A clever solution to aid in estimating distances to things.
If you know the length of your arm, the distance between your pupils, and you then measure the perceived parallax between each eye when lining up an object held at arms length, you can determine the distance to that thing.
Thomas Achtemichuk put up a generator to print out a card to make this simple. https://tomchuk.com/rf/
This has been helpful getting familiar with an camera that has no rangefinder.
