is it normal that you can make your eyesight blurry whenever you want to all by yourself and then make it go back to normal?

Yes

Explanation:

But if I remember correctly, it does strain your eyes which can damage them or cause headaches

the more fit you are, the faster the recovery. your heart rate drops most sharply in the first minute after you stop exercising; it should then fall about 20 beats a minute—a drop of less than 12 beats a minute is considered abnormal. this “recovery heart rate” is measured as part of an exercise stress test.