try this: take a picture, same object, no flash, ... one under normal bright sun and another in a room under normal bright lighting. which one is sharper?
set phone brightness to lowest, another on highest. try to read something and measure your arm's length. adjust brightness so you can still focus at farthest arm's length as comfort dictates. don't save on power juice. your eyes are more important.
drive outside and then go to underground parking lot. you'll notice it's harder to look.
use your normal glasses in a normal lighting room. go outside and swith to lower power (say your previous glasses). notice the difference.
the problem was i used the same power glasses all the time, out of habit. the very same glasses for reading far (chalkboards) and for reading books. it's somewhat not practical to switch glasses every time you copy something from board and write to your notes. using lower powered glasses and move my seat to the front mitigate this problem.
tldr; try to get maximum amount of light to your eyes as comfort dictates. if you really have to use glasses, choose the one with lower power that's sufficient for your need. practice to never use glasses for most tasks (adjust brightness on smartphone). you'll be surprised on what ur eyes can do without glasses.