Strength means lens magnification.
If you need text to be bigger when reading, select a number higher than zero (eg. +2.50)
You can head to the link here to test your eyes – https://tbceyewear.com/blogs/welcome-to-the-club/find-your-strength
If you have perfect 20:20 sight but want some blue light protection, select +0.00 ONLY (all other numbers will magnify the world for you, which could be confusing slash dizzying!)
All of our frames have the same level of blue light protection, reducing transmittance by 30%