Our optical world is pretty incredible, in that the further we are from objects the smaller they are in our field of view. Our brain accounts for this. It takes this relative height into account when judging the size of distance objects. It’s not something you have to think about, it happens automatically.
Next time you’re on the golf course, you can use it to your benefit. The flagstick is always seven feet high, regardless of how far you are from it. But in your field of vision, it may appear much smaller.
With your arm fully extended, thumb up, and body facing the flag, close one eye and observe the height of the flag relative to your thumb.
When you’re a hundred yards out, from this position, use a pen and mark the relative size of the flag on your thumb. At a hundred and fifty yards, do the same thing. At two hundred yards, do the same thing. As long as you stand the same way relative to the flag, you have a makeshift rangefinder. And it can be as accurate as you want it to be. Of course, it’s unlikely that you’ll be down to the yard accurate, but it should be possible to narrow the distance to ten to twenty yards with a properly ‘calibrated thumb.’
If you want to take it up a notch, there is no real reason to even know yardages anyways. We only use those to pick clubs. So next time you do it, just write the club number down instead of the yardage.