As for feeling old, try to grasp this 
.
This is an old manual-focus lens:
https://images.app.goo.gl/jkAsrRNuTeSGcCiX6
When you focus it at a certain distance (for example, the 8 feet the lens is focused at in this image), you’ll see another set of numbers (the 22 - 16 - 8 - 4 - 4 - 8 - 16 - 22).
That is to indicate to you that when you are focused at this distance, at f4 the indicated distance is in focus, at f8 that indicated distance is in focus, etc…
So, when you focus at 8ft, at f16 you’ll have something in focus between +/- 5ft and 15ft according to this image.
It’s used in street photography (but can be used for everything of course) to just keep your camera and F stop set at a certain setting, and then remember that everything between ‘1m and 4m is sort of in focus’. And if you then just remember to be at give or take 2.5m distance, you can just point your camera quickly and take the shot and keep walking.
Once again, the tolerances for ‘in focus’ this way have become stricter in the modern era, but the principle still stays .