiculously good.
I can also (through conscious effort) expand my pupil size. Looking in the mirror if, instead of focusing on one point, I try to look at a larger area (without moving my eyes) my pupils will expand.
There are hidden picture books which look like random noise until you manage to focus your eyes 'behind' the book. I could never do that but others could. So I would say yes, you can with some effort.
I mean with just one eye, being able to consciously refocus that eye.
For example, looking at this writing on your screen able to make that go in and out of focus.
I can bring my finger into focus, drawing my focal point in front of the screen, and if then remove my finger I can hold that focus (with about a 50% success rate). Perhaps that is cheating a little ;).
I can do this. It took me a very, very long time to figure out how to explain it to people who have "normal" vision. For what it's worth, I also have diplopia and I'm convinced that they are related.
This image[1] is pretty good. Look at the two squares, and squint a lot, so you see double, and the try to make the pretty far. Now gradually squint less and try to make the two center squares align with each other so you see three squares. When you manage to do that you have the correct focus. Shift your gaze to the image.
I think your friend isn't as lucky as you think. The eye/brain should automatically make the focus as sharp as possible so all that you can do consciously is make it worse.
Then again, if your vision isn't perfect there are tricks like placing pressure on the eyeball or squinting which can change the focus, but are probably not too healthy in the long term.
Best advice is to see an optometrist instead. Although, I have found that they aim to correct vision for 6 metres in red-green light. If you're reasonably young and myopic, ask for a slightly stronger minus power to get better blue vision at infinity (at the risk of more eye strain close up). The extra -0.25 makes a huge difference for me, really allows depth perception to "click" properly.
20/20 (or 6/6 in metres) is awful. I didn't know that until optometrists started moving from cards at a distance (where I could choose which line I wanted to be able to read, thank you very much) to the retinal projection thingies most of them use now. The difference is, as you noted, about -0.25 for myopia in the -3 to -4 dioptre correction range, and it's huge. I'll assume that it's a better set-up than the old way for astigmatism (which was never a problem for me), but settling for "normal" vision when there's an option... I don't think so.
If you know a bit about optics it's not hard to derive. The difference between correction at 6m vs infinity is 1/6 - 1/inf = 0.166.. dioptres.
On top of this, one thing that surprised me was the difference in correction between red, green and blue light. The refractive index of the eye varies according to light frequency such that blue is focused approximately 2 dioptres more than red. If your myopic eyes are corrected for red at infinity, you won't be able to focus on any blue light further than 0.5m away! Not really a concern in the natural world but very noticeable in a server room full of blue LEDs or on a screen with blue text on black.
The difference in power between red and green is less but still useful enough for some vision tests. If you can focus on red traffic lights but green ones are blurry, this is why - consider a stronger power as long as it doesn't strain your near vision.
I'm slightly farsighted, and while I can see without my glasses, I have to make a conscious effort to bring the world into focus if I'm not wearing them and headaches eventually result.
Only trick I know is getting the right prescription glasses...
Are you able to consciously refocus your eye?
By this I mean stare at an object (with one eye), and consciously make it go in and out of focus, whilst always staring straight at it.
I find that I can do this to a certain extent - is this something that most people can do?