Hold out your arm and stare your fist, then focus on something 1 or 2 meters away, then stare your fist again and repeat 30 times. What happens is when you look at your fist, that image is clear but whataver else far away is blurred and when you focus on the canvas which is gar away, that image is clear but the fist is blurred. That means the crystaline or natural glasses of your eyes are adapting and you're forcing them to expand and contract, they are doing "exercises".

Also when you have to read something with nano-size letters, it's better using magnified glasses to increase the size of the letters on the paper .... than wearing glasses which obviously are closer to your eyes. Glasses are actually doing your eyes'job and that's why after a time you need more powerful glasses because your eyes GOT WORST.

Try to adapt your eyes watching in the darkness at night at least 20 minutes. Drink natural juices without sugar. Spin your eyes fast in one direction and then opposite way.